.. _settings-common:

Commonly Used Settings
======================

.. setting:: autodiscover

``autodiscover``
----------------

:type: :class:`~typing.Any`
:default: :const:`False`

Automatic discovery of agents, tasks, timers, views and commands.

Faust has an API to add different :mod:`asyncio` services and other
user extensions, such as "Agents", HTTP web views,
command-line commands, and timers to your Faust workers.
These can be defined in any module, so to discover them at startup,
the worker needs to traverse packages looking for them.

.. warning::

   The autodiscovery functionality uses the :pypi:`Venusian` library
   to scan wanted packages for ``@app.agent``, ``@app.page``,
   ``@app.command``, ``@app.task`` and ``@app.timer`` decorators,
   but to do so, it's required to traverse the package path and import
   every module in it.

   Importing random modules like this can be dangerous so make sure
   you follow Python programming best practices. Do not start
   threads; perform network I/O; do test monkey-patching for mocks or
   similar, as a side effect of importing a module.  If you encounter
   a case such as this then please find a way to perform your
   action in a lazy manner.

.. warning::

   If the above warning is something you cannot fix, or if it's out
   of your control, then please set ``autodiscover=False`` and make
   sure the worker imports all modules where your
   decorators are defined.

The value for this argument can be:

``bool``
   If ``App(autodiscover=True)`` is set, the autodiscovery will
   scan the package name described in the ``origin`` attribute.

   The ``origin`` attribute is automatically set when you start
   a worker using the :program:`faust` command line program,
   for example:

   .. sourcecode:: console

       faust -A example.simple worker

   The :option:`-A <faust -A>`, option specifies the app, but you
   can also create a shortcut entry point by calling ``app.main()``:

   .. sourcecode:: python

       if __name__ == '__main__':
           app.main()

   Then you can start the :program:`faust` program by executing for
   example ``python myscript.py worker --loglevel=INFO``, and it
   will use the correct application.

``Sequence[str]``
   The argument can also be a list of packages to scan::

       app = App(..., autodiscover=['proj_orders', 'proj_accounts'])

``Callable[[], Sequence[str]]``
   The argument can also be a function returning a list of packages
   to scan::

       def get_all_packages_to_scan():
           return ['proj_orders', 'proj_accounts']

       app = App(..., autodiscover=get_all_packages_to_scan)

``False``
   If everything you need is in a self-contained module, or you
   import the stuff you need manually, just set ``autodiscover``
   to False and don't worry about it :-)

.. admonition:: Django

   When using :pypi:`Django` and the :envvar:`DJANGO_SETTINGS_MODULE`
   environment variable is set, the Faust app will scan all packages
   found in the ``INSTALLED_APPS`` setting.

   If you're using Django you can use this to scan for
   agents/pages/commands in all packages
   defined in ``INSTALLED_APPS``.

   Faust will automatically detect that you're using Django
   and do the right thing if you do::

       app = App(..., autodiscover=True)

   It will find agents and other decorators in all of the
   reusable Django applications. If you want to manually control
   what packages are traversed, then provide a list::

       app = App(..., autodiscover=['package1', 'package2'])

   or if you want exactly :const:`None` packages to be traversed,
   then provide a False:

       app = App(.., autodiscover=False)

   which is the default, so you can simply omit the argument.

.. tip::

   For manual control over autodiscovery, you can also call the
   :meth:`@discover` method manually.


.. setting:: datadir

``datadir``
-----------

:type: :class:`str` / :class:`~pathlib.Path`
:default: ``'{conf.name}-data'``
:environment: :envvar:`APP_DATADIR`
:related-command-options: :option:`faust -`, :option:`faust -`, :option:`faust d`, :option:`faust a`, :option:`faust t`, :option:`faust a`, :option:`faust d`, :option:`faust i`, :option:`faust r`

Application data directory.

The directory in which this instance stores the data used by
local tables, etc.

.. seealso::

   - The data directory can also be set using the
     :option:`faust --datadir` option, from the command-line,
     so there is usually no reason to provide a default value
     when creating the app.


.. setting:: tabledir

``tabledir``
------------

:type: :class:`str` / :class:`~pathlib.Path`
:default: ``'tables'``
:environment: :envvar:`APP_TABLEDIR`

Application table data directory.

