[GitHub] [airflow] potiuk commented on a diff in pull request #28300: Add Public Interface description to Airflow documentation

[GitBox <gi...@apache.org>]

potiuk commented on code in PR #28300:
URL: https://github.com/apache/airflow/pull/28300#discussion_r1066811953


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docs/apache-airflow/public-airflow-interface.rst:
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+ .. Licensed to the Apache Software Foundation (ASF) under one
+    or more contributor license agreements.  See the NOTICE file
+    distributed with this work for additional information
+    regarding copyright ownership.  The ASF licenses this file
+    to you under the Apache License, Version 2.0 (the
+    "License"); you may not use this file except in compliance
+    with the License.  You may obtain a copy of the License at
+
+ ..   http://www.apache.org/licenses/LICENSE-2.0
+
+ .. Unless required by applicable law or agreed to in writing,
+    software distributed under the License is distributed on an
+    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+    KIND, either express or implied.  See the License for the
+    specific language governing permissions and limitations
+    under the License.
+
+Public Interface of Airflow
+...........................
+
+The Public Interface of Apache Airflow is a set of interfaces that allow developers to interact
+with and access certain features of the Apache Airflow system. This includes operations such as
+creating and managing DAGs (directed acyclic graphs), managing tasks and their dependencies,
+and extending Airflow capabilities by writing new executors, plugins, operators and providers. The
+Public Interface can be useful for building custom tools and integrations with other systems,
+and for automating certain aspects of the Airflow workflow.
+
+Using Airflow Public Interfaces
+===============================
+
+Using Airflow Public Interfaces is needed when you want to interact with Airflow programmatically:
+
+* When you are writing new (or extending existing) custom Python classes (Operators, Hooks) - basic building
+  blocks of DAGs and can be done DAG Authors to add missing functionality in their DAGs or by those who write
+  reusable custom operators for other DAG authors.
+* When writing new :doc:`Plugins <authoring-and-scheduling/plugins>` that extend Airflow's functionality beyond
+  DAG building blocks. Secrets, Timetables, Triggers, Listeners are all examples of such functionality. This
+  is usually done by users who manage Airflow instances.
+* Bundling custom Operators, Hooks, Plugins and releasing them together via
+  :doc:`provider packages <apache-airflow-providers:index>` - this is usually done by those who intend to
+  provide reusable set of functionality for external service or application Airflow integrates with.
+
+All the ways above involve extending or using Airflow Python classes and functions. The classes
+and functions mentioned below can be relied to keep backwards-compatible signatures and behaviours within
+MAJOR version of Airflow. On the other hand, classes and method started with ``_`` (also known
+as protected Python methods) and ``__`` (also known as private Python methods) are not part of the Public
+Airflow Interface and might change any time.
+
+You can also use Airflow's Public Interface via the `Stable REST API <stable-rest-api-ref>`_ (based on the
+OpenAPI specification). For specific needs you can also use the
+`Airflow Command Line Interface (CLI) <cli-and-env-variables-ref.rst>`_ though it's behaviour might change
+in details (such as output format and available flags) so if you want to rely on those in programmatic
+way, the Stable REST API is recommended.
+
+
+Using Public Interface by DAG Authors
+=====================================
+
+DAGS
+----
+
+The DAG is Airflow's core entity that represents a recurring workflow. You can create a DAG by
+instantiating the :class:`~airflow.models.dag.DAG` class in your DAG file.
+
+Airflow has a set of Example DAGs that you can use to learn how to write DAGs
+
+.. toctree::
+  :includehidden:
+  :glob:
+  :maxdepth: 1
+
+  _api/airflow/example_dags/index
+
+You can read more about DAGs in :doc:`DAGs <core-concepts/dags>`.
+
+.. _pythonapi:operators:
+
+Operators
+---------
+
+Operators allow for generation of certain types of tasks that become nodes in
+the DAG when instantiated.
+
+There are 3 main types of operators:
+
+- Operators that performs an **action**, or tell another system to
+  perform an action
+- **Transfer** operators move data from one system to another
+- **Sensors** are a certain type of operator that will keep running until a
+  certain criterion is met. Examples include a specific file landing in HDFS or
+  S3, a partition appearing in Hive, or a specific time of the day. Sensors
+  are derived from :class:`~airflow.sensors.base.BaseSensorOperator` and run a poke
+  method at a specified :attr:`~airflow.sensors.base.BaseSensorOperator.poke_interval` until it
+  returns ``True``.
+
+All operators are derived from :class:`~airflow.models.baseoperator.BaseOperator` and acquire much
+functionality through inheritance. Since this is the core of the engine,
+it's worth taking the time to understand the parameters of :class:`~airflow.models.baseoperator.BaseOperator`
+to understand the primitive features that can be leveraged in your DAGs.
+
+Airflow has a set of Operators that are considered public. You are also free to extend their functionality
+by extending them:
+
+.. toctree::
+  :includehidden:
+  :glob:
+  :maxdepth: 1
+
