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Posted to commits@nifi.apache.org by mc...@apache.org on 2015/01/30 13:22:45 UTC
[1/3] incubator-nifi git commit: NIFI-152: Initial checkin of
developer guide
Repository: incubator-nifi
Updated Branches:
refs/heads/NIFI-250 1ebaf1d29 -> e7beef8d2
NIFI-152: Initial checkin of developer guide
Project: http://git-wip-us.apache.org/repos/asf/incubator-nifi/repo
Commit: http://git-wip-us.apache.org/repos/asf/incubator-nifi/commit/5830f005
Tree: http://git-wip-us.apache.org/repos/asf/incubator-nifi/tree/5830f005
Diff: http://git-wip-us.apache.org/repos/asf/incubator-nifi/diff/5830f005
Branch: refs/heads/NIFI-250
Commit: 5830f005f526acbc228eab5201260b0063b8ade6
Parents: 6b560b9
Author: Mark Payne <ma...@hotmail.com>
Authored: Thu Jan 29 08:42:49 2015 -0500
Committer: Mark Payne <ma...@hotmail.com>
Committed: Thu Jan 29 08:42:49 2015 -0500
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1 file changed, 1775 insertions(+), 16 deletions(-)
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http://git-wip-us.apache.org/repos/asf/incubator-nifi/blob/5830f005/nifi/nifi-docs/src/main/asciidoc/developer-guide.adoc
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diff --git a/nifi/nifi-docs/src/main/asciidoc/developer-guide.adoc b/nifi/nifi-docs/src/main/asciidoc/developer-guide.adoc
index bfaa669..1e9e509 100644
--- a/nifi/nifi-docs/src/main/asciidoc/developer-guide.adoc
+++ b/nifi/nifi-docs/src/main/asciidoc/developer-guide.adoc
@@ -19,27 +19,1786 @@ NiFi Developer's Guide
Apache NiFi Team <de...@nifi.incubator.apache.org>
:homepage: http://nifi.incubator.apache.org
-The designed points of extension
---------------------------------
-Processor, Prioritizer, ...
-The NiFi Archive (NAR) and NiFi Classloading
---------------------------------------------
-A NAR is ...
+== Introduction
-How to build extensions
------------------------
-Understanding Nars the first step is...
+This Developer Guide is written by developers for developers. It is expected that before reading this
+guide, you have a basic understanding of Apache NiFi (incubating) and the concepts of dataflow. If not,
+please see the link:overview.html[NiFi Overview] and the link:user-guide.html[NiFi User Guide] to familiar
+yourself with the concepts of NiFi.
-Design considerations
----------------------
-The right abstraction ...
+This guide also assumes that the reader is familiar with Java 7 and Apache Maven.
+
+The intent of this guide is to provide the reader with the information needed to understand how NiFi extensions
+are developed and help to explain the thought process behind developing the components.
+
+
+[[components]]
+== NiFi Components
+
+NiFi provides several extension points to provide developers the
+ability to add functionality
+to the application to meet their needs. The following list provides a
+high-level description
+of the most common extension points:
+
+- Processor
+ - The Processor interface is the mechanism through which NiFi
+exposes access to
+ FlowFiles, their attributes, and their content. The Processor is
+the basic building
+ block used to comprise a NiFi dataflow. This interface is used
+to accomplish
+ all of the following tasks:
+
+ - Create FlowFiles
+ - Read FlowFile content
+ - Write FlowFile content
+ - Read FlowFile attributes
+ - Update FlowFile attributes
+ - Ingest data
+ - Egress data
+ - Route data
+ - Extract data
+ - Modify data
+
+- ReportingTask
+ - The ReportingTask interface is a mechanism that NiFi exposes to allow metrics,
+ monitoring information, and internal NiFI state to be published to external
+ sources, such as log files, e-mail, and remote web services.
+
+- ControllerService
+ - A ControllerService provides shared state across Processors, other ControllerServices,
+ and ReportingTasks within a single JVM. An example use case may include loading a very
+ large dataset into memory. By performing this work in a ControllerService, the data
+ can be loaded once and be exposed to all Processors via this service.
+
+- FlowFilePrioritizer
+ - The FlowFilePrioritizer interface provides a mechanism by which FlowFiles
+ in a queue can be prioritized, or sorted, so that the FlowFiles can be processed in an order
+ that is most effective for a particular use case.
+
+- AuthorityProvider
+ - An AuthorityProvide is responsible for determining which privileges and roles, if any,
+ a given user should be granted.
+
+
+[[processor_api]]
+== Processor API
+
+The Processor is the most widely used Component available in NiFi.
+Processors are the only Component
+to which access is given to create, remove, modify, or inspect
+FlowFiles (data and attributes).
+
+All Processors are loaded and instantiated using Java's ServiceLoader
+mechanism. This means that all
+Processors must adhere to the following rules:
+
+ - The Processor must have a default constructor.
+ - The Processor's JAR file must contain an entry in the META-INF/services directory named
+ `org.apache.nifi.processor.Processor`. This is a text file where each line contains the
+ fully-qualified class name of a Processor.
+
+While `Processor` is an interface that can be implemented directly, it
+will be extremely rare to do so, as
+the `org.apache.nifi.processor.AbstractProcessor` is the base class
+for almost all Processor
+implementations. The `AbstractProcessor` class provides a significant
+amount of functionality, which makes
+the task of developing a Processor much easier and more convenient.
+For the scope of this document,
+we will focus primarily on the `AbstractProcessor` class when dealing
+with the Processor API.
+
+.Concurrency Note
+NiFi is a highly concurrent framework. This means that all extensions
+must be thread-safe.
+If unfamiliar with writing concurrent software in Java, it is highly
+recommended that you familiarize
+yourself with the principles of Java concurrency.
+
+
+[[supporting_api]]
+=== Supporting API
+
+In order to understand the Processor API, we must first understand -
+at least at a high level - several supporting classes and interfaces, which are discussed below.
+
+[[flowfile]]
+==== FlowFile
+A FlowFile is a logical notion that correlates a piece of data with a
+set of Attributes about that data.
+Such attributes include a FlowFile's unique identifier, as well as its
+name, size, and any number of other
+flow-specific values. While the contents and attributes of a FlowFile
+can change, the FlowFile object is
+immutable. Modifications to a FlowFile are made possible by the ProcessSession.
+
+[[process_session]]
+==== ProcessSession
+The ProcessSession, often referred to as simply a "session," provides
+a mechanism by which FlowFiles
+can be created, destroyed, examined, cloned, and transferred to other
+Processors. Additionally, a
+ProcessSession provides mechanism for creating modified versions of
+FlowFiles, by adding or removing
+attributes, or by modifying the FlowFile's content. The ProcessSession
+also exposes a mechanism for
+emitting provenance events that provide for the ability to track the
+lineage and history of a FlowFile.
+After operations are performed on one or more FlowFiles, a
+ProcessSession can be either committed or
+rolled back.
+
+[[process_context]]
+==== ProcessContext
+The ProcessContext provides a bridge between a Processor and the
+framework. It provides information
+about how the Processor is currently configured and allows the
+Processor to perform
+Framework-specific tasks, such as yielding its resources so that the
+framework will schedule other
+Processors to run without consuming resources unnecessarily.
+
+
+[[property_descriptor]]
+==== PropertyDescriptor
+PropertyDescriptor defines a property that is to be used by a
+Processor, ReportingTask, or ControllerService.
+The definition of a property includes its name, a description of the
+property, an optional default value,
+validation logic, and an indicator as to whether or not the property
+is required in order for the Processor
+to be valid. PropertyDescriptors are created by instantiating an
+instance of the `PropertyDescriptor.Builder`
+class, calling the appropriate methods to fill in the details about
+the property, and finally calling
+the `build` method.
+
+
+[[validator]]
+==== Validator
+A PropertyDescriptor may specify one or more Validators that can be
+used to ensure that the user-entered value
+for a property is valid. If a Validator indicates that a property
+value is invalid, the Component will not be
+able to be run or used until the property becomes valid.
+
+
+[[validation_context]]
+==== ValidationContext
+When validating property values, a ValidationContext can be used to
+obtain ControllerServices,
+create PropertyValue objects, and compile and evaluate property values
+using the Expression Language.
+
+
+[[property_value]]
+==== PropertyValue
+All property values returned to a Processor are returned in the form
+of a PropertyValue object. This
+object has convenience methods for converting the value from a String
+to other forms, such as numbers
+and time periods, as well as providing an API for evaluating the
+Expression Language.
+
+
+[[relationship]]
+==== Relationship
+Relationships define the routes to which a FlowFile may be transfered
+from a Processor. Relationships
+are created by instantiating an instance of the `Relationship.Builder`
+class, calling the appropriate methods
+to fill in the details of the Relationship, and finally calling the
+`build` method.
+
+
+[[processor_initialization_context]]
+==== ProcessorInitializationContext
+After a Processor is created, its `initialize` method will be called
+with an `InitializationContext` object.
+This object exposes configuration to the Processor that will not
+change throughout the life of the Processor,
+such as the unique identifier of the Processor.
+
+[[ProcessorLog]]
+==== ProcessorLog
+Processors are encouraged to perform their logging via the
+`ProcessorLog` interface, rather than obtaining
+a direct instance of a third-party logger. This is because logging via
+the ProcessorLog allows the framework
+to render log messages that exceed s a configurable severity level to
+the User Interface, allowing those who
+monitor the dataflow to be notified when important events occur.
+Additionally, it provides a consistent logging
+format for all Processors by logging stack traces when in DEBUG mode
+and providing the Processor's unique
+identifier in log messages.
+
+
+
+
+
+[[AbstractProcessor]]
+=== AbstractProcessor API
+
+Since the vast majority of Processors will be created by extending the
+AbstractProcessor, it is the
+abstract class that we will examine in this section. The
+AbstractProcessor provides several methods that
+will be of interest to Processor developers.
+
+
+==== Processor Initialization
+
+When a Processor is created, before any other methods are invoked, the
+`init` method of the
+AbstractProcessor will be invoked. The method takes a single argument,
+which is of type
+`ProcessorInitializationContext`. The context object supplies the
+Processor with a ProcessorLog,
+the Processor's unique identifier, and a ControllerServiceLookup that
+can be used to interact with the
+configured ControllerServices. Each of these objects is stored by the
+AbstractProcessor and may be obtained by
+subclasses via the `getLogger`, `getIdentifier`, and
+`getControllerServiceLookup` methods, respectively.
+
+
+==== Exposing Processor's Relationships
+
+In order for a Processor to transfer a FlowFile to a new destination
+for follow-on processing, the
+Processor must first be able to expose to the Framework all of the
+Relationships that it currently supports.
+This allows users of the application to connect Processors to one
+another by creating
+Connections between Processors and assigning the appropriate
+Relationships to those Connections.
+
+A Processor exposes the valid set of Relationships by overriding the
+`getRelationships` method.
+This method takes no arguments and returns a `Set` of `Relationship`
+objects. For most Processors, this Set
+will be static, but other Processors will generate the Set
+dynamically, based on user configuration.
+For those Processors for which the Set is static, it is advisable to
+create an immutable Set in the Processor's
+constructor or init method and return that value, rather than
+dynamically generating the Set. This
+pattern lends itself to cleaner code and better performance.
+
+
+==== Exposing Processor Properties
+
+Most Processors will require some amount of user configuration before
+they are able to be used. The properties
+that a Processor supports are exposed to the Framework via the
+`getSupportedPropertyDescriptors` method.
+This method takes no arguments and returns a `List` of
+`PropertyDescriptor` objects. The order of the objects in the
+List is important in that it dictates the order in which the
+properties will be rendered in the User Interface.
+
+A `PropertyDescriptor` object is constructed by creating a new
+instance of the `PropertyDescriptor.Builder` object,
+calling the appropriate methods on the builder, and finally calling
+the `build` method.
+
+While this method covers most of the use cases, it is sometimes
+desirable to allow users to configure
+additional properties whose name are not known. This can be achieved
+by overriding the
+`getSupportedDynamicPropertyDescriptor` method. This method takes a
+`String` as its only argument, which
+indicates the name of the property. The method returns a
+`PropertyDescriptor` object that can be used to validate
+both the name of the property, as well as the value. Any
+PropertyDescriptor that is returned from this method
+should be built setting the value of `isDynamic` to true in the
+`PropertyDescriptor.Builder` class. The default
+behavior of AbstractProcessor is to not allow any dynamically created
+properties.