The directory in which this instance stores local table data.
Usually you will want to configure the :setting:`datadir` setting,
but if you want to store tables separately you can configure this one.

If the path provided is relative (it has no leading slash), then the
path will be considered to be relative to the :setting:`datadir`
setting.


.. setting:: debug

``debug``
---------

:type: :class:`bool`
:default: :const:`False`
:environment: :envvar:`APP_DEBUG`
:related-command-options: :option:`faust -`, :option:`faust -`, :option:`faust d`, :option:`faust e`, :option:`faust b`, :option:`faust u`, :option:`faust g`

Use in development to expose sensor information endpoint.


.. tip::

   If you want to enable the sensor statistics endpoint in production,
   without enabling the :setting:`debug` setting, you can do so
   by adding the following code:

   .. sourcecode:: python

       app.web.blueprints.add(
           '/stats/', 'faust.web.apps.stats:blueprint')


.. setting:: env_prefix

``env_prefix``
--------------

.. versionadded:: 1.11

:type: :class:`str`
:default: :const:`None`
:environment: :envvar:`APP_ENV_PREFIX`

Environment variable prefix.

When configuring Faust by environent variables,
this adds a common prefix to all Faust environment value names.


.. setting:: id_format

``id_format``
-------------

:type: :class:`str`
:default: ``'{id}-v{self.version}'``
:environment: :envvar:`APP_ID_FORMAT`

Application ID format template.

The format string used to generate the final :setting:`id` value
by combining it with the :setting:`version` parameter.


.. setting:: origin

``origin``
----------

:type: :class:`str`
:default: :const:`None`

The reverse path used to find the app.

For example if the app is located in::

   from myproj.app import app

Then the ``origin`` should be ``"myproj.app"``.

The :program:`faust worker` program will try to automatically set
the origin, but if you are having problems with auto generated names
then you can set origin manually.


.. setting:: timezone

``timezone``
------------

.. versionadded:: 1.4

:type: :class:`~datetime.tzinfo`
:default: ``datetime.timezone.utc``
:environment: :envvar:`TIMEZONE`

Project timezone.

The timezone used for date-related functionality such as cronjobs.


.. setting:: version

``version``
-----------

:type: :class:`int`
:default: ``1``
:environment: :envvar:`APP_VERSION`

App version.

Version of the app, that when changed will create a new isolated
instance of the application. The first version is 1,
the second version is 2, and so on.

.. admonition:: Source topics will not be affected by a version change.

   Faust applications will use two kinds of topics: source topics, and
   internally managed topics. The source topics are declared by the
   producer, and we do not have the opportunity to modify any
   configuration settings, like number of partitions for a source
   topic; we may only consume from them. To mark a topic as internal,
   use: ``app.topic(..., internal=True)``.


.. setting:: blocking_timeout

``blocking_timeout``
--------------------

:type: :class:`float` / :class:`~datetime.timedelta`
:default: :const:`None`
:environment: :envvar:`BLOCKING_TIMEOUT`
:related-command-options: :option:`faust -`, :option:`faust -`, :option:`faust b`, :option:`faust l`, :option:`faust o`, :option:`faust c`, :option:`faust k`, :option:`faust i`, :option:`faust n`, :option:`faust g`, :option:`faust -`, :option:`faust t`, :option:`faust i`, :option:`faust m`, :option:`faust e`, :option:`faust o`, :option:`faust u`, :option:`faust t`

Blocking timeout (in seconds).

When specified the worker will start a periodic signal based
timer that only triggers when the loop has been blocked
for a time exceeding this timeout.

This is the most safe way to detect blocking, but could have
adverse effects on libraries that do not automatically
retry interrupted system calls.

Python itself does retry all interrupted system calls
since version 3.5 (see :pep:`475`), but this might not
be the case with C extensions added to the worker by the user.

The blocking detector is a background thread
that periodically wakes up to either arm a timer, or cancel
an already armed timer. In pseudocode:

.. sourcecode:: python

   while True:
       # cancel previous alarm and arm new alarm
       signal.signal(signal.SIGALRM, on_alarm)
       signal.setitimer(signal.ITIMER_REAL, blocking_timeout)
       # sleep to wakeup just before the timeout
       await asyncio.sleep(blocking_timeout * 0.96)

   def on_alarm(signum, frame):
       logger.warning('Blocking detected: ...')

