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Logging for Tasks
=================

Airflow writes logs for tasks in a way that allows you to see the logs for each task separately in the Airflow UI.
Core Airflow provides an interface FileTaskHandler, which writes task logs to file, and includes a mechanism to serve them from workers while tasks are running. The Apache Airflow Community also releases providers for many
services (:doc:`apache-airflow-providers:index`) and some of them provide handlers that extend the logging
capability of Apache Airflow. You can see all of these providers in :doc:`apache-airflow-providers:core-extensions/logging`.

When using S3, GCS, WASB, HDFS or OSS remote logging service, you can delete the local log files after
they are uploaded to the remote location, by setting the config:

.. code-block:: ini

    [logging]
    remote_logging = True
    remote_base_log_folder = schema://path/to/remote/log
    delete_local_logs = True

Configuring logging
-------------------

For the default handler, FileTaskHandler, you can specify the directory to place log files in ``airflow.cfg`` using
``base_log_folder``. By default, logs are placed in the ``AIRFLOW_HOME``
directory.

.. note::
    For more information on setting the configuration, see :doc:`/howto/set-config`

The default pattern is followed while naming log files for tasks:

- For normal tasks: ``dag_id={dag_id}/run_id={run_id}/task_id={task_id}/attempt={try_number}.log``.
- For dynamically mapped tasks: ``dag_id={dag_id}/run_id={run_id}/task_id={task_id}/map_index={map_index}/attempt={try_number}.log``.

These patterns can be adjusted by :ref:`config:logging__log_filename_template`.

In addition, you can supply a remote location to store current logs and backups.

Writing to task logs from your code
-----------------------------------

Airflow uses the standard Python `logging <https://docs.python.org/3/library/logging.html>`_ framework to
write logs, and for the duration of a task, the root logger is configured to write to the task's log.

Most operators will write logs to the task log automatically. This is because they
have a ``log`` logger that you can use to write to the task log.
This logger is created and configured by :class:`~airflow.utils.log.LoggingMixin` that all
operators derive from. But also due to the root logger handling, any standard logger (using default settings) that
propagates logging to the root will also write to the task log.

So if you want to log to the task log from custom code of yours you can do any of the following:

* Log with the ``self.log`` logger from BaseOperator
* Use standard ``print`` statements to print to ``stdout`` (not recommended, but in some cases it can be useful)
* Use the standard logger approach of creating a logger using the Python module name
  and using it to write to the task log

This is the usual way loggers are used directly in Python code:

.. code-block:: python

  import logging

  logger = logging.getLogger(__name__)
  logger.info("This is a log message")

Grouping of log lines
---------------------

.. versionadded:: 2.9.0

Like CI pipelines also Airflow logs can be quite large and become hard to read. Sometimes therefore it is useful to group sections of log areas
and provide folding of text areas to hide non relevant content. Airflow therefore implements a compatible log message grouping like
`GitHub <https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#grouping-log-lines>`_ and
`Azure DevOps <https://learn.microsoft.com/en-us/azure/devops/pipelines/scripts/logging-commands?view=azure-devops&tabs=powershell#formatting-commands>`_
such that areas of text can be folded. The implemented scheme is compatible such that tools making output in CI can leverage the same experience
in Airflow directly.

By adding log markers with the starting and ending positions like for example below log messages can be grouped:

.. code-block:: python

   print("Here is some standard text.")
   print("::group::Non important details")
   print("bla")
   print("debug messages...")
   print("::endgroup::")
   print("Here is again some standard text.")

When displaying the logs in web UI, the display of logs will be condensed:

.. code-block:: text

   [2024-03-08, 23:30:18 CET] {logging_mixin.py:188} INFO - Here is some standard text.
   [2024-03-08, 23:30:18 CET] {logging_mixin.py:188} ⯈ Non important details
   [2024-03-08, 23:30:18 CET] {logging_mixin.py:188} INFO - Here is again some standard text.

If you click on the log text label, the detailed log lines will be displayed.

.. code-block:: text

   [2024-03-08, 23:30:18 CET] {logging_mixin.py:188} INFO - Here is some standard text.
   [2024-03-08, 23:30:18 CET] {logging_mixin.py:188} ⯆ Non important details
   [2024-03-08, 23:30:18 CET] {logging_mixin.py:188} INFO - bla
   [2024-03-08, 23:30:18 CET] {logging_mixin.py:188} INFO - debug messages...
   [2024-03-08, 23:30:18 CET] {logging_mixin.py:188} ⯅⯅⯅ Log group end
   [2024-03-08, 23:30:18 CET] {logging_mixin.py:188} INFO - Here is again some standard text.

Interleaving of logs
--------------------

Airflow's remote task logging handlers can broadly be separated into two categories: streaming handlers (such as ElasticSearch, AWS Cloudwatch, and GCP operations logging, formerly stackdriver) and blob storage handlers (e.g. S3, GCS, WASB).