+
+
+==== Validating Processor Properties
+
+A Processor is not able to be started if its configuration is not
+valid. Validation of a Processor property can
+be achieved by setting a Validator on a PropertyDescriptor or by
+restricting the allowable values for a
+property via the PropertyDescriptor.Builder's `allowableValues` method
+or `identifiesControllerService` method.
+
+There are times, though, when validating a Processor's properties
+individually is not sufficient. For this purpose,
+the AbstractProcessor exposes a `customValidate` method. The method
+takes a single argument of type `ValidationContext`.
+The return value of this method is a `Collection` of
+`ValidationResult` objects that describe any problems that were
+found during validation. Only those ValidationResult objects whose
+`isValid` method returns `false` should be returned.
+This method will be invoked only if all properties are valid according
+to their associated Validators and Allowable Values.
+I.e., this method will be called only if all properties are valid
+in-and-of themselves, and this method allows for
+validation of a Processor's configuration as a whole.
+
+
+==== Responding to Changes in Configuration
+
+It is sometimes desirable to have a Processor eagerly react when its
+properties are changed. The `onPropertyModified`
+method allows a Processor to do just that. When a user changes the
+property values for a Processor, the
+`onPropertyModified` method will be called for each modified property.
+The method takes three arguments: the PropertyDescriptor that
+indicates which property was modified,
+the old value, and the new value. If the property had no previous
+value, the second argument will be `null`. If the property
+was removed, the third argument will be `null`. It is important to
+note that this method will be called regardless of whether
+or not the values are valid. This method will be called only when a
+value is actually modified, rather than being
+called when a user updates a Processor without changing its value. At
+the point that this method is invoked, it is guaranteed
+that the thread invoking this method is the only thread currently
+executing code in the Processor, unless the Processor itself
+creates its own threads.
+
+
+==== Performing the Work
+
+When a Processor has work to do, it is scheduled to do so by having
+its `onTrigger` method called by the framework.
+The method takes two arguments: a `ProcessContext` and a
+`ProcessSession`. The first step in the `onTrigger` method
+is often to obtain a FlowFile on which the work is to be performed by
+calling one of the `get` methods on the ProcessSession.
+For Processors that ingest data into NiFi from external sources, this
+step is skipped. The Processor is then free to examine
+FlowFile attributes; add, remove, or modify attributes; read or modify
+FlowFile content; and transfer FlowFiles to the appropriate
+Relationships.
+
+
+==== When Processors are Triggered
+
+A Processor's `onTrigger` method will be called only when it is
+scheduled to run and when work exists for the Processor.
+Work is said to exist for a Processor if any of the following conditions is met:
+
+- A Connection whose destination is the Processor has at least one
+FlowFile in its queue
+- The Processors has no incoming Connections
+- The Processor is annotated with the @TriggerWhenEmpty annotation
+
+Several factors exist that will contribute to when a Processor's
+`onTrigger` method is invoked. First, the Processor will not
+be triggered unless a user has configured the Processor to run. If a
+Processor is scheduled to run, the Framework periodically
+(the period is configured by users in the User Interface) checks if
+there is work for the Processor to do, as described above.
+If so, the Framework will check downstream destinations of the
+Processor. If any of the Processor's outbound Connections is full,
+by default, the Processor will not be scheduled to run.
+
+However, the `@TriggerWhenAnyDestinationAvailable` annotation may be
+added to the Processor's class. In this case, the requirement
+is changed so that only one downstream destination must be "available"
+(a destination is considered "available" if the Connection's
+queue is not full), rather than requiring that all downstream
+destinations be available.
+
+Also related to Processor scheduling is the `@TriggerSerially`
+annotation. Processors that use this Annotation will never have more
+than one thread running the `onTrigger` method simultaneously. It is
+crucial to note, though, that the thread executing the code
+may change from invocation to invocation. Therefore, care must still
+be taken to ensure that the Processor is thread-safe!
+
+
+
+=== Component Lifecycle
+
+The NiFi API provides lifecycle support through use of Java
+Annotations. The `org.apache.nifi.annotations.lifecycle` package
+contains
+several annotations for lifecycle management. The following
+Annotations may be applied to Java methods in a NiFi component to
+indicate to
+the framework when the methods should be called. For the discussion of
+Component Lifecycle, we will define a NiFi component as a
+Processor, ControllerServices, or ReportingTask.
+
+==== @OnAdded
+
+The `@OnAdded` annotation causes a method to be invoked as soon as a
+component is created. The
+component's `initialize` method (or `init` method, if subclasses
+`AbstractProcessor`) will be invoked after the component is
+constructed,
+followed by methods that are annotated with `@OnAdded`. If any method
+annotated with `@OnAdded` throws an Exception, an error will
+be returned to the user, and that component will not be added to the
+flow. Furthermore, other methods with this
+Annotation will not be invoked. This method will be called only once
+for the lifetime of a component.
+Methods with this Annotation must take zero arguments.
+
+
+==== @OnRemoved
+
+The `@OnRemoved` annotation causes a method to be invoked before a
+component is removed from the flow.
+This allows resources to be cleaned up before removing a component.
+Methods with this annotation must take zero arguments.
+If a method with this annotation throws an Exception, the component
+will still be removed.
+
+==== @OnScheduled
+
+This annotation indicates that a method should be called every time
+the component is scheduled to run. Because ControllerServices
+are not scheduled, using this annotation on a ControllerService does
+not make sense and will not be honored. It should be
+used only for Processors and Reporting Tasks. If any method with this
+annotation throws an Exception, other methods with this
+annotation will not be invoked, and a notification will be presented
+to the user. In this case, methods annotated with
+`@OnUnscheduled` are then triggered, followed by methods with the
+`@OnStopped` annotation (during this state, if any of these
+methods throws an Exception, those Exceptions are ignored). The
+component will then yield its execution for some period of time,
+referred to as the "Administrative Yield Duration," which is a value
+that is configured in the `nifi.properties` file. Finally, the
+process will start again, until all of the methods annotated with
+`@OnScheduled` have returned without throwing any Exception.
+Methods with this annotation may take zero arguments or may take a
+single argument. If the single argument variation is used,
+the argument must be of type `ProcessContext` if the component is a
+Processor or `ConfigurationContext` if the component
+is a ReportingTask.
+
+==== @OnUnscheduled
+
+Methods with this annotation will be called whenever a Processor or
+ReportingTask is no longer scheduled to run. At that time, many threads
+may still be active in the Processor's `onTrigger` method. If such a method
+throws an Exception, a log message will be generated, and the
+Exception will be otherwise
+ignored and other methods with this annotation will still be invoked.
+Methods with this annotation may take zero arguments or may take a
+single argument.
+If the single argument variation is used, the argument must be of type
+`ProcessContext` if the component is a Processor or
+`ConfigurationContext` if the
+component is a ReportingTask.
+
+
+==== @OnStopped
+
+Methods with this annotation will be called when a Processor or
+ReportingTask is no longer scheduled to run
+and all threads have returned from the `onTrigger` method. If such a
+method throws an Exception,
+a lot message will be generated, and the Exception will otherwise be
+ignored; other methods with
+this annotation will still be invoked. Methods with this annotation
+must take zero arguments.
+
+
+==== @OnShutdown
+
+Any method that is annotated with the `@OnShutdown` annotation will be
+called when NiFi is successfully
+shut down. If such a method throws an Exception, a log message will be
+generated, and the
+Exception will be otherwise ignored and other methods with this
+annotation will still be invoked.
+Methods with this annotation must take zero arguments. Note: while
+NiFi will attempt to invoke methods
+with this annotation on all components that use it, this is not always
+possible. For example, the process
+may be killed unexpectedly, in which case it does not have a chance to
+invoke these methods. Therefore,
+while methods using this annotation can be used to clean up resources,
+for instance, they should not be
+relied upon for critical business logic.
+
+
+
+
+=== Reporting Processor Activity
+
+Processors are responsible for reporting their activity so that users
+are able to understand what happens
+to their data. Processors should log events via the ProcessorLog,
+which is accessible via the InitializationContext
+or by calling the `getLogger` method of `AbstractProcessor`.
+
+Additionally, Processors should use the `ProvenanceReporter`
+interface, obtained via the ProcessSession's
+`getProvenanceReporter` method. The ProvenanceReoprter should be used
+to indicate any time that content is
+received from an external source or sent to an external location. The
+ProvenanceReporter also has methods for
+reporting when a FlowFile is cloned, forked, or modified, and when
+multiple FlowFiles are merged into a single FlowFile
+as well as associating a FlowFile with some other identifier. However,
+these functions are less critical to report, as
+the framework is able to detect these things and emit appropriate
+events on the Processor's behalf. Yet, it is a best practice
+for the Processor developer to emit these events, as it becomes
+explicit in the code that these events are being emitted, and
+the developer is able to provide additional details to the events,
+such as the amount of time that the action took or
+pertinent information about the action that was taken. If the
+Processor emits an event, the framework will not emit a duplicate
+event. Instead, it always assumes that the Processor developer knows
+what is happening in the context of the Processor
+better than the framework does. The framework may, however, emit a
+different event. For example, if a Processor modifies both the
+content of a FlowFile and its attributes and then emits only an
+ATTRIBUTES_MODIFIED event, the framework will emit a CONTENT_MODIFIED
+event. The framework will not emit an ATTRIBUTES_MODIFIED event if any
+other event is emitted for that FlowFile (either by the
+Processor or the framework). This is due to the fact that all
+Provenance Events know about the attributes of the FlowFile before the
+event occurred as well as those attributes that occurred as a result
+of the processing of that FlowFile, and as a result the
+ATTRIBUTES_MODIFIED is generally considered redundant and would result
+in a rendering of the FlowFile lineage being very verbose.
+It is, however, acceptable for a Processor to emit this event along
+with others, if the event is considered pertinent from the
+perspective of the Processor.
+
+
+
+
+
+
+
+== Documenting a Component
+
+NiFi attempts to make the user experience as simple and convenient as
+possible by providing significant amount of documentation
+to the user from within the NiFi application itself via the User
+Interface. In order for this to happen, of course, Processor
+developers must provide that documentation to the framework. NiFi
+exposes a few different mechanisms for supplying documentation to
+the framework.
+
+
+=== Documenting Properties
+
+Individual properties can be documented by calling the `description`
+method of a PropertyDescriptor's builder as such:
+
+[source,java]
+----
+public static final PropertyDescriptor MY_PROPERTY = new PropertyDescriptor.Builder()
+ .name("My Property")
+ .description("Description of the Property")
+ ...
+ .build();
+----
+
+If the property is to provide a set of allowable values, those values
+are presented to the user in a drop-down field in the UI.
+Each of those values can also be given a description:
+
+[source,java]
+----
+public static final AllowableValue EXTENSIVE = new AllowableValue("Extensive", "Extensive",
+ "Everything will be logged - use with caution!");
+public static final AllowableValue VERBOSE = new AllowableValue("Verbose", "Verbose",
+ "Quite a bit of logging will occur");
+public static final AllowableValue REGULAR = new AllowableValue("Regular", "Regular",
+ "Typical logging will occur");
+
+public static final PropertyDescriptor LOG_LEVEL = new PropertyDescriptor.Builder()
+ .name("Amount to Log")
+ .description("How much the Processor should log")
+ .allowableValues(REGULAR, VERBOSE, EXTENSIVE)
+ .defaultValue(REGULAR.getValue())
+ ...
+ .build();
+----
+
+
+=== Documenting Relationships
+
+Processor Relationships are documented in much the same way that
+properties are - by calling the `description` method of a
+Relationship's builder:
+
+[source,java]
+----
+public static final Relationship MY_RELATIONSHIP = new Relationship.Builder()
+ .name("My Relationship")
+ .description("This relationship is used only if the Processor fails to process the data.")
+ .build();
+----
+
+
+=== Documenting Capability and Keywords
+
+The `org.apache.nifi.annotations.documentation` package provides Java
+annotations that can be used to document components. The
+CapabilityDescription
+annotation can be added to a Processor, Reporting Task, or Controller
+Service and is intended to provide a brief description of the
+functionality
+provided by the component. The Tags annotation has a `value` variable
+that is defined to be an Array of Strings. As such, it is used
+by providing multiple values as a comma-separated list of `String`s
+with curly braces. These values are then incorporated into the UI by
+allowing
+users to filter the components based on a tag (i.e., a keyword).