If the sleep does not wake up in time the alarm signal
will be sent to the process and a traceback will be logged.


.. setting:: broker

``broker``
----------

:type: :class:`str` / :class:`~yarl.URL` / ``[`` :class:`str` ``]``
:default: :const:`None`
:environment: :envvar:`BROKER_URL`

Broker URL, or a list of alternative broker URLs.

Faust needs the URL of a "transport" to send and receive messages.

Currently, the only supported production transport is ``kafka://``.
This uses the :pypi:`aiokafka` client under the hood, for consuming and
producing messages.

You can specify multiple hosts at the same time by separating them
using the semi-comma:

.. sourcecode:: text

   kafka://kafka1.example.com:9092;kafka2.example.com:9092

Which in actual code looks like this:

.. sourcecode:: python

   BROKERS = 'kafka://kafka1.example.com:9092;kafka2.example.com:9092'
   app = faust.App(
       'id',
       broker=BROKERS,
   )

You can also pass a list of URLs:

.. sourcecode:: python

   app = faust.App(
       'id',
       broker=['kafka://kafka1.example.com:9092',
               'kafka://kafka2.example.com:9092'],
   )

.. seealso::

   You can configure the transport used for consuming and producing
   separately, by setting the :setting:`broker_consumer` and
   :setting:`broker_producer` settings.

   This setting is used as the default.

**Available Transports**

- ``kafka://``

   Alias to ``aiokafka://``

- ``aiokafka://``

   The recommended transport using the :pypi:`aiokafka` client.

   Limitations: None

- ``confluent://``

   Experimental transport using the :pypi:`confluent-kafka` client.

   Limitations: Does not do sticky partition assignment (not
       suitable for tables), and do not create any necessary internal
       topics (you have to create them manually).


.. setting:: broker_credentials

``broker_credentials``
----------------------

.. versionadded:: 1.5

:type: :class:`~faust.types.auth.CredentialsT`
:default: :const:`None`
:environment: :envvar:`BROKER_CREDENTIALS`

Broker authentication mechanism.

Specify the authentication mechanism to use when connecting to
the broker.

The default is to not use any authentication.

SASL Authentication
   You can enable SASL authentication via plain text:

   .. sourcecode:: python

       app = faust.App(
           broker_credentials=faust.SASLCredentials(
               username='x',
               password='y',
           ))

   .. warning::

       Do not use literal strings when specifying passwords in
       production, as they can remain visible in stack traces.

       Instead the best practice is to get the password from
       a configuration file, or from the environment:

       .. sourcecode:: python

           BROKER_USERNAME = os.environ.get('BROKER_USERNAME')
           BROKER_PASSWORD = os.environ.get('BROKER_PASSWORD')

           app = faust.App(
               broker_credentials=faust.SASLCredentials(
                   username=BROKER_USERNAME,
                   password=BROKER_PASSWORD,
               ))

GSSAPI Authentication
   GSSAPI authentication over plain text:

   .. sourcecode:: python

       app = faust.App(
           broker_credentials=faust.GSSAPICredentials(
               kerberos_service_name='faust',
               kerberos_domain_name='example.com',
           ),
       )

   GSSAPI authentication over SSL:

   .. sourcecode:: python

       import ssl
       ssl_context = ssl.create_default_context(
           purpose=ssl.Purpose.SERVER_AUTH, cafile='ca.pem')
       ssl_context.load_cert_chain(
           'client.cert', keyfile='client.key')

       app = faust.App(
           broker_credentials=faust.GSSAPICredentials(
               kerberos_service_name='faust',
               kerberos_domain_name='example.com',
               ssl_context=ssl_context,
           ),
       )

SSL Authentication
   Provide an SSL context for the Kafka broker connections.

   This allows Faust to use a secure SSL/TLS connection for the
   Kafka connections and enabling certificate-based authentication.

