[Pig Wiki] Update of "PigErrorHandlingDesign" by SanthoshSrinivasan

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This document describes the design for the error handling feature in Pig. The design follows the [#errReq requirements] and [#errFuncSpec functional specification]. The error handling design is influenced by Mika Raento's excellent online [#mika resource].

Pig's architecture is designed to support several back-ends. Currently, the only supported back-end is Hadoop. The front-end is ideally back-end agnostic. For a breakdown of the front-end components refer to the [#errFuncSpec functional specification].

== Error Handling ==

=== Front-end ===

`PigException` will serve as the base class for all the frontend exceptions. `PigException` will also be the exception thrown by Pig to external systems. Presently, the Pig APIs throw `IOException`. As a result, `PigException` will extend `IOException` in order to maintain backward compatibility. `FrontendException` will extend `PigException` and serve as the umbrella for all front-end exceptions. The task specific exceptions from the front-end components will subclass `FrontendException` to ensure clarity and enable extensions in the future.

`PigException` contains the following attributes and methods.

==== Attributes ====

   1. retriable: A boolean variable to indicate if an exception is retriable or not
   2. errorSouce: An enum to represent the source of the error. The enum can be extended in the future to include more information like sub-component. The values for this enum type will be:
      i. User input
      ii. Bug
      iii. User environment
      iv. Remote environment
   3. errorCode: An integer that represents the error. Used for documentation and automation
   4. detailedMsg: A string that holds detailed information that is pertinent to the Pig developer. It will contain details that are not required by the user

==== Methods ====
   1. retirable: Return true if the exception is retirable; false otherwise
   2. getErrorCode: Returns the error code associated with this exception
   3. getDetailedMessage: Returns the string detailedMsg

==== Methods of interest from IOException and its super classes ====
   1. getMessage: User facing message will be reported using getMessage()
   2. getCause: When exceptions are chained, the cause of each exception is retrieved using getCause()
   3. getStackTrace: Useful for determining the root cause and contains details required by the developer
   4. initCause: Used for chaining exceptions

As mentioned earlier, the source of the exception is classified into the four categories. Each exception should report the appropriate source based on the context. Absence of the source will be treated as a bug by default.

=== Back-end ===
   Hadoop reports errors via strings instead of exceptions. Since Hadoop does not support any APIs to query the exception thrown, Pig will make an assumption on the format of the string. The error messages contain the string format of an exception, i.e., name of the exception along with an error message followed by the stack trace. Pig will parse these error messages and report the appropriate error message and source (i.e., stack trace). An example of an error string reported by Hadoop is shown below.

{{{#!java
java.lang.RuntimeException: Unexpected data type 74 found in stream.                                                   
        at org.apache.pig.data.DataReaderWriter.readDatum(DataReaderWriter.java:115)
        at org.apache.pig.builtin.BinStorage.bytesToInteger(BinStorage.java:169)
        at org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.POCast.getNext(POCast.java:143)
        at org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperators.POForEach.processPlan(POForEach.java:242)
        at org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperators.POForEach.getNext(POForEach.java:180)
        at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigMapBase.runPipeline(PigMapBase.java:170)
        at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigMapBase.map(PigMapBase.java:158)
        at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigMapOnly$Map.map(PigMapOnly.java:65)
        at org.apache.hadoop.mapred.MapRunner.run(MapRunner.java:47)
        at org.apache.hadoop.mapred.MapTask.run(MapTask.java:227)
        at org.apache.hadoop.mapred.TaskTracker$Child.main(TaskTracker.java:2207)                              
}}}

   Hadoop reports multiple failures due to retries. The retries have to be ignored and only the final failure has to be reported.

== Warning message aggregation ==

=== Back-end ===

Hadoop provides the ability to aggregate counters for the entire application. The change in counter values has to be performed via the Hadoop reporter. A new interface, `PigLogger` will be used to abstract logging of warning messages. A back-end specific `PigHadoopLogger` will implement this interface and provide the functionality of warning message aggregation using Hadoop counters if the warning message aggregation is turned on. If the warning message aggregation is turned off, the warning messages are sent to STDERR which will appear in Hadoop's STDERR logs.

=== Front-end ===

Currently, the type checker uses a collector to collect error and warning messages. The use of the collector has to be extended for each subsystem in the front-end.


== Open questions ==

   1. `ParseException` is throw by the parser. Ensuring that `ParseException` is subclassed from `FrontendException` requires the generated `ParseException.java` file to be checked into the source repository.
   2. Lexical errors in Grunt will result in a `TokenMgrError`, resulting in an exit from Grunt. What should be the strategy around lexical error handling.

== References ==

 1. [[Anchor(errReq)]] Santhosh Srinivasan, Pig Error Handling Requirements October 30, 2008, http://wiki.apache.org/pig/PigErrorHandling
 2. [[Anchor(errFuncSpec)]] Santhosh Srinivasan, Pig Error Handling Functional Specification, November 26, 2008, http://wiki.apache.org/pig/PigErrorHandlingFunctionalSpecification
 3. [[Anchor(mika)]] Mika Raento, "What should Exceptions look like?" July 30, 2006, http://www.errorhandling.org/wordpress/