+Additionally, the UI provides a tag cloud that allows users to select
+the tags that
+they want to filter by. The tags that are largest in the cloud are
+those tags that exist the most on the components in that instance of
+NiFi. An
+example of using these annotations is provided below:
+
+[source, java]
+----
+@Tags({"example", "documentation", "developer guide", "processor", "tags"})
+@CapabilityDescription("Example Processor that provides no real functionality but is provided" +
+ " for an example in the Developer Guide")
+public static final ExampleProcessor extends Processor {
+ ...
+}
+----
+
+
+=== Advanced Documentation
+
+When the documentation methods above are not sufficient, NiFi provides
+the ability to expose more advanced documentation to the user via the
+"Usage" documentation. When a user right-clicks on a Processor, NiFi
+provides a "Usage" menu item in the context menu. Additionally, the
+UI exposes a "Help" link in the top-right corner, from which the same
+Usage information can be found.
+
+The advanced documentation of a Processor is provided as an HTML file.
+This file should exist within a directory whose name is the
+fully-qualified
+name of the component, and this directory's parent should be named
+`docs` and exist in the root of the Processor's jar.
+The mechanism provided for this will be changing as of the 0.1.0
+release. At that time, this section will be updated to reflect
+the new procedures for providing this advanced documentation.
+
+
+
+== Common Processor Patterns
+
+While there are many different Processors available to NiFi users, the
+vast majority of them fall into
+one of several common design patterns. Below, we discuss these
+patterns, when the patterns are appropriate,
+reasons we follow these patterns, and things to watch out for when
+applying such patterns. Note that the patterns
+and recommendations discussed below are general guidelines and not
+hardened rules.
+
+
+[[ingress]]
+=== Data Ingress
+
+A Processor that ingests data into NiFi has a single Relationship
+names `success`. This Processor generates
+new FlowFiles via the ProcessSession `create` method and does not pull
+FlowFiles from incoming Connections.
+The Processor name starts with "Get" or "Listen," depending on whether
+it polls an external source or exposes
+some interface to which external sources can connect. The name ends
+with the protocol used for communications.
+Processors that follow this pattern include `GetFile`, `GetSFTP`,
+`ListenHTTP`, and `GetHTTP`.
+
+This Processor may create or initialize a Connection Pool in a method
+that uses the `@OnScheduled` annotation.
+However, because communications problems may prevent connections from
+being established or cause connections
+to be terminated, connections themselves are not created at this
+point. Rather, the connections are
+created or leased from the pool in the `onTrigger` method.
+
+The `onTrigger` method of this Processor begins by leasing a
+connection from the Connection Pool, if possible,
+or otherwise creates a connection to the external service. When no
+data is available from the
+external source, the `yield` method of the ProcessContext is called by
+the Processor and the method returns so
+that this Processor avoids continually running and depleting resources
+without benefit. Otherwise, this
+Processor then creates a FlowFile via the ProcessSession's `create`
+method and assigns an appropriate
+filename and path to the FlowFile (by adding the `filename` and `path`
+attributes), as well as any other
+attributes that may be appropriate. An OutputStream to the FlowFile's content is
+obtained via the ProcessSession's `write` method, passing a new
+OutputStreamCallback (which is usually
+an anonymous inner class). From within this callback, the Processor is
+able to write to the FlowFile and streams
+the content from the external resource to the FlowFile's OutputStream.
+If the desire is to write the entire contents
+of an InputStream to the FlowFile, the `importFrom` method of
+ProcessSession may be more convenient to use than the
+`write` method.
+
+When this Processor expects to receive many small files, it may be
+advisable to create several FlowFiles from a
+single session before committing the session. Typically, this allows
+the Framework to treat the content of the
+newly created FlowFiles much more efficiently.
+
+This Processor generates a Provenance event indicating that it has
+received data and specifies from
+where the data came. This Processor should log the creation of the
+FlowFile so that the FlowFile's
+origin can be determined by analyzing logs, if necessary.
+
+This Processor acknowledges receipt of the data and/or removes the
+data from the external source in order
+to prevent receipt of duplicate files. *This is done only after the
+ProcessSession by which the FlowFile was
+created has been committed!* Failure to adhere to this principle may
+result in data loss, as restarting NiFi
+before the session has been committed will result in the temporary
+file being deleted. Note, however, that it
+is possible using this approach to receive duplicate data because the
+application could be restarted after
+committing the session and before acknowledging or removing the data
+from the external source. In general, though,
+potential data duplication is preferred over potential data loss. The
+connection is finally returned or added to
+the Connection Pool, depending on whether the connection was leased
+from the Connection Pool to begin with or
+was created in the `onTrigger` method.
+
+If there is a communications problem, the connection is typically
+terminated and not returned (or added) to
+the Connection Pool. Connections to remote systems are torn down and
+the Connection Pool shutdown in a method
+annotated with the `@OnStopped` annotation so that resources can be reclaimed.
+
+
+=== Data Egress
+
+A Processor that publishes data to an external source has two
+Relationships: `success` and `failure`. The
+Processor name starts with "Put" followed by the protocol that is used
+for data transmission. Processors
+that follow this pattern include `PutEmail`, `PutSFTP`, and
+`PostHTTP` (note that the name does not
+begin with "Put" because this would lead to confusion, since PUT and
+POST have special meanings when dealing with
+HTTP).
+
+This Processor may create or initialize a Connection Pool in a method
+that uses the `@OnScheduled` annotation.
+However, because communications problems may prevent connections from
+being established or cause connections
+to be terminated, connections themselves are not created at this
+point. Rather, the connections are
+created or leased from the pool in the `onTrigger` method.
+
+The `onTrigger` method first obtains a FlowFile from the
+ProcessSession via the `get` method. If no FlowFile is
+available, the method returns without obtaining a connection to the
+remote resource.
+
+If at least one FlowFile is available, the Processor obtains a
+connection from the Connection Pool, if possible,
+or otherwise creates a new connection. If the Processor is neither
+able to lease a connection from the Connection Pool
+nor create a new connection, the FlowFile is routed to `failure`, the
+event is logged, and the method returns.
+
+If a connection was obtained, the Processor obtains an InputStream to
+the FlowFile's content by invoking the
+`read` method on the ProcessSession and passing an InputStreamCallback
+(which is often an anonymous inner class)
+and from within that callback transmits the contents of the FlowFile
+to the destination. The event is logged
+along with the amount of time taken to transfer the file and the data
+rate at which the file was transferred.
+A SEND event is reported to the ProvenanceReporter by obtaining the
+reporter from the ProcessSession via the
+`getProvenanceReporter` method and calling the `send` method on the
+reporter. The connection is returned or added
+to the Connection Pool, depending on whether the connection was leased
+from the pool or newly created by the
+`onTrigger` method.
+
+If there is a communications problem, the connection is typically
+terminated and not returned (or added) to
+the Connection Pool. If there is an issue sending the data to the
+remote resource, the desired approach for handling the
+error depends on a few considerations. If the issue is related to a
+network condition, the FlowFile is generally
+routed to `failure`. The FlowFile is not penalized because there is
+not necessary a problem with the data. Unlike the
+case of the <<ingress>> Processor, we typically do not call `yield` on
+the ProcessContext. This is because in the case of
+ingest, the FlowFile does not exist until the Processor is able to
+perform its function. However, in the case of a Put Processor,
+the DataFlow Manager may choose to route `failure` to a different
+Processor. This can allow for a "backup" system to be
+used in the case of problems with one system or can be used for load
+distribution across many systems.
+
+If a problem occurs that is data-related, one of two approaches should
+be taken. First, if the problem is likely to
+sort itself out, the FlowFile is penalized and then routed to
+`failure`. This is the case, for instance, with PutFTP,
+when a FlowFile cannot be transferred because of a file naming
+conflict. The presumption is that the file will eventually
+be removed from the directory so that the new file can be transferred.
+As a result, we penalize the FlowFile and route to
+`failure` so that we can try again later. In the other case, if there
+is an actual problem with the data (such as the data does
+not conform to some required specification), a different approach may
+be taken. In this case, it may be advantageous
+to break apart the `failure` relationship into a `failure` and a
+`communications failure` relationship. This allows the
+DataFlow Manager to determine how to handle each of these cases
+individually. It is important in these situations to document
+well the differences between the two Relationships by clarifying it in
+the "description" when creating the Relationship.
+
+Connections to remote systems are torn down and the Connection Pool
+shutdown in a method
+annotated with `@OnStopped` so that resources can be reclaimed.
+
+
+=== Route Based on Content (One-to-One)
+
+A Processor that routes data based on its content will take one of two
+forms: Route an incoming FlowFile to exactly
+one destination, or route incoming data to 0 or more destinations.
+Here, we will discuss the first case.
+
+This Processor has two relationships: `matched` and `unmatched`. If a
+particular data format is expected, the Processor
+will also have a `failure` relationship that is used when the input is
+not of the expected format. The Processor exposes
+a Property that indicates the routing criteria.
+
+If the Property that specifies routing criteria requires processing,
+such as compiling a Regular Expression, this processing
+is done in a method annotated with `@OnScheduled`, if possible. The
+result is then stored in a member variable that is marked
+as `volatile`.
+
+The `onTrigger` method obtains a single FlowFile. The method reads the
+contents of the FlowFile via the ProcessSession's `read`
+method, evaluating the Match Criteria as the data is streamed. The
+Processor then determines whether the FlowFile should be
+routed to `matched` or `unmatched` based on whether or not the
+criteria matched, and routes the FlowFile to the appropriate
+relationship.
+
+The Processor then emits a Provenance ROUTE event indicating which
+Relationship to which the Processor routed the FlowFile.
+
+This Processor is annotated with the `@SideEffectFree` and
+`@SupportsBatching` annotations from the `org.apache.nifi.annotations.behavior`
+package.
+
+
+=== Route Based on Content (One-to-Many)
+
+If a Processor will route a single FlowFile to potentially many
+relationships, this Processor will be slightly different than
+the above-described Processor for Routing Data Based on Content. This
+Processor typically has Relationships that are dynamically
+defined by the user as well as an `unmatched` relationship.
+
+In order for the user to be able to define additionally Properties,
+the `getSupportedDynamicPropertyDescriptor` method must be
+overridden. This method returns a PropertyDescriptor with the supplied
+name and an applicable Validator to ensure that the
+user-specified Matching Criteria is valid.
+
+In this Processor, the Set of Relationships that is returned by the
+`getRelationships` method is a member variable that is
+marked `volatile`. This Set is initially constructed with a single
+Relationship named `unmatched`. The `onPropertyModified` method
+is overridden so that when a Property is added or removed, a new
+Relationship is created with the same name. If the Processor has
+Properties that are not user-defined, it is important to check if the
+specified Property is user-defined. This can be achieved by
+calling the `isDynamic` method of the PropertyDescriptor that is
+passed to this method. If this Property is dynamic,
+a new Set of Relationships is then created, and the previous set of
+Relationships is copied into it. This new Set
+either has the newly created Relationship added to it or removed from
+it, depending on whether a new Property was added
+to the Processor or a Property was removed (Property removal is
+detected by check if the third argument to this function is `null`).
+The member variable holding the Set of Relationships is then updated
+to point to this new Set.
+
+If the Properties that specify routing criteria require processing,
+such as compiling a Regular Expression, this processing is done
+in a method annotated with `@OnScheduled`, if possible. The result is
+then stored in a member variable that is marked as `volatile`.
+This member variable is generally of type `Map` where the key is of
+type `Relationship` and the value's type is defined by the result of
+processing the property value.
+
+The `onTrigger` method obtains a FlowFile via the `get` method of
+ProcessSession. If no FlowFile is available, it returns immediately.
+Otherwise, a Set of type Relationship is created. The method reads the
+contents of the FlowFile via the ProcessSession's `read` method,
+evaluating each of the Match Criteria as the data is streamed. For any
+criteria that matches, the relationship associated with that Match
+Criteria is added to the Set of Relationships.
+
+After reading the contents of the FlowFile, the method checks if the
+Set of Relationships is empty. If so, the original FlowFile has
+an attribute added to it to indicate the Relationship to which it was
+routed and is routed to the `unmatched`. This is logged, a
+Provenance ROUTE event is emitted, and the method returns. If the size
+of the Set is equal to 1, the original FlowFile has an attribute
+added to it to indicate the Relationship to which it was routed and
+is routed to the Relationship specified by the entry in the Set.
+This is logged, a Provenance ROUTE event is emitted for the FlowFile,
+and the method returns.
+
+In the event that the Set contains more than 1 Relationship, the
+Processor creates a clone of the FlowFile for each Relationship,
+except
+for the first. This is done via the `clone` method of the
+ProcessSession. There is no need to report a CLONE Provenance Event,
+as the
+framework will handle this for you. The original FlowFile and each
+clone are routed to their appropriate Relationship with attribute
+indicating the name of the Relationship. A Provenance ROUTE event is
+emitted for each FlowFile. This is logged, and the method returns.