   .. sourcecode:: python

       import ssl

       ssl_context = ssl.create_default_context(
           purpose=ssl.Purpose.SERVER_AUTH, cafile='ca.pem')
       ssl_context.load_cert_chain(
           'client.cert', keyfile='client.key')
       app = faust.App(..., broker_credentials=ssl_context)


.. setting:: ssl_context

``ssl_context``
---------------

:type: :class:`~ssl.SSLContext`
:default: :const:`None`

SSL configuration.

See :setting:`credentials`.


.. setting:: logging_config

``logging_config``
------------------

.. versionadded:: 1.5

:type: :class:`dict`
:default: :const:`None`

Logging dictionary configuration.

Optional dictionary for logging configuration, as supported
by :func:`logging.config.dictConfig`.


.. setting:: loghandlers

``loghandlers``
---------------

:type: ``[`` :class:`~logging.Handler` ``]``
:default: :const:`None`

List of custom logging handlers.

Specify a list of custom log handlers to use in worker instances.


.. setting:: processing_guarantee

``processing_guarantee``
------------------------

.. versionadded:: 1.5

:type: :class:`str`
:default: ``<ProcessingGuarantee.AT_LEAST_ONCE: 'at_least_once'>``
:environment: :envvar:`PROCESSING_GUARANTEE`

The processing guarantee that should be used.

Possible values are "at_least_once" (default) and "exactly_once".

Note that if exactly-once processing is enabled consumers are
configured with ``isolation.level="read_committed"`` and producers
are configured with ``retries=Integer.MAX_VALUE`` and
``enable.idempotence=true`` per default.

Note that by default exactly-once processing requires a cluster of
at least three brokers what is the recommended setting for production.
For development you can change this, by adjusting broker setting
``transaction.state.log.replication.factor`` to the number of brokers
you want to use.


.. setting:: store

``store``
---------

:type: :class:`str` / :class:`~yarl.URL`
:default: ``'memory://'``
:environment: :envvar:`APP_STORE`

Table storage backend URL.

The backend used for table storage.

Tables are stored in-memory by default, but you should
not use the ``memory://`` store in production.

In production, a persistent table store, such as ``rocksdb://`` is
preferred.


.. setting:: cache

``cache``
---------

.. versionadded:: 1.2

:type: :class:`str` / :class:`~yarl.URL`
:default: ``'memory://'``
:environment: :envvar:`CACHE_URL`

Cache backend URL.

Optional backend used for Memcached-style caching.
URL can be:

+ ``redis://host``
+ ``rediscluster://host``, or
+ ``memory://``.



.. _settings-agent:

Advanced Agent Settings
=======================

.. setting:: agent_supervisor

``agent_supervisor``
--------------------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'mode.OneForOneSupervisor'``
:environment: :envvar:`AGENT_SUPERVISOR`

Default agent supervisor type.

An agent may start multiple instances (actors) when
the concurrency setting is higher than one (e.g.
``@app.agent(concurrency=2)``).

Multiple instances of the same agent are considered to be in the same
supervisor group.

The default supervisor is the :class:`mode.OneForOneSupervisor`:
if an instance in the group crashes, we restart that instance only.

These are the supervisors supported:

+ :class:`mode.OneForOneSupervisor`

   If an instance in the group crashes we restart only that instance.

+ :class:`mode.OneForAllSupervisor`

   If an instance in the group crashes we restart the whole group.

+ :class:`mode.CrashingSupervisor`

   If an instance in the group crashes we stop the whole application,
   and exit so that the Operating System supervisor can restart us.

+ :class:`mode.ForfeitOneForOneSupervisor`

   If an instance in the group crashes we give up on that instance
   and never restart it again (until the program is restarted).

+ :class:`mode.ForfeitOneForAllSupervisor`

   If an instance in the group crashes we stop all instances
   in the group and never restarted them again (until the program is
   restarted).



.. _settings-broker:

Advanced Broker Settings
========================

.. setting:: broker_consumer

``broker_consumer``
-------------------

.. versionadded:: 1.7

:type: :class:`str` / :class:`~yarl.URL` / ``[`` :class:`str` ``]``
:default (alias to setting): :setting:`broker`
:environment: :envvar:`BROKER_CONSUMER_URL`

Consumer broker URL.