+  _api/airflow/operators/index
+
+  _api/airflow/sensors/index
+
+
+You can read more about the operators in :doc:`core-concepts/operators`, :doc:`core-concepts/sensors`.
+Also you can learn how to write a custom operator in :doc:`howto/custom-operator`.
+
+.. _pythonapi:hooks:
+
+Hooks
+-----
+
+Hooks are interfaces to external platforms and databases, implementing a common
+interface when possible and acting as building blocks for operators. All hooks
+are derived from :class:`~airflow.hooks.base.BaseHook`.
+
+Airflow has a set of Hooks that are considered public. You are free to extend their functionality
+by extending them:
+
+.. toctree::
+  :includehidden:
+  :glob:
+  :maxdepth: 1
+
+  _api/airflow/hooks/index
+
+Public Airflow utilities
+------------------------
+
+When writing or extending Hooks and Operators, DAG authors and developers of those Operators and Hooks can
+use the following classes:
+
+* The :class:`~airflow.models.connection.Connection`, which provides access to external service credentials and configuration.
+* The :class:`~airflow.models.variable.Variable`, which provides access to Airflow configuration variables.
+* The :class:`~airflow.models.xcom.XCom` which are used to access to inter-task communication data.
+
+You can read more about the public Airflow utilities in :doc:`howto/connection`,
+:doc:`core-concepts/variables`, :doc:`core-concepts/xcoms`
+
+Public Exceptions
+-----------------
+
+When writing the custom Operators and Hooks, you can handle and raise public Exceptions that Airflow
+exposes:
+
+.. toctree::
+  :includehidden:
+  :glob:
+  :maxdepth: 1
+
+  _api/airflow/exceptions/index
+
+
+Using Public Interface to extend Airflow capabilities
+=====================================================
+
+Airflow uses Plugin mechanism as a way to extend Airflow platform capabilities. They allow to extend
+Airflow UI but also they are the way to expose the below customizations (Triggers, Timetables, Listeners).
+Providers can also implement plugin endpoints and customize Airflow UI and the customizations.
+
+You can read more about plugins in :doc:`authoring-and-scheduling/plugins`. You can read how to extend
+Airflow UI in :doc:`howto/custom-view-plugin`. Note that there are some simple customizations of the UI
+that do not require plugins - you can read more about them in :doc:`howto/customize-ui`.
+
+Here are the ways how Plugins can be used to extend Airflow:
+
+Triggers
+--------
+
+Airflow might be configure to use Triggers in order to implement ``asyncio`` compatible Deferrable Operators.
+All Triggers derive from :class:`~airflow.triggers.base.BaseTrigger`.
+
+Airflow has a set of Triggers that are considered public. You are free to extend their functionality
+by extending them:
+
+.. toctree::
+  :includehidden:
+  :glob:
+  :maxdepth: 1
+
+  _api/airflow/triggers/index
+
+You can read more about Triggers in :doc:`authoring-and-scheduling/deferring`.
+
+Timetables
+----------
+
+Custom timetable implementations provide Airflow's scheduler additional logic to
+schedule DAG runs in ways not possible with built-in schedule expressions.
+All Timetables derive from :class:`~airflow.timetables.base.Timetable`.
+
+Airflow has a set of Timetables that are considered public. You are free to extend their functionality
+by extending them:
+
+.. toctree::
+  :includehidden:
+  :maxdepth: 1
+
+  _api/airflow/timetables/index
+
+You can read more about Timetables in :doc:`howto/timetable`.
+
+Listeners
+---------
+
+Listeners are the way that airflow platform as a whole can be extended to respond to DAG/Task lifecycle events.
+
+This is implemented via :class:`~airflow.listeners.listener.ListenerManager` class that provides hooks that
+can be implemented to respond to DAG/Task lifecycle events.
+
+.. versionadded:: 2.5
+
+   Listener public interface has been added in version 2.5.
+
+You can read more about Listeners in :doc:`administration-and-deployment/listeners`.
+
+Extra Links
+-----------
+
+Extra links are dynamic links that could be added to Airflow independently from custom Operators. Normally
+they can be defined by the Operators, but plugins allow to override the links on aa global level.
+
+You can read more about the Extra Links in :doc:`/howto/define-extra-link`.
+
+Using Public Interface to integrate with external services and applications
+===========================================================================
+
+In order to integrate Airflow with external services and applications you can develop Hooks and Operators,
+similarly to DAG Authors, but also you can also extend the core functionality of Airflow by integrating
+them with those external services. This can be done by just adding the extension classes to PYTHONPATH
+on your system,and configuring Airflow to use them (there is no need to use plugin mechanism). However,
+you can also package and release the Hooks, Operators and core extensions in the form of
+:doc:`provider packages <apache-airflow-providers:index>`. You can read more about core extensions
+delivered by providers via :doc:`provider packages <apache-airflow-providers:core-extensions/index>`.

Review Comment:
   Yes it is that. But I would also leave the links to detailed documents of the providers and "core-extensions" provided by the providers. I find it really important to inter-link our documentation and refer to more details when we are explaining something as it is the way how the reader can look easily for more information.
   
   I will add a fixup after I bulk-merge your corrections.



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