+
+This Processor is annotated with the `@SideEffectFree` and
+`@SupportsBatching` annotations from the
+`org.apache.nifi.annotations.behavior`
+package.
+
+
+=== Route Streams Based on Content (One-to-Many)
+
+The previous description of Route Based on Content (One-to-Many)
+provides an abstraction
+for creating a very powerful Processor. However, it assumes that each
+FlowFile will be routed
+in its entirety to zero or more Relationships. What if the incoming
+data format is a "stream" of
+many different pieces of information - and we want to send different
+pieces of this stream to
+different Relationships? For example, imagine that we want to have a
+RouteCSV Processor such that
+it is configured with multiple Regular Expressions. If a line in the
+CSV file matches a Regular
+Expression, that line should be included in the outbound FlowFile to
+the associated relationship.
+If a Regular Expression is associated with the Relationship
+"has-apples" and that Regular Expression
+matches 1,000 of the lines in the FlowFile, there should be one outbound
+FlowFile for the "has-apples" relationship that has 1,000 lines in it.
+If a different Regular Expression
+is associated with the Relationship "has-oranges" and that Regular
+Expression matches 50 lines in the
+FlowFile, there should be one outbound FlowFile for the "has-oranges"
+relationship that has 50 lines in it.
+I.e., one FlowFile comes in and two FlowFiles come out. The two
+FlowFiles may contain some of the same lines
+of text from the original FlowFile, or they may be entirely different.
+This is the type of Processor that
+we will discuss in this section.
+
+This Processor's name starts with "Route" and ends with the name of
+the data type that it routes. In our
+example here, we are routing CSV data, so the Processor is named
+RouteCSV. This Processor supports dynamic
+properties. Each user-defined property has a name that maps to the
+name of a Relationship. The value of
+the Property is in the format necessary for the "Match Criteria." In
+our example, the value of the property
+must be a valid Regular Expression.
+
+This Processor maintains an internal `ConcurrentMap` where the key is
+a `Relationship` and the value is of
+a type dependent on the format of the Match Criteria. In our example,
+we would maintain a
+`ConcurrentMap<Relationship, Pattern>`. This Processor overrides the
+`onPropertyModified` method.
+If the new value supplied to this method (the third argument) is null,
+the Relationship whose name is
+defined by the property name (the first argument) is removed from the
+ConcurrentMap. Otherwise, the new value
+is processed (in our example, by calling `Pattern.compile(newValue)`)
+and this value is added to the ConcurrentMap
+with the key again being the Relationship whose name is specified by
+the property name.
+
+This Processor will override the `customValidate` method. In this
+method, it will retrieve all Properties from
+the `ValidationContext` and count the number of PropertyDescriptors
+that are dynamic (by calling `isDynamic()`
+on the PropertyDescriptor). If the number of dynamic
+PropertyDescriptors is 0, this indicates that the user
+has not added any Relationships, so the Processor returns a
+`ValidationResult` indicating that the Processor
+is not valid because it has no Relationships added.
+
+The Processor returns all of the Relationships specified by the user
+when its `getRelationships` method is
+called and will also return an `unmatched` Relationship. Because this
+Processor will have to read and write to the
+Content Repository (which can be relatively expensive), if this
+Processor is expected to be used for very high
+data volumes, it may be advantageous to add a Property that allows the
+user to specify whether or not they care
+about the data that does not match any of the Match Criteria.
+
+When the `onTrigger` method is called, the Processor obtains a
+FlowFile via `ProcessSession.get`. If no data
+is available, the Processor returns. Otherwise, the Processor creates
+a `Map<Relationship, FlowFile>`. We will
+refer to this Map as `flowFileMap`. The Processor reads the incoming
+FlowFile by calling `ProcessSession.read`
+and provides an `InputStreamCallback`.
+From within the Callback, the Processor reads the first piece of data
+from the FlowFile. The Processor then
+evaluates each of the Match Criteria against this piece of data. If a
+particular criteria (in our example,
+a Regular Expression) matches, the Processor obtains the FlowFile from
+`flowFileMap` that belongs to the appropriate
+Relationship. If no FlowFile yet exists in the Map for this
+Relationship, the Processor creates a new FlowFile
+by calling `session.create(incomingFlowFile)` and then adds the new
+FlowFile to `flowFileMap`. The Processor then
+writes this piece of data to the FlowFile by calling `session.append`
+with an `OutputStreamCallback`. From within
+this OutputStreamCallback, we have access to the new FlowFile's
+OutputStream, so we are able to write the data
+to the new FlowFile. We then return from the OutputStreamCallback.
+After iterating over each of the Match Criteria,
+if none of them match, we perform the same routines as above for the
+`unmatched` relationship (unless the user
+configures us to not write out unmatched data). Now that we have
+called `session.append`, we have a new version of
+the FlowFile. As a result, we need to update our `flowFileMap` to
+associate the Relationship with the new FlowFile.
+
+If at any point, an Exception is thrown, we will need to route the
+incoming FlowFile to `failure`. We will also
+need to remove each of the newly created FlowFiles, as we won't be
+transferring them anywhere. We can accomplish
+this by calling `session.remove(flowFileMap.values())`. At this point,
+we will log the error and return.
+
+Otherwise, if all is successful, we can now iterate through the
+`flowFileMap` and transfer each FlowFile to the
+corresponding Relationship. The original FlowFile is then either
+removed or routed to an `original` relationship.
+For each of the newly created FlowFiles, we also emit a Provenance
+ROUTE event indicating which Relationship
+the FlowFile went to. It is also helpful to include in the details of
+the ROUTE event how many pieces of information
+were included in this FlowFile. This allows DataFlow Managers to
+easily see when looking at the Provenance
+Lineage view how many pieces of information went to each of the
+relationships for a given input FlowFile.
+
+Additionally, some Processors may need to "group" the data that is
+sent to each Relationship so that each FlowFile
+that is sent to a relationship has the same value. In our example, we
+may wan to allow the Regular Expression
+to have a Capturing Group and if two different lines in the CSV match
+the Regular Expression but have different
+values for the Capturing Group, we want them to be added to two
+different FlowFiles. The matching value could then
+be added to each FlowFile as an Attribute. This can be accomplished by
+modifying the `flowFileMap` such that
+it is defined as `Map<Relationship, Map<T, FlowFile>>` where `T` is
+the type of the Grouping Function (in our
+example, the Group would be a `String` because it is the result of
+evaluating a Regular Expression's
+Capturing Group).
+
+
+
+=== Route Based on Attributes
+
+This Processor is almost identical to the Route Data Based on Content
+Processors described above. It takes two different forms: One-to-One
+and
+One-to-Many, as do the Content-Based Routing Processors. This
+Processor, however, does not make any call to ProcessSession's `read`
+method,
+as it does not read FlowFile content. This Processor is typically very
+fast, so the `@SupportsBatching` annotation can be very important
+in this case.
+
+
+
+=== Split Content (One-to-Many)
+
+This Processor generally requires no user configuration, with the
+exception of the size of each Split to create. The `onTrigger` method
+obtains
+a FlowFile from its input queues. A List of type FlowFile is created.
+The original FlowFile is read via the ProcessSession's `read` method,
+and an InputStreamCallback is used. Within the InputStreamCallback,
+the content is read until a point is reached at which the FlowFile
+should be
+split. If no split is needed, the Callback returns, and the original
+FlowFile is routed to `success`. In this case, a Provenance ROUTE
+event
+is emitted. Typically, ROUTE events are not emitted when routing a
+FlowFile to `success` because this generates a very verbose lineage
+that
+becomes difficult to navigate. However, in this case,the event is
+useful because we would otherwise expect a FORK event and the absence
+of
+any event is likely to cause confusion. The fact that the FlowFile was
+not split but was instead transferred to `success` is logged, and the
+method returns.
+
+If a point is reached at which a FlowFile needs to be split, a new
+FlowFile is created via the ProcessSession's `create(FlowFile)` method
+or the
+`clone(FlowFile, long, long)` method. The next section of code depends
+on whether the `create` method is used or the `clone` method is used.
+Both methods are described below. Which solution is appropriate must
+be determined on a case-by-case basis.
+
+The Create Method is most appropriate when the data will not be
+directly copied from the original FlowFile to the new FlowFile.
+For example, if only some of the data will be copied, or if the data
+will be modified in some way before being copied to the new
+FlowFile, this method is necessary. However, if the content of the new
+FlowFile will be an exact copy of a portion of the original
+FlowFile, the Clone Method is much preferred.
+
+*Create Method*
+If using the `create` method, the method is called with the original
+FlowFile as the argument so that the newly created FlowFile will
+inherit
+the attributes of the original FlowFile and a Provenance FORK event
+will be created by the framework.
+
+The code then enters a `try/finally` block. Within the `finally`
+block, the newly created FlowFile is added to the List of FlowFiles
+that have
+been created. This is done within a `finally` block so that if an
+Exception is thrown, the newly created FlowFile will be appropriately
+cleaned up.
+Within the `try` block, the callback initiates a new callback by
+calling the ProcessSession's `write` method with an
+OutputStreamCallback.
+The appropriate data is then copied from the InputStream of the
+original FlowFile to the OutputStream for the new FlowFile.
+
+*Clone Method*
+If the content of the newly created created FlowFile is to be only a
+contiguous subset of the bytes of the original FlowFile, it is
+preferred
+to use the `clone(FlowFile, long, long)` method instead of the
+`create(FlowFile)` method of the ProcessSession. In this case, the
+offset
+of the original FlwoFile at which the new FlowFile's content should
+begin is passed as the second argument to the `clone` method. The
+length
+of the new FlowFile is passed as the third argument to the `clone`
+method. For example, if the original FlowFile was 10,000 bytes
+and we called `clone(flowFile, 500, 100)`, the FlowFile that would be
+returned to us would be identical to `flowFile` with respect to its
+attributes. However, the content of the newly created FlowFile would
+be 100 bytes in length and would start at offset 500 of the original
+FlowFile. That is, the contents of the newly created FlowFile would be
+the same as if you had copied bytes 500 through 599 of the original
+FlowFile.
+
+After the clone has been created, it is added to the List of FlowFiles.
+
+This method is much more highly preferred than the Create method, when
+applicable,
+because no disk I/O is required. The framework is able to simply
+create a new FlowFile
+that references a subset of the original FlowFile's content, rather
+than actually copying
+the data. However, this is not always possible. For example, if header
+information must be copied
+from the beginning of the original FlowFile and added to the beginning
+of each Split,
+then this method is not possible.
+
+
+*Both Methods*
+Regardless of whether the Clone Method or the Create Method is used,
+the following is applicable.
+
+If at any point in the InputStreamCallback, a condition is reached in
+which processing cannot continue
+(for example, the input is malformed), a `ProcessException` should be
+thrown. The call to the
+ProcesssSession's `read` method is wrapped in a `try/catch` block
+where `ProcessException` is
+caught. If an Exception is caught, a log message is generated
+explaining the error. The List of
+newly created FlowFiles is removed via the ProcessSession's `remove`
+method. The original FlowFile
+is routed to `failure`.
+
+If no problems arise, the original FlowFile is routed to `original`
+and all newly created FlowFiles
+are updated to include the following attributes:
+
+[options="header"]
+|===
+| Attribute Name | Description
+| `split.parent.uuid` | The UUID of the original FlowFile
+| `split.index` | A one-up number indicating which FlowFile in the list this is (the first FlowFile
+ created will have a value `0`, the second will have a value `1`, etc.)
+| `split.count` | The total number of split FlowFiles that were created
+|===
+
+The newly created FlowFiles are routed to `success`; this event is
+logged; and the method returns.
+
+
+=== Update Attributes Based on Content
+
+This Processor is very similar to the Route Based on Content
+Processors discussed above. Rather than
+routing a FlowFile to `matched` or `unmatched`, the FlowFile is
+generally routed to `success` or `failure`
+and attributes are added to the FlowFile as appropriate. The
+attributes to be added are configured in a
+manner similar to that of the Route Based on Content (One-to-Many),
+with the user defining their own
+properties. The name of the property indicates the name of an
+attribute to add. The value of the
+property indicates some Matching Criteria to be applied to the data.
+If the Matching Criteria matches
+the data, an attribute is added with the name the same as that of the
+Property. The value of the
+attribute is the criteria from the content that matched.