You can use this setting to configure the transport used for
producing and consuming separately.

If not set the value found in :setting:`broker` will be used.


.. setting:: broker_producer

``broker_producer``
-------------------

.. versionadded:: 1.7

:type: :class:`str` / :class:`~yarl.URL` / ``[`` :class:`str` ``]``
:default (alias to setting): :setting:`broker`
:environment: :envvar:`BROKER_PRODUCER_URL`

Producer broker URL.

You can use this setting to configure the transport used for
producing and consuming separately.

If not set the value found in :setting:`broker` will be used.


.. setting:: broker_api_version

``broker_api_version``
----------------------

.. versionadded:: 1.10

:type: :class:`str`
:default: ``'auto'``
:environment: :envvar:`BROKER_API_VERSION`

Broker API version,.

This setting is also the default for :setting:`consumer_api_version`,
and :setting:`producer_api_version`.

Negotiate producer protocol version.

The default value - "auto" means use the latest version supported
by both client and server.

Any other version set means you are requesting a specific version of
the protocol.

Example Kafka uses:

**Disable sending headers for all messages produced**

Kafka headers support was added in Kafka 0.11, so you can specify
``broker_api_version="0.10"`` to remove the headers from messages.


.. setting:: broker_check_crcs

``broker_check_crcs``
---------------------

:type: :class:`bool`
:default: :const:`True`
:environment: :envvar:`BROKER_CHECK_CRCS`

Broker CRC check.

Automatically check the CRC32 of the records consumed.


.. setting:: broker_client_id

``broker_client_id``
--------------------

:type: :class:`str`
:default: ``'faust-1.11.0a1'``
:environment: :envvar:`BROKER_CLIENT_ID`

Broker client ID.

There is rarely any reason to configure this setting.

The client id is used to identify the software used, and is not usually
configured by the user.


.. setting:: broker_commit_every

``broker_commit_every``
-----------------------

:type: :class:`int`
:default: ``10000``
:environment: :envvar:`BROKER_COMMIT_EVERY`

Broker commit message frequency.

Commit offset every n messages.

See also :setting:`broker_commit_interval`, which is how frequently
we commit on a timer when there are few messages being received.


.. setting:: broker_commit_interval

``broker_commit_interval``
--------------------------

:type: :class:`float` / :class:`~datetime.timedelta`
:default: ``2.8``
:environment: :envvar:`BROKER_COMMIT_INTERVAL`

Broker commit time frequency.

How often we commit messages that have been
fully processed (:term:`acked`).


.. setting:: broker_commit_livelock_soft_timeout

``broker_commit_livelock_soft_timeout``
---------------------------------------

:type: :class:`float` / :class:`~datetime.timedelta`
:default: ``300.0``
:environment: :envvar:`BROKER_COMMIT_LIVELOCK_SOFT_TIMEOUT`

Commit livelock timeout.

How long time it takes before we warn that the Kafka commit offset has
not advanced (only when processing messages).


.. setting:: broker_heartbeat_interval

``broker_heartbeat_interval``
-----------------------------

.. versionadded:: 1.0.11

:type: :class:`float` / :class:`~datetime.timedelta`
:default: ``3.0``
:environment: :envvar:`BROKER_HEARTBEAT_INTERVAL`

Broker heartbeat interval.

How often we send heartbeats to the broker, and also how often
we expect to receive heartbeats from the broker.

If any of these time out, you should increase this setting.


.. setting:: broker_max_poll_interval

``broker_max_poll_interval``
----------------------------

.. versionadded:: 1.7

:type: :class:`float` / :class:`~datetime.timedelta`
:default: ``1000.0``
:environment: :envvar:`BROKER_MAX_POLL_INTERVAL`

Broker max poll interval.

The maximum allowed time (in seconds) between calls to consume messages
If this interval is exceeded the consumer
is considered failed and the group will rebalance in order to reassign
the partitions to another consumer group member. If API methods block
waiting for messages, that time does not count against this timeout.

See `KIP-62`_ for technical details.