For blob storage handlers, depending on the state of the task, logs could be in a lot of different places and in multiple different files.  For this reason, we need to check all locations and interleave what we find.  To do this we need to be able to parse the timestamp for each line.  If you are using a custom formatter you may need to override the default parser by providing a callable name at Airflow setting ``[logging] interleave_timestamp_parser``.

For streaming handlers, no matter the task phase or location of execution, all log messages can be sent to the logging service with the same identifier so generally speaking there isn't a need to check multiple sources and interleave.

Troubleshooting
---------------

If you want to check which task handler is currently set, you can use the ``airflow info`` command as in
the example below.

.. code-block:: bash

    $ airflow info

    Apache Airflow
    version                | 2.9.0.dev0
    executor               | LocalExecutor
    task_logging_handler   | airflow.utils.log.file_task_handler.FileTaskHandler
    sql_alchemy_conn       | postgresql+psycopg2://postgres:airflow@postgres/airflow
    dags_folder            | /files/dags
    plugins_folder         | /root/airflow/plugins
    base_log_folder        | /root/airflow/logs
    remote_base_log_folder |

    [skipping the remaining outputs for brevity]

The output of ``airflow info`` above is truncated to only display the section that pertains to the logging configuration.
You can also run ``airflow config list`` to check that the logging configuration options have valid values.

Advanced configuration
----------------------

You can configure :doc:`advanced features </administration-and-deployment/logging-monitoring/advanced-logging-configuration>`
- including adding your own custom task log handlers (but also log handlers for all airflow components), and creating
custom log handlers per operators, hooks and tasks.

.. _serving-worker-trigger-logs:

Serving logs from workers and triggerer
---------------------------------------

Most task handlers send logs upon completion of a task. In order to view logs in real time, Airflow starts an HTTP server to serve the logs in the following cases:

- If ``LocalExecutor`` is used, then when ``airflow scheduler`` is running.
- If ``CeleryExecutor`` is used, then when ``airflow worker`` is running.

In triggerer, logs are served unless the service is started with option ``--skip-serve-logs``.

The server is running on the port specified by ``worker_log_server_port`` option in ``[logging]`` section, and option ``triggerer_log_server_port`` for triggerer.  Defaults are 8793 and 8794, respectively.
Communication between the webserver and the worker is signed with the key specified by ``secret_key`` option  in ``[api]`` section. You must ensure that the key matches so that communication can take place without problems.

We are using `Gunicorn <https://gunicorn.org/>`__ as a WSGI server. Its configuration options can be overridden with the ``GUNICORN_CMD_ARGS`` env variable. For details, see `Gunicorn settings <https://docs.gunicorn.org/en/latest/settings.html#settings>`__.

Implementing a custom file task handler
---------------------------------------

.. note:: This is an advanced topic and most users should be able to just use an existing handler from :doc:`apache-airflow-providers:core-extensions/logging`.

In our providers we have a healthy variety of options with all the major cloud providers.  But should you need to implement logging with a different service, and should you then decide to implement a custom FileTaskHandler, there are a few settings to be aware of, particularly in the context of trigger logging.

Triggers require a shift in the way that logging is set up.  In contrast with tasks, many triggers run in the same process, and with triggers, since they run in asyncio, we have to be mindful of not introducing blocking calls through the logging handler.  And because of the variation in handler behavior (some write to file, some upload to blob storage, some send messages over network as they arrive, some do so in thread), we need to have some way to let triggerer know how to use them.

To accomplish this we have a few attributes that may be set on the handler, either the instance or the class.  Inheritance is not respected for these parameters, because subclasses of FileTaskHandler may differ from it in the relevant characteristics.  These params are described below:

- ``trigger_should_wrap``: Controls whether this handler should be wrapped by TriggerHandlerWrapper.  This is necessary when each instance of handler creates a file handler that it writes all messages to.
- ``trigger_should_queue``: Controls whether the triggerer should put a QueueListener between the event loop and the handler, to ensure blocking IO in the handler does not disrupt the event loop.
- ``trigger_send_end_marker``: Controls whether an END signal should be sent to the logger when trigger completes. It is used to tell the wrapper to close and remove the individual file handler specific to the trigger that just completed.
- ``trigger_supported``: If ``trigger_should_wrap`` and ``trigger_should_queue`` are not True, we generally assume that the handler does not support triggers.  But if in this case the handler has ``trigger_supported`` set to True, then we'll still move the handler to root at triggerer start so that it will process trigger messages.  Essentially, this should be true for handlers that "natively" support triggers. One such example of this is the StackdriverTaskHandler.

External Links
--------------

When using remote logging, you can configure Airflow to show a link to an external UI within the Airflow Web UI. Clicking the link redirects you to the external UI.

Some external systems require specific configuration in Airflow for redirection to work but others do not.