+
+For example, a Processor that evaluates XPath Expressions may allow
+user-defined XPaths to be
+entered. If the XPath matches the content of a FlowFile, that FlowFile
+will have an attribute added with
+the name being equal to that of the Property name and a value equal to
+the textual content of the XML Element or
+Attribute that matched the XPath. The `failure` relationship would
+then be used if the incoming FlowFile
+was not valid XML in this example. The `success` relationship would be
+used regardless of whether or not
+any matches were found. This can then be used to route the FlowFile
+when appropriate.
+
+This Processor emits a Provenance Event of type ATTRIBUTES_MODIFIED.
+
+
+=== Enrich/Modify Content
+
+The Enrich/Modify Content pattern is very common and very generic.
+This pattern is responsible for any
+general content modification. For the majority of cases, this
+Processor is marked with the
+`@SideEffectFree` and `@SupportsBatching` annotations. The Processor
+has any number of required and optional
+Properties, depending on the Processor's function. The Processor
+generally has a `success` and `failure` relationship.
+The `failure` relationship is generally used when the input file is
+not in the expected format.
+
+This Processor obtains a FlowFile and updates it using the
+ProcessSession's `write(StreamCallback)` method
+so that it is able to both read from the FlowFile's content and write
+to the next version of the FlowFile's
+content. If errors are encountered during the callback, the callback
+will throw a `ProcessException`. The
+call to the ProcessSession's `write` method is wrapped in a
+`try/catch` block that catches `ProcessException`
+and routes the FlowFile to failure.
+
+If the callback succeeds, a CONTENT_MODIFIED Provenance Event is emitted.
+
+
+
+== Error Handling
+
+=== Exceptions within a callback: IOException, RuntimeException
+
+=== Exceptions within the Processor: ProcessException, others.
+
+=== Catching ProcessException from callback
+
+=== Penalization vs. Yielding
+
+=== Session Rollback: with and within penalization
+
+=== Administrative Yielding
+
+
+
+== General Design Considerations
+
+=== Cohesion / Reusability : Do one thing and do it well!
+
+=== Naming Conventions
+
+=== Processor Behavior Annotations
+
+=== Data Buffering
+
+
+
+
+
+
+[[controller-services]]
+== Controller Services
+
+The `ControllerService` interface allows developers to share
+functionality and state across the JVM in a clean
+and consistent manner. The interface resembles that of the `Processor`
+interface but does not
+have an `onTrigger` method because Controller Services are not
+scheduled to run periodically, and
+Controller Services do not have Relationships because they are not
+integrated into the flow directly. Rather,
+they are used Processors, Reporting Tasks, and other Controller Services.
+
+[[developing-controller-service]]
+=== Developing a ControllerService
+
+Just like with the Processor interface, the ControllerService
+interface exposes methods for configuration,
+validation, and initialization. These methods are all identical to
+those of the Processor interface
+except that the `initialize` method is passed a
+`ControllerServiceInitializationContext`, rather
+than a `ProcessorInitializationContext`.
+
+Controller Services come with an additional constraint that Processors
+do not have. A Controller Service
+must be comprised of an interface that extends `ControllerService`.
+Implementations can then be interacted
+with only through their interface. A Processor, for instance, will
+never be given a concrete implementation of
+a ControllerService and therefore must reference the service only via
+interfaces that extends `ControllerService`.
+
+This constraint is in place mainly because a Processor can exist in
+one NiFi Archive (NAR) while the implementation
+of the Controller Service that the Processor lives in can exist in a
+different NAR. This is accomplished by
+the framework by dynamically implementing the exposed interfaces in
+such a way that the framework can
+switch to the appropriate ClassLoader and invoke the desired method on
+the concrete implementation. However,
+in order to make this work, the Processor and the Controller Service
+implementation must share the same definition
+of the Controller Service interface. Therefore, both of these NARs
+must depend on the NAR that houses the
+Controller Service's interface. See <<nars>> for more information.
+
+
+[[interacting-with-controller-service]]
+=== Interacting with a ControllerService
+
+ControllerServices may be obtained by a Processor, another
+ControllerService, or a ReportingTask
+by means of the ControllerServiceLookup or by using the
+`identifiesControllerService` method of the
+PropertyDescriptor's Builder class. The ControllerServiceLookup can be
+obtained by a Processor from the
+ProcessorInitializationContext that is passed to the `initialize`
+method. Likewise, it is obtained by
+a ControllerService from the ControllerServiceInitializationContext
+and by a ReportingTask via the
+ReportingConfiguration object passed to the `initialize` method.
+
+For most use cases, though, using the `identifiesControllerService`
+method of a PropertyDescriptor Builder
+is preferred and is the least complicated method. In order to use this
+method, we create a PropertyDescriptor
+that references a Controller Service as such:
+
+[source,java]
+----
+public static final PropertyDescriptor SSL_CONTEXT_SERVICE = new PropertyDescriptor.Builder()
+ .name("SSL Context Service")
+ .details("Specified the SSL Context Service that can be used to create secure connections")
+ .required(true)
+ .identifiesControllerService(SSLContextService.class)
+ .build();
+----
+
+Using this method, the user will be prompted to supply the SSL Context
+Service that should be used. This is
+done by providing the user with a drop-down menu from which they are
+able to choose any of the SSLContextService
+configurations that have been configured, regardless of the implementation.
+
+In order to make use of this service, the Processor can use code such as:
+
+[source,java]
+----
+final SSLContextService sslContextService = context.getProperty(SSL_CONTEXT_SERVICE)
+ .asControllerService(SSLContextService.class);
+----
+
+Note here that `SSLContextService` is an interface that extends
+ControllerService. The only implementation
+at this time is the `StandardSSLContextService`. However, the
+Processor developer need not worry about this
+detail.
+
+
+
+
+
+
+
+== Reporting Tasks
+
+So far, we have mentioned little about how to convey to the outside
+world how NiFi and its components
+are performing. Is the system able to keep up with the incoming data
+rate? How much more can
+the system handle? How much data is processed at the peak time of day
+versus the least busy time of day?
+
+In order to answer these questions, and many more, NiFi provides a
+capability for reporting status,
+statistics, metrics, and monitoring information to external services
+by means of the `ReportingTask`
+interface. ReportingTasks are given access to a host of information to
+determine how the system is performing.
+
+
+=== Developing a Reporting Task
+
+Just like with the Processor and ControllerService interfaces, the
+ReportingTask interface exposes methods for
+configuration, validation, and initialization. These methods are all
+identical to those of the
+Processor and ControllerService interfaces except that the
+`initialize` method is passed a `ReportingConfiguration`
+object, as opposed to the initialization objects received by the other
+Components. The ReportingTask also has
+an `onTrigger` method that is invoked by the framework to trigger the
+task to perform its job.
+
+Within the `onTrigger` method, the ReportingTask is given access to a
+ReportingContext, from which configuration
+and information about the NiFi instance can be obtained. The
+BulletinRepository allows Bulletins to be queried
+and allows the ReportingTask to submit its own Bulletins, so that
+information will be rendered to users. The
+ControllerServiceLookup that is accessible via the Context provides
+access to ControllerServices that have been
+configured. However, this method of obtaining Controller Services is
+not the preferred method. Rather, the
+preferred method for obtaining a Controller Service is to reference
+the Controller Service in a PropertyDescriptor,
+as is discussed in the <<interacting-with-controller-service>> section.
+
+The `EventAccess` object that is exposed via the ReportingContext
+provides access to the `ProcessGroupStatus`,
+which exposes statistics about the amount of data processed in the
+past five minutes by Process Groups,
+Processors, Connections, and other Components. Additionally, the
+EventAccess object provides access to
+the `ProvenanceEventRecord`s
+that have been stored in the `ProvenanceEventRepository`. These
+Provenance Events are emitted by Processors when
+data is received from external sources, emitted to external services,
+removed from the system, modified,
+or routed according to some decision that was made.
+
+Each ProvenanceEvent has the ID of the FlowFile, the type of Event,
+the creation time of the Event, and
+all FlowFile attributes associated with the FlowFile at the time that
+the FlowFile was accessed by the component
+as well as the FlowFile attributes that were associated with the
+FlowFile as a result of the processing that the
+event describes. This provides a great deal of information to
+ReportingTasks, allowing reports to be generated
+in many different ways to expose metrics and monitoring capabilities
+needed for any number of operational concerns.
+
+
+
+
+
+
+== Testing
+
+Testing the components that will be used within a larger framework can often be very cumbersome
+and tricky. With NiFi, we strive to make testing components as easy as possible. In order to do this,
+we have created a `nifi-mock` module that can be used in conjunction with JUnit to provide extensive
+testing of components.
+
+The Mock Framework is mostly aimed at testing Processors, as these are by far the most commonly
+developed extension point. However, the framework does provide the ability to test Controller Services
+as well.
+
+Components have typically been tested by creating functional tests to verify component behavior. This is
+done because often a Processor will consist of a handful of helper methods but the logic will largely be
+encompassed within the `onTrigger` method. The `TestRunner` interface allows us to test Processors
+and Controller Services by converting more "primitive" objects such as files and byte arrays into
+FlowFiles and handles creating the ProcessSessions and ProcessContexts needed for a Processor to do its job,
+as well as invoking the necessary lifecycle methods in order to ensure that the Processor behaves the
+same way in the unit tests as it does in production.
+
+
+=== Instantiate TestRunner
+
+Most unit tests for a Processor or a Controller Service start by creating an instance of the `TestRunner`
+class. In order to add the necessary classes to your Processor,
+you can use the Maven dependency:
+
+[source]
+----
+<dependency>
+ <groupId>org.apache.nifi</groupId>
+ <artifactId>nifi-mock</artifactId>
+ <version>${nifi version}</version>
+</dependency>
+----
+
+We create a new `TestRunner` by calling the static `newTestRunner` method of the `TestRunners` class
+(located in the `org.apache.nifi.util` package). This method takes a single argument. That argument can
+either be the class of the Processor to test or can be an instance of a Processor.
+
+=== Add ControllerServices
+
+After creating a new Test Runner, we can add any Controller Services to the Test Runner that our Processor
+will need in order to perform its job. We do this by calling the `addControllerService` method and supply
+both an identifier for the Controller Service and an instance of the Controller Service.
+
+If the Controller Service needs to be configured, its properties can be set by
+calling the `setProperty(ControllerService, PropertyDescriptor, String)`, `setProperty(ControllerService, String, String)`,
+or `setProperty(ControllerService, PropertyDescriptor, AllowableValue)` method. Each of these methods returns a
+`ValidationResult`. This object can then be inspected to ensure that the property is valid by calling `isValid`.
+Annotation data can be set by calling the `setAnnotationData(ControllerService, String)` method.
+
+We can now ensure that the Controller Service is valid by calling `assertValid(ControllerService)` - or ensure
+that the configured values are not valid, if testing the Controller Service itself, by calling
+`assertNotValid(ControllerService)`.
+
+Once a Controller Service has been added to the Test Runner and configured, it can now be enabled by calling the
+`enableControllerService(ControllerService)` method. If the Controller Service is not valid, this method
+will throw an IllegalStateException. Otherwise, the service is now ready to use.
+
+
+
+=== Set Property Values
+
+After configuring any necessary Controller Services, we need to configure our Processor. We can do this by
+calling the same methods as we do for Controller Services, without specifying any Controller Service. I.e.,
+we can call `setProperty(PropertyDescriptor, String)`, and so on. Each of the `setProperty` methods again
+returns a `ValidationResult` property that can be used to ensure that the property value is valid.
+
+Similarly, we can also call `assertValid()` and `assertNotValid()` to ensure that the configuration of the
+Processor is valid or not, according to our expectations.
+
+
+=== Enqueue FlowFiles
+
+
+
+
+=== Run the Processor
+
+
+
+=== Validate Output Counts
+
+
+
+=== Validate Output Results
+
+
+
+=== Mocking Connections
+
+
+
+=== Additional Testing Capabilities
+In addition to the above-mentioned capabilities provided by the
+testing framework, the TestRunner provides several
+convenience methods for verifying the behavior of a Processor. Methods
+are provided for ensuring that the Processor's
+Input Queue has been emptied. Unit Tests are able to obtain the
+ProcessContext, ProcessSessionFactory, ProvenanceReporter,
+and other framework-specific entities that will be used by the
+TestRunner. The `shutdown` method provides the ability to
+test Processor methods that are annotated to be run only on shutdown
+of NiFi. Annotation Data can be set for Processors
+that make use of Custom User Interfaces. Finally, the number of
+threads that should be used to run the Processor can
+be set via the `setThreadCount(int)` method.