.. _`KIP-62`:
   https://cwiki.apache.org/confluence/display/KAFKA/KIP-62%3A+Allow+consumer+to+send+heartbeats+from+a+background+thread


.. setting:: broker_max_poll_records

``broker_max_poll_records``
---------------------------

.. versionadded:: 1.4

:type: :class:`int`
:default: :const:`None`
:environment: :envvar:`BROKER_MAX_POLL_RECORDS`

Broker max poll records.

The maximum number of records returned in a single call to ``poll()``.
If you find that your application needs more time to process
messages you may want to adjust :setting:`broker_max_poll_records`
to tune the number of records that must be handled on every
loop iteration.


.. setting:: broker_rebalance_timeout

``broker_rebalance_timeout``
----------------------------

.. versionadded:: 1.10

:type: :class:`float` / :class:`~datetime.timedelta`
:default: ``60.0``
:environment: :envvar:`BROKER_REBALANCE_TIMEOUT`

Broker rebalance timeout.

How long to wait for a node to finish rebalancing before the broker
will consider it dysfunctional and remove it from the cluster.

Increase this if you experience the cluster being in a state of
constantly rebalancing, but make sure you also increase the
:setting:`broker_heartbeat_interval` at the same time.

.. note::

   The session timeout must not be greater than the
   :setting:`broker_request_timeout`.


.. setting:: broker_request_timeout

``broker_request_timeout``
--------------------------

.. versionadded:: 1.4

:type: :class:`float` / :class:`~datetime.timedelta`
:default: ``90.0``
:environment: :envvar:`BROKER_REQUEST_TIMEOUT`

Kafka client request timeout.

.. note::

   The request timeout must not be less than the
   :setting:`broker_session_timeout`.


.. setting:: broker_session_timeout

``broker_session_timeout``
--------------------------

.. versionadded:: 1.0.11

:type: :class:`float` / :class:`~datetime.timedelta`
:default: ``60.0``
:environment: :envvar:`BROKER_SESSION_TIMEOUT`

Broker session timeout.

How long to wait for a node to finish rebalancing before the broker
will consider it dysfunctional and remove it from the cluster.

Increase this if you experience the cluster being in a state of
constantly rebalancing, but make sure you also increase the
:setting:`broker_heartbeat_interval` at the same time.

.. note::

   The session timeout must not be greater than the
   :setting:`broker_request_timeout`.



.. _settings-consumer:

Advanced Consumer Settings
==========================

.. setting:: consumer_api_version

``consumer_api_version``
------------------------

.. versionadded:: 1.10

:type: :class:`str`
:default (alias to setting): :setting:`broker_api_version`
:environment: :envvar:`CONSUMER_API_VERSION`

Consumer API version.

Configures the broker API version to use for consumers.
See :setting:`broker_api_version` for more information.


.. setting:: consumer_max_fetch_size

``consumer_max_fetch_size``
---------------------------

.. versionadded:: 1.4

:type: :class:`int`
:default: ``1048576``
:environment: :envvar:`CONSUMER_MAX_FETCH_SIZE`

Consumer max fetch size.

The maximum amount of data per-partition the server will return.
This size must be at least as large as the maximum message size.

Note: This is PER PARTITION, so a limit of 1Mb when your
workers consume from 10 topics having 100 partitions each,
means a fetch request can be up to a gigabyte (10 * 100 * 1Mb),
This limit being too generous may cause
rebalancing issues: if the amount of time required
to flush pending data stuck in socket buffers exceed
the rebalancing timeout.

You must keep this limit low enough to account
for many partitions being assigned to a single node.


.. setting:: consumer_auto_offset_reset

``consumer_auto_offset_reset``
------------------------------

.. versionadded:: 1.5

:type: :class:`str`
:default: ``'earliest'``
:environment: :envvar:`CONSUMER_AUTO_OFFSET_RESET`

Consumer auto offset reset.

Where the consumer should start reading messages from when
there is no initial offset, or the stored offset no longer
exists, e.g. when starting a new consumer for the first time.

Options include 'earliest', 'latest', 'none'.


.. setting:: consumer_group_instance_id

``consumer_group_instance_id``
------------------------------

.. versionadded:: 2.1

:type: :class:`str`
:default: :const:`None`
:environment: :envvar:`CONSUMER_GROUP_INSTANCE_ID`

Consumer group instance id.