+
+
+
+
+
+
+[[nars]]
+== NiFi Archives (NARs)
+
+When software from many different organizations is all hosted within
+the same environment, Java ClassLoaders quickly
+become a concern. If multiple components have a dependency on the same
+library but each depends on a different
+version, many problems arise, typically resulting in unexpected
+behavior or `NoClassDefFoudnError` errors occurring.
+In order to prevent these issues from becoming problematic, NiFi
+introduces the notion of a NiFi Archive, or NAR.
+
+A NAR allows several components and their dependencies to be packaged
+together into a single package.
+The NAR package is then provided ClassLoader isolation from other NAR
+packages. Developers should always deploy
+their NiFi components as NAR packages.
+
+To achieve this, a developer creates a new Maven Artifact, which we
+refer to as the NAR artifact. The packaging is
+set to `nar`. The `dependencies` section of the POM is then created so
+that the NAR has a dependency on all NiFi
+Components that are to be included within the NAR.
+
+TODO: DISCUSS HOW TO INCLUDE THE NAR MAVEN PLUGIN.
+
+The NAR is able to have one dependency that is of type `nar`. If more
+than one dependency is specified that is of type
+`nar`, then the nifi-nar-maven-plugin will error. If NAR A adds a
+dependency on NAR B, this will *not* result in
+NAR B packaging all of the components of NAR A. Rather, this will add
+a `Nar-Dependency-Id` element to the `MANIFEST.MF`
+file of NAR A. This will result in setting the ClassLoader of NAR B as
+the Parent ClassLoader of NAR A. In this case,
+we refer to NAR B as the _Parent_ of NAR A.
+
+This linkage of Parent ClassLoaders is the mechanism that NiFi uses in
+order to enable Controller Services to be shared
+across all NARs. As mentioned in the <<developing-controller-service>>
+section, A Controller Service must be separated
+into an interface that extends `ControllerService` and an
+implementation that implements that interface. Controller Services
+can be referenced from any Processor, regardless of which NAR it is
+in, as long as both the Controller Service Implementation
+and the Processor share the same definition of the Controller Service interface.
+
+In order to share this same definition, both the Processor's NAR and
+the Controller Service Implementation's NAR must have
+as a Parent the Controller Service definition's NAR. An example
+hierarchy may look like this:
+
+.Controller Service NAR Layout
+------------------------------------------------------------------------------------------------
+root
+├── my-controller-service-api
+│ ├── pom.xml
+│ └── src
+│ └── main
+│ └── java
+│ └── org
+│ └── my
+│ └── services
+│ └── MyService.java
+│
+├── my-controller-service-api-nar
+│ └── pom.xml <1>
+│
+│
+│
+├── my-controller-service-impl
+│ ├── pom.xml <2>
+│ └── src
+│ ├── main
+│ │ ├── java
+│ │ │ └── org
+│ │ │ └── my
+│ │ │ └── services
+│ │ │ └── MyServiceImpl.java
+│ │ └── resources
+│ │ └── META-INF
+│ │ └── services
+│ │ └── org.apache.nifi.controller.ControllerService
+│ └── test
+│ └── java
+│ └── org
+│ └── my
+│ └── services
+│ └── TestMyServiceImpl.java
+│
+│
+├── my-controller-service-nar
+│ └── pom.xml <3>
+│
+│
+└── other-processor-nar
+ └── pom.xml <3>
+
+
+
+
+------------------------------------------------------------------------------------------------
+<1> This POM file has a type of `nar`. It has a dependency on
+`nifi-standard-services-api-nar`.
+<2> This POM file is of type `jar`. It has a dependency on
+`my-controller-service-api`. It does *not* have a dependency on any
+`nar` artifacts.
+<3> This POM file has a type of `nar`. It has a dependency on
+`my-controller-service-api-nar`.
+
+
+While these may seem very complex at first, after creating such a
+hierarchy once or twice, it becomes
+far less complicated. Note here that the
+`my-controller-service-api-nar` has a dependency on
+`nifi-standard-services-api-nar`.
+This is done so that any NAR that has a dependency on
+`my-controller-service-api-nar` will also be able to access
+all of the Controller Services that are provided by the
+`nifi-standard-services-api-nar`, such as the SSLContextService.
+In this same vane, it is not necessary to create a different
+"service-api" NAR for each service. Instead, it often
+makes sense to have a single "service-api" NAR that encapsulates the
+API's for many different Controller Services,
+as is done by the `nifi-standard-services-api-nar`. Generally, the API
+will not include extensive dependencies, and as a result,
+ClassLoader isolation may be less important, so lumping together many
+API artifacts into the same NAR is often acceptable.
+
+
+
+
+
+
+
+== Consider the User Experience
-Consider the User Experience
-----------------------------
The user...
-How to contribute to Apache NiFi
---------------------------------
+
+
+
+== How to contribute to Apache NiFi
+
Git, Maven, ASF processes, NiFi processes, ...
[2/3] incubator-nifi git commit: NIFI-152: Updated developer guide
Posted by mc...@apache.org.
NIFI-152: Updated developer guide
Project: http://git-wip-us.apache.org/repos/asf/incubator-nifi/repo
Commit: http://git-wip-us.apache.org/repos/asf/incubator-nifi/commit/bafa945a
Tree: http://git-wip-us.apache.org/repos/asf/incubator-nifi/tree/bafa945a
Diff: http://git-wip-us.apache.org/repos/asf/incubator-nifi/diff/bafa945a
Branch: refs/heads/NIFI-250
Commit: bafa945a763380062e5b65838d61735c9c364d26
Parents: 5830f00
Author: Mark Payne <ma...@hotmail.com>
Authored: Thu Jan 29 15:46:45 2015 -0500
Committer: Mark Payne <ma...@hotmail.com>
Committed: Thu Jan 29 15:46:45 2015 -0500
----------------------------------------------------------------------
.../src/main/asciidoc/developer-guide.adoc | 475 ++++++++++++++++---
1 file changed, 398 insertions(+), 77 deletions(-)
----------------------------------------------------------------------
http://git-wip-us.apache.org/repos/asf/incubator-nifi/blob/bafa945a/nifi/nifi-docs/src/main/asciidoc/developer-guide.adoc
----------------------------------------------------------------------
diff --git a/nifi/nifi-docs/src/main/asciidoc/developer-guide.adoc b/nifi/nifi-docs/src/main/asciidoc/developer-guide.adoc
index 1e9e509..17264c8 100644
--- a/nifi/nifi-docs/src/main/asciidoc/developer-guide.adoc
+++ b/nifi/nifi-docs/src/main/asciidoc/developer-guide.adoc
@@ -22,64 +22,60 @@ Apache NiFi Team <de...@nifi.incubator.apache.org>
== Introduction
-This Developer Guide is written by developers for developers. It is expected that before reading this
-guide, you have a basic understanding of Apache NiFi (incubating) and the concepts of dataflow. If not,
-please see the link:overview.html[NiFi Overview] and the link:user-guide.html[NiFi User Guide] to familiar
-yourself with the concepts of NiFi.
-
+The intent of this Developer Guide is to provide the reader with the information needed to understand how Apache NiFi (incubating)
+extensions are developed and help to explain the thought process behind developing the components. It provides an introduction to
+and explanation of the API that is used to develop extensions. It does not, however, go into great detail about each
+of the methods in the API, as this guide is intended to supplement the JavaDocs of the API rather than replace them.
This guide also assumes that the reader is familiar with Java 7 and Apache Maven.
-The intent of this guide is to provide the reader with the information needed to understand how NiFi extensions
-are developed and help to explain the thought process behind developing the components.
+This guide is written by developers for developers. It is expected that before reading this
+guide, you have a basic understanding of NiFi and the concepts of dataflow. If not, please see the link:overview.html[NiFi Overview]
+and the link:user-guide.html[NiFi User Guide] to familiarize yourself with the concepts of NiFi.
[[components]]
== NiFi Components
NiFi provides several extension points to provide developers the
-ability to add functionality
-to the application to meet their needs. The following list provides a
-high-level description
-of the most common extension points:
+ability to add functionality to the application to meet their needs. The following list provides a
+high-level description of the most common extension points:
- Processor
- - The Processor interface is the mechanism through which NiFi
-exposes access to
- FlowFiles, their attributes, and their content. The Processor is
-the basic building
- block used to comprise a NiFi dataflow. This interface is used
-to accomplish
+ * The Processor interface is the mechanism through which NiFi exposes access to
+ <<flowfile>>s, their attributes, and their content. The Processor is the basic building
+ block used to comprise a NiFi dataflow. This interface is used to accomplish
all of the following tasks:
- - Create FlowFiles
- - Read FlowFile content
- - Write FlowFile content
- - Read FlowFile attributes
- - Update FlowFile attributes
- - Ingest data
- - Egress data
- - Route data
- - Extract data
- - Modify data
+ ** Create FlowFiles
+ ** Read FlowFile content
+ ** Write FlowFile content
+ ** Read FlowFile attributes
+ ** Update FlowFile attributes
+ ** Ingest data
+ ** Egress data
+ ** Route data
+ ** Extract data
+ ** Modify data
- ReportingTask
- - The ReportingTask interface is a mechanism that NiFi exposes to allow metrics,
- monitoring information, and internal NiFI state to be published to external
- sources, such as log files, e-mail, and remote web services.
+ * The ReportingTask interface is a mechanism that NiFi exposes to allow metrics,
+ monitoring information, and internal NiFi state to be published to external
+ endpoints, such as log files, e-mail, and remote web services.
- ControllerService
- - A ControllerService provides shared state across Processors, other ControllerServices,
+ * A ControllerService provides shared state and functionality across Processors, other ControllerServices,
and ReportingTasks within a single JVM. An example use case may include loading a very
large dataset into memory. By performing this work in a ControllerService, the data
- can be loaded once and be exposed to all Processors via this service.
+ can be loaded once and be exposed to all Processors via this service, rather than requiring
+ many different Processors to load the dataset themselves.
- FlowFilePrioritizer
- - The FlowFilePrioritizer interface provides a mechanism by which FlowFiles
+ * The FlowFilePrioritizer interface provides a mechanism by which <<flowfile>s
in a queue can be prioritized, or sorted, so that the FlowFiles can be processed in an order
that is most effective for a particular use case.
- AuthorityProvider
- - An AuthorityProvide is responsible for determining which privileges and roles, if any,
+ * An AuthorityProvide is responsible for determining which privileges and roles, if any,
a given user should be granted.
@@ -103,20 +99,15 @@ Processors must adhere to the following rules:
While `Processor` is an interface that can be implemented directly, it
will be extremely rare to do so, as
the `org.apache.nifi.processor.AbstractProcessor` is the base class
-for almost all Processor
-implementations. The `AbstractProcessor` class provides a significant
-amount of functionality, which makes
-the task of developing a Processor much easier and more convenient.
-For the scope of this document,
-we will focus primarily on the `AbstractProcessor` class when dealing
+for almost all Processor implementations. The `AbstractProcessor` class provides a significant
+amount of functionality, which makes the task of developing a Processor much easier and more convenient.
+For the scope of this document, we will focus primarily on the `AbstractProcessor` class when dealing
with the Processor API.
.Concurrency Note
NiFi is a highly concurrent framework. This means that all extensions
-must be thread-safe.
-If unfamiliar with writing concurrent software in Java, it is highly
-recommended that you familiarize
-yourself with the principles of Java concurrency.
+must be thread-safe. If unfamiliar with writing concurrent software in Java, it is highly
+recommended that you familiarize yourself with the principles of Java concurrency.
[[supporting_api]]
@@ -138,27 +129,18 @@ immutable. Modifications to a FlowFile are made possible by the ProcessSession.
[[process_session]]
==== ProcessSession
The ProcessSession, often referred to as simply a "session," provides
-a mechanism by which FlowFiles
-can be created, destroyed, examined, cloned, and transferred to other
-Processors. Additionally, a
-ProcessSession provides mechanism for creating modified versions of
-FlowFiles, by adding or removing
-attributes, or by modifying the FlowFile's content. The ProcessSession
-also exposes a mechanism for
-emitting provenance events that provide for the ability to track the
-lineage and history of a FlowFile.
-After operations are performed on one or more FlowFiles, a
-ProcessSession can be either committed or
-rolled back.
+a mechanism by which FlowFiles can be created, destroyed, examined, cloned, and transferred to other
+Processors. Additionally, a ProcessSession provides mechanism for creating modified versions of
+FlowFiles, by adding or removing attributes, or by modifying the FlowFile's content. The ProcessSession
+also exposes a mechanism for emitting provenance events that provide for the ability to track the
+lineage and history of a FlowFile. After operations are performed on one or more FlowFiles, a
+ProcessSession can be either committed or rolled back.
[[process_context]]
==== ProcessContext
-The ProcessContext provides a bridge between a Processor and the
-framework. It provides information
-about how the Processor is currently configured and allows the
-Processor to perform
-Framework-specific tasks, such as yielding its resources so that the
-framework will schedule other
+The ProcessContext provides a bridge between a Processor and the framework. It provides information
+about how the Processor is currently configured and allows the Processor to perform
+Framework-specific tasks, such as yielding its resources so that the framework will schedule other
Processors to run without consuming resources unnecessarily.
@@ -1337,30 +1319,248 @@ If the callback succeeds, a CONTENT_MODIFIED Provenance Event is emitted.
== Error Handling
+When writing a Processor, there are several different unexpected cases that can occur.
+It is important that Processor developers understand the mechanics of how the NiFi framework
+behaves if Processors do not handle errors themselves, and it's important to understand
+what error handling is expected of Processors. Here, we will discuss how Processors should
+handle unexpected errors during the course of their work.
+
+
+=== Exceptions within the Processor
+
+During the execution of the `onTrigger` method of a Processor, many things can potentially go
+awry. Common failure conditions include:
+
+ - Incoming data is not in the expected format.
+ - Network connections to external services fail.
+ - Reading or writing data to a disk fails.
+ - There is a bug in the Processor or a dependent library.
+
+Any of these conditions can result in an Exception being thrown from the Processor. From the framework
+perspective, there are two types of Exceptions that can escape a Processor: `ProcessException` and
+all others.
+
+If a ProcessException is thrown from the Processor, the framework will assume that this is a failure that
+is a known outcome. Moreover, it is a condition where attempting to process the data again later may
+be successful. As a result, the framework will roll back the session that was being processed and penalize
+the FlowFiles that were being processed.
+
+If any other Exception escapes the Processor, though, the framework will assume that it is a failure that
+was not taken into account by the developer. In this case, the framework will also roll back the session
+and penalize the FlowFiles. However, in this case, we can get into some very problematic cases. For example,
+the Processor may be in a bad state and may continually run, depleting system resources, without providing
+any useful work. This is fairly common, for instance, when a NullPointerException is thrown continually.
+In order to avoid this case, if an Exception other than ProcessException is able to escape the Processor's
+`onTrigger` method, the framework will also "Administratively Yield" the Processor. This means that the
+Processor will not be triggered to run again for some amount of time. The amount of time is configured
+in the `nifi.properties` file but is 10 seconds by default.
+
+
=== Exceptions within a callback: IOException, RuntimeException
-=== Exceptions within the Processor: ProcessException, others.
+More often than not, when an Exception occurs in a Processor, it occurs from within a callback (I.e.,
+`InputStreamCallback`, `OutputStreamCallback`, or `StreamCallback`). That is, during the processing of a
+FlowFile's content. Callbacks are allowed to throw either `RuntimeException` or `IOException`. In the case
+of RuntimeException, this Exception will propagate back to the `onTrigger` method. In the case of an
+`IOException`, the Exception will be wrapped within a ProcessException and this ProcessException will then
+be thrown from the Framework.
+
+For this reason, it is recommended that Processors that use callbacks do so within a `try/catch` block
+and catch `ProcessException` as well as any other `RuntimeException` that they expect their callback to
+throw. It is *not* recommended that Processors catch the general `Exception` or `Throwable` cases, however.
+This is discouraged for two reasons.
+
+First, if an unexpected RuntimeException is thrown, it is likely a bug
+and allowing the framework to rollback the session will ensure no data loss and ensures that DataFlow Managers
+are able to deal with the data as they see fit by keeping the data queued up in place.
+
+Second, when an IOException is thrown from a callback, there really are two types of IOExceptions: those thrown
+from Processor code (for example, the data is not in the expected format or a network connection fails), and
+those that are thrown from the Content Repository (where the FlowFile content is stored). If the latter is the case,
+the framework will catch this IOException and wrap it into a `FlowFileAccessException`, which extends `RuntimeException`.
+This is done explicitly so that the Exception will escape the `onTrigger` method and the framework can handle this
+condition appropriately. Catching the general Exception prevents this from happening.
-=== Catching ProcessException from callback
=== Penalization vs. Yielding
-=== Session Rollback: with and within penalization
+When an issue occurs during processing, the framework exposes two methods to allow Processor developers to avoid performing
+unnecessary work: "penalization" and "yielding." These two concepts can become confusing for developers new to the NiFi API.
+A developer is able to penalize a FlowFile by calling the `penalize(FlowFile)` method of ProcessSession. This causes the
+FlowFile itself to be inaccessible to downstream Processors for a period of time. The amount of time that the FlowFile is
+inaccessible is determined by the DataFlow Manager by setting the "Penalty Duration" setting in the Processor Configuration
+dialog. The default value is 30 seconds. Typically, this is done when a Processor determines that the data cannot be processed
+due to environmental reasons that are expected to sort themselves out. A great example of this is the PutSFTP processor, which
+will penalize a FlowFile if a file already exists on the SFTP server that has the same filename. In this case, the Processor
+penalizes the FlowFile and routes it to failure. A DataFlow Manager can then route failure back to the same PutSFTP Processor.
+This way, if a file exists with the same filename, the Processor will not attempt to send the file again for 30 seconds
+(or whatever period the DFM has configured the Processor to use). In the meantime, it is able to continue to process other
+FlowFiles.
+
+On the other hand, yielding allows a Processor developer to indicate to the framework that it will not be able to perform
+any useful function for some period of time. This commonly happens with a Processor that is communicating with a remote
+resource. If the Processor cannot connect to the remote resource, or if the remote resource is expected to provide data
+but reports that it has none, the Processor should call `yield` on the `ProcessContext` object and then return. By doing
+this, the Processor is telling the framework that it should not waste resources triggering this Processor to run, because
+there's nothing that it can do - it's better to use those resources to allow other Processors to run.
+
+
+=== Session Rollback
+
+Thus far, when we have discussed the `ProcessSession`, we have typically referred to it simply as a mechanism for accessing
+FlowFiles. However, it provides another very important capability, which is transactionality. All methods that are called
+on a ProcessSession happen as a transaction. When we decided to end the transaction, we can do so either by calling
+`commit()` or by calling `rollback()`. Typically, this is handled by the `AbstractProcessor` class: if the `onTrigger` method
+throws an Exception, the AbstractProcessor will catch the Exception, call `session.rollback()`, and then re-throw the Exception.
+Otherwise, the AbstractProcessor will call `commit()` on the ProcessSession.
+
+There are times, however, that developers will want to roll back a session explicitly. This can be accomplished at any time
+by calling the `rollback()` or `rollback(boolean)` method. If using the latter, the boolean indicates whether or not those
+FlowFiles that have been pulled from queues (via the ProcessSession `get` methods) should be penalized before being added
+back to their queues.
+
+When `rollback` is called, any modification that has occurred to the FlowFiles in that session are discarded, to included
+both content modification and attribute modification. Additionally, all Provenance Events are rolled back (with the exception
+of any SEND event that was emitted by passing a value of `true` for the `force` argument). The FlowFiles that were pulled from
+the input queues are then transferred back to the input queues (and optionally penalized) so that they can be processed again.
+
+On the other hand, when the `commit` method is called, the FlowFile's new state is persisted in the FlowFile Repository, and
+any Provenance Events that occurred are persisted in the Provenance Repository. The previous content is destroyed (unless
+another FlowFile references the same piece of content), and the FlowFiles are transferred to the outbound queues so that the
+next Processors can operate on the data.
+
+It is also important to note how this behavior is affected by using the `org.apache.nifi.annotations.behavior.SupportsBatching`
+annotation. If a Processor utilizes this annotation, calls to `ProcessSession.commit` may not take affect immediately. Rather,
+these commits may be batched together in order to provide higher throughput. However, if at any point, the Processor rolls back
+the ProcessSession, all changes since the last call to `commit` will be discarded and all "batched" commits will take affect.
+These "batched" commits are not rolled back.
-=== Administrative Yielding
== General Design Considerations
-=== Cohesion / Reusability : Do one thing and do it well!
+When designing a Processor, there are a few important design considering to keep in mind. This section of the Developer Guide
+brings to the forefront some of the ideas that a developer should be thinking about when creating a Processor.
+=== Consider the User
+
+One of the most important concepts to keep in mind when developing a Processor (or any other component) is the user
+experience that you are creating. It's important to remember that as the developer of such a component, you may have
+important knowledge about the context that others do not have. Documentation should always be supplied so that those
+less familiar with the process are able to use it with ease.
+
+When thinking about the user experience, it is also important to note that consistency is very important. It is best
+to stick with the standard <<naming-convensions>>. This is true for Processor names, Property names and value, Relationship
+names, and any other aspect that the user will experience.
+
+Simplicity is crucial! Avoid adding properties that you don't expect users to understand or change. As developers, we are
+told that hard-coding values is bad. But this sometimes results in developers exposing properties that, when asked for clarification,
+tell users to just leave the default value. This leads to confusion and complexity.
+
+
+=== Cohesion and Reusability
+
+For the sake of making a single, cohesive unit, developers are sometimes tempted to combine several functions into a single Processor.
+This is very true for the case when a Processor expects input data to be in format X so that the Processor can convert the data into
+format Y and send the newly-formatted data to some external service.
+
+Taking this approach of formatting the data for a particular endpoint and then sending the data to that endpoint within the same Processor
+has several drawbacks:
+
+ - The Processor becomes very complex, as it has to perform the data translation task as well as the task of
+ sending the data to the remote service.
+ - If the Processor is unable to communicate with the remote service, it will route the data to a `failure` Relationship. In this case,
+ the Processor will be responsible to perform the data translation again. And if it fails again, the translation is done yet again.
+ - If we have five different Processors that translate the incoming data into this new format before sending the data, we have a great
+ deal of duplicated code. If the schema changes, for instance, many Processors must be updated.
+ - This intermediate data is thrown away when the Processor finishes sending to the remote service. The intermediate data format
+ may well be useful to other Processors.
+
+In order to avoid these issues, and make Processors more reusable, a Processor should always stick to the principal of "do one thing and do
+it well." Such a Processor should be broken into two separate Processors: one to convert the data from Format X to Format Y, and another
+Processor to send data to the remote resource.
+
+
+[[naming-convensions]]
=== Naming Conventions
+In order to deliver a consistent look and feel to users, it is advisable that Processors keep with standard naming conventions. The following
+is a list of standard conventions that are used:
+
+ - Processors that pull data from a remote system are named Get<Service> or Get<Protocol>, depending on if they poll data from arbitrary
+ sources over a known Protocol (such as GetHTTP or GetFTP) or if they pull data from a known service (such as GetKafka)
+ - Processors that push data to a remote system are named Put<Service> or Put<Protocol>.
+ - Relationship names are lower-cased and use spaces to delineated words.
+ - Property names capitalize significant words, as would be done with the title of a book.
+
+
+
=== Processor Behavior Annotations
+When creating a Processor, the developer is able to provide hints to the framework about how to utilize the Processor most
+effectively. This is done by applying annotations to the Processor's class. The annotations that can be applied to a
+Processor exist in three sub-packages of `org.apache.nifi.annotations`. Those in the `documentation` sub-package are used
+to provide documentation to the user. Those in the `lifecycle` sub-package instruct the framework which methods should
+be called on the Processor in order to respond to the appropriate life-cycle events. Those in the `behavior` package
+help the framework understand how to interact with the Processor in terms of scheduling and general behavior.
+
+The following annotations from the `org.apache.nifi.annotations.behavior` package can be used to modify how the framework
+will handle your Processor:
+
+ - `EventDriven`: Instructs the framework that the Processor can be scheduled using the Event-Driven scheduling
+ strategy. This strategy is still experimental at this point, but can result in reduced resource utilization
+ on dataflows that do not handle extremely high data rates.
+
+ - `SideEffectFree`: Indicates that the Processor does not have any side effects external to NiFi. As a result, the
+ framework is free to invoke the Processor many times with the same input without causing any unexpected
+ results to occur. This implies idempotent behavior. This can be used by the framework to improve efficiency by
+ performing actions such as transferring a ProcessSession from one Processor to another, such that if
+ a problem occurs many Processors' actions can be rolled back and performed again.
+
+ - `SupportsBatching`: This annotation indicates that it is okay for the framework to batch together multiple
+ ProcessSession commits into a single commit. If this annotation is present, the user will be able to choose
+ whether they prefer high throughput or lower latency in the Processor's Scheduling tab. This annotation should
+ be applied to most Processors, but it comes with a caveat: if the Processor calls `ProcessSession.commit`,
+ there is no guarantee that the data has been safely stored in NiFi's Content, FlowFile, and Provenance Repositories.
+ As a result, it is not appropriate for those Processors that receive data from an external source, commit the session,
+ and then delete the remote data or confirm a transaction with a remote resource.
+
+ - `TriggerSerially`: When this annotation is present, the framework will not allow the user to schedule more than one
+ concurrent thread to execute the `onTrigger` method at a time. Instead, the number of thread ("Concurrent Tasks")
+ will always be set to `1`. This does *not*, however, mean that the Processor does not have to be thread-safe,
+ as the thread that is executing `onTrigger` may change between invocations.
+
+ - `TriggerWhenAnyDestinationAvailable`: By default, NiFi will not schedule a Processor to run if any of its outbound
+ queues is full. This allows back-pressure to be applied all the way a chain of Processors. However, some Processors
+ may need to run even if one of the outbound queues is full. This annotations indicates that the Processor should run
+ if any Relationship is "available." A Relationship is said to be "available" if none of the connections that use
+ that Relationship is full. For example, the DistributeLoad Processor makes use of this annotation. If the "round robin"
+ scheduling strategy is used, the Processor will not run if any outbound queue is full. However, if the "next available"
+ scheduling strategy is used, the Processor will run if any Relationship at all is available and will route FlowFiles
+ only to those relationships that are available.
+
+ - `TriggerWhenEmpty`: The default behavior is to trigger a Processor to run only if its input queue has at least one
+ FlowFile or if the Processor has no input queues (which is typical of a "source" Processor). Applying this annotation
+ will cause the framework to ignore the size of the input queues and trigger the Processor regardless of whether or
+ not there is any data on an input queue. This is useful, for example, if the Processor needs to be triggered to run
+ periodically to time out a network connection.
+
+
=== Data Buffering
+An important point to keep in mind is that NiFi provides a generic data processing capability. Data can be in any format. Processors
+are generally scheduled with several threads. A common mistake that developers new to NiFi make is to buffer all the contents of a
+FlowFile in memory. While there are cases when this is required, it should be avoided if at all possible, unless it is well-known
+what format the data is in. For example, a Processor responsible for executing XPath against an XML document will need to load the
+entire contents of the data into memory. This is generally acceptable, as XML is not expected to be extremely large. However, a Processor
+that searches for a specific byte sequence may be used to search files that are hundreds of gigabytes or more. Attempting to load this
+into memory can cause a lot of problems - especially if multiple threads are processing different FlowFiles simultaneously.
+
+Instead of buffering this data into memory, it is advisable to instead evaluate the data as it is streamed from the Content Repository
+(i.e., scan the content from the `InputStream` that is provided to your callback by `ProcessSession.read`). Of course, in this case,
+we don't want to read from the Content Repository for each byte, so we would use a BufferedInputStream or somehow buffer some small
+amount of data, as appropriate.
@@ -1618,26 +1818,104 @@ Processor is valid or not, according to our expectations.
=== Enqueue FlowFiles
+Before triggering a Processor to run, it is usually necessary to enqueue FlowFiles for the Processor to process.
+This can be achieved by using the `enqueue` methods of the `TestRunner` class. The `enqueue` method has several
+different overrides, and allows data to be added in the form of a `byte[]`, `InputStream`, or `Path`. Each of these
+methods also supports a variation that allows a `Map<String, String>` to be added to support FlowFile attributes.
+Additionally, there is an `enqueue` method that takes a var-args of FlowFile objects. This can be useful, for example,
+to obtain the output of a Processor and then feed this to the input of the Processor.
=== Run the Processor
+After configuring the Controller Services and enqueuing the necessary FlowFiles, the Processor can be triggered
+to run by calling the `run` method of `TestRunner`. If this method is called without any arguments, it will
+invoke any method in the Processor with an `@OnScheduled` annotation, call the Processor's `onTrigger` method once,
+and then run the `@OnUnscheduled` and finally `@OnStopped` methods.
+
+If it is desirable to run several iterations of the `onTrigger` method before the other `@OnUnscheduled` and
+`@OnStopped` life-cycle events are triggered, the `run(int)` method can be used to specify now many iterations
+of `onTrigger` should be called.
+There are times when we want to trigger the Processor to run but not trigger the `@OnUnscheduled` and `@OnStopped`
+life-cycle events. This is useful, for instance, to inspect the Processor's state before these events occur. This
+can be achieved using the `run(int, boolean)` and passing `false` as the second argument. After doing this, though,
+calling the `@OnScheduled` life-cycle methods could cause an issue. As a result, we can now run `onTrigger` again
+without causing these events to occur by using the `run(int,boolean,boolean)` version of the `run` method and
+passing `false` as the third argument.
-=== Validate Output Counts
+If it is useful to test behavior that occurs with multiple threads, this can also be achieved by calling the
+`setThreadCount` method of `TestRunner`. The default is 1 thread. If using multiple threads, it is important
+to remember that the `run` call of `TestRunner` specifies how many times the Processor should be triggered, not
+the number of times that the Processor should be triggered per thread. So, if the thread count is set to 2 but
+`run(1)` is called, only a single thread will be used.
+=== Validate Output
-=== Validate Output Results
+After a Processor has finished running, a unit test will generally want to validate that the FlowFiles went where
+they were expected to go. This can be achieved using the `TestRunners` `assertAllFlowFilesTransferred` and
+`assertTransferCount` methods. The former method takes as arguments a Relationship and an integer to dictate how many
+FlowFiles should have been transferred to that Relationship. The method will fail the unit test unless this number of
+FlowFiles were transferred to the given Relationship *or* if any FlowFile was transferred to any other Relationship.
+The `assertTransferCount` method validates only that the FlowFile count was the expected number for the given Relationship.
+After validating the counts, we can then obtain the actual output FlowFiles via the `getFlowFilesForRelationship` method.
+This method returns a `List<MockFlowFile>`. It's important to note that the type of the List is `MockFlowFile`, rather
+than the `FlowFile` interface. This is done because `MockFlowFile` comes with many methods for validating the contents.
+For example, `MockFlowFile` has methods for asserting that FlowFile Attributes exist (`assertAttributeExists`), asserting
+that other attributes are not present (`assertAttributeNotExists`), or that Attributes have the correct value
+(`assertAttributeEquals`, `assertAttributeNotEquals`). Similar methods exist for verifying the contents of the FlowFile.
+The contents of a FlowFile can be compared to a `byte[]`, and `InputStream`, a file, or a String. If the data is expected
+to be textual, the String version is preferred, as it provides a more intuitive error message if the output is not
+as expected.
-=== Mocking Connections
+=== Mocking External Resources
+
+One of the biggest problems when testing a NiFi processor that connects to a remote resource is that we don't want to
+actually connect to some remote resource from a unit test. We can stand up a simple server ourselves in the unit test
+and configure the Processor to communicate with it, but then we have to understand and implement the server-specific
+specification and may not be able to properly send back error messages, etc. that we would like for testing.
+
+Generally, the approach taken here is to have a method in the Processor that is responsible for obtaining a connection
+or a client to a remote resource. We generally mark this method as protected. In the unit test, instead of creating
+the `TestRunner` by calling `TestRunners.newTestRunner(Class)` and providing the Processor class, we instead create
+a subclass of the Processor in our unit test and use this:
+
+[source,java]
+----
+@Test
+public void testConnectionFailure() {
+ final TestRunner runner = TestRunners.newTestRunner(new MyProcessor() {
+ protected Client getClient() {
+ // Return a mocked out client here.
+ return new Client() {
+ public void connect() throws IOException {
+ throw new IOException();
+ }
+
+ // ...
+ // other client methods
+ // ...
+ };
+ }
+ });
+
+ // rest of unit test.
+}
+----
+
+This allows us to implement a Client that mocks out all of the network communications and returns the different
+error results that we want to test, as well as ensure that our logic is correct for handling successful calls
+to the client.
+
=== Additional Testing Capabilities
+
In addition to the above-mentioned capabilities provided by the
testing framework, the TestRunner provides several
convenience methods for verifying the behavior of a Processor. Methods
@@ -1678,10 +1956,28 @@ their NiFi components as NAR packages.
To achieve this, a developer creates a new Maven Artifact, which we
refer to as the NAR artifact. The packaging is
set to `nar`. The `dependencies` section of the POM is then created so
-that the NAR has a dependency on all NiFi
-Components that are to be included within the NAR.
+that the NAR has a dependency on all NiFi Components that are to be included within the NAR.
-TODO: DISCUSS HOW TO INCLUDE THE NAR MAVEN PLUGIN.
+In order to use a packaging of `nar`, we must use the `nifi-nar-maven-plugin` module.
+This is included by adding the following snippet to the NAR's pom.xml:
+
+[source,xml]
+----
+<build>
+ <plugins>
+ <plugin>
+ <groupId>org.apache.nifi</groupId>
+ <artifactId>nifi-nar-maven-plugin</artifactId>
+ <version>1.0.0-incubating</version>
+ <extensions>true</extensions>
+ </plugin>
+ </plugins>
+</build>
+----
+
+In the Apache NiFi codebase, this exists in the NiFi root POM from which all other NiFi artifacts
+(with the exception of the nifi-nar-maven-plugin itself) inherit, so that we do not need to include
+this in any of our other POM files.
The NAR is able to have one dependency that is of type `nar`. If more
than one dependency is specified that is of type
@@ -1789,16 +2085,41 @@ API artifacts into the same NAR is often acceptable.
+== How to contribute to Apache NiFi
+We are always excited to have contributions from the community - especially from new contributors!
+We are interested in accepting contributions of code, as well as documentation and even artwork that
+can be applied as icons or styling to the application.
-== Consider the User Experience
-The user...
+=== Technologies
+The back end of Apache NiFi is written in Java. The web tier makes use of JAX-RS and JavaScript is extensively
+used to provide a user interface. We depend on several third-party JavaScript libraries, including D3 and JQuery,
+among others. We make use of Apache Maven for our builds and Git for our version control system.
+=== Where to Start?
-== How to contribute to Apache NiFi
+link:http://issues.apache.org/jira/browse/NIFI[NiFi's Jira page] can be used to find tickets that are tagged as "beginner",
+or you can dig into any of the tickets for creating Processors. Processors should be self-contained and not rely on other
+outside components (except for Controller Services), so they make for excellent starting points for new NiFi developers to
+get started. This exposes the developer to the NiFi API and is the most extensible part of the dataflow system.
+
+
+=== Supplying a contribution
+
+Contributions can be provided either by creating a patch:
+
+`git format-patch`
+
+and attaching that patch to a ticket, or by generating a Pull Request.
+
+
+=== Contact Us
-Git, Maven, ASF processes, NiFi processes, ...
+The developer mailing list (dev@nifi.incubator.apache.org) is monitored pretty closely, and we tend to respond pretty
+quickly. If you have a question, don't hesitate to shoot us an e-mail - we're here to help! Unfortunately, though, e-mails
+can get lost in the shuffle, so if you do send an e-mail and don't get a response within a day or two, it's our fault - don't
+worry about bothering us. Just ping the mailing list again.
[3/3] incubator-nifi git commit: Merge branch 'develop' into NIFI-250
Posted by mc...@apache.org.
Merge branch 'develop' into NIFI-250
Project: http://git-wip-us.apache.org/repos/asf/incubator-nifi/repo
Commit: http://git-wip-us.apache.org/repos/asf/incubator-nifi/commit/e7beef8d
Tree: http://git-wip-us.apache.org/repos/asf/incubator-nifi/tree/e7beef8d
Diff: http://git-wip-us.apache.org/repos/asf/incubator-nifi/diff/e7beef8d
Branch: refs/heads/NIFI-250
Commit: e7beef8d224797c057650787c4041b15dbef87af
Parents: 1ebaf1d bafa945
Author: Matt Gilman <ma...@gmail.com>
Authored: Fri Jan 30 07:22:25 2015 -0500
Committer: Matt Gilman <ma...@gmail.com>
Committed: Fri Jan 30 07:22:25 2015 -0500
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.../src/main/asciidoc/developer-guide.adoc | 2116 +++++++++++++++++-
1 file changed, 2098 insertions(+), 18 deletions(-)
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