The group_instance_id for static partition assignment.

If not set, default assignment strategy is used. Otherwise,
each consumer instance has to have a unique id.


.. setting:: ConsumerScheduler

``ConsumerScheduler``
---------------------

.. versionadded:: 1.5

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust.transport.utils:DefaultSchedulingStrategy'``

Consumer scheduler class.

A strategy which dictates the priority of topics and
partitions for incoming records.
The default strategy does first round-robin over topics and then
round-robin over partitions.

Example using a class::

   class MySchedulingStrategy(DefaultSchedulingStrategy):
       ...

   app = App(..., ConsumerScheduler=MySchedulingStrategy)

Example using the string path to a class::

   app = App(..., ConsumerScheduler='myproj.MySchedulingStrategy')



.. _settings-serialization:

Serialization Settings
======================

.. setting:: key_serializer

``key_serializer``
------------------

:type: :class:`str` / :class:`~faust.types.codecs.CodecT`
:default: ``'raw'``
:environment: :envvar:`APP_KEY_SERIALIZER`

Default key serializer.

Serializer used for keys by default when no serializer
is specified, or a model is not being used.

This can be the name of a serializer/codec, or an actual
:class:`faust.serializers.codecs.Codec` instance.

.. seealso::

   - The :ref:`codecs` section in the model guide -- for
     more information about codecs.


.. setting:: value_serializer

``value_serializer``
--------------------

:type: :class:`str` / :class:`~faust.types.codecs.CodecT`
:default: ``'json'``
:environment: :envvar:`APP_VALUE_SERIALIZER`

Default value serializer.

Serializer used for values by default when no serializer
is specified, or a model is not being used.

This can be string, the name of a serializer/codec, or an actual
:class:`faust.serializers.codecs.Codec` instance.

.. seealso::

   - The :ref:`codecs` section in the model guide -- for
     more information about codecs.



.. _settings-producer:

Advanced Producer Settings
==========================

.. setting:: producer_acks

``producer_acks``
-----------------

:type: :class:`int`
:default: ``-1``
:environment: :envvar:`PRODUCER_ACKS`

Producer Acks.

The number of acknowledgments the producer requires the leader to have
received before considering a request complete. This controls the
durability of records that are sent. The following settings are common:

* ``0``: Producer will not wait for any acknowledgment from
        the server at all. The message will immediately be
        considered sent (Not recommended).
* ``1``: The broker leader will write the record to its local
        log but will respond without awaiting full acknowledgment
        from all followers. In this case should the leader fail
        immediately after acknowledging the record but before the
        followers have replicated it then the record will be lost.
* ``-1``: The broker leader will wait for the full set of in-sync
         replicas to acknowledge the record. This guarantees that
         the record will not be lost as long as at least one in-sync
         replica remains alive. This is the strongest
         available guarantee.


.. setting:: producer_api_version

``producer_api_version``
------------------------

.. versionadded:: 1.5.3

:type: :class:`str`
:default (alias to setting): :setting:`broker_api_version`
:environment: :envvar:`PRODUCER_API_VERSION`

Producer API version.

Configures the broker API version to use for producers.
See :setting:`broker_api_version` for more information.


.. setting:: producer_compression_type

``producer_compression_type``
-----------------------------

:type: :class:`str`
:default: :const:`None`
:environment: :envvar:`PRODUCER_COMPRESSION_TYPE`

Producer compression type.

The compression type for all data generated by the producer.
Valid values are `gzip`, `snappy`, `lz4`, or :const:`None`.


.. setting:: producer_linger

``producer_linger``
-------------------

:type: :class:`float` / :class:`~datetime.timedelta`
:default: :const:`None`
:environment: :envvar:`PRODUCER_LINGER`

Producer batch linger configuration.

Minimum time to batch before sending out messages from the producer.

Should rarely have to change this.


.. setting:: producer_max_batch_size

``producer_max_batch_size``
---------------------------

:type: :class:`int`
:default: ``16384``
:environment: :envvar:`PRODUCER_MAX_BATCH_SIZE`

Producer max batch size.

Max size of each producer batch, in bytes.


.. setting:: producer_max_request_size

``producer_max_request_size``
-----------------------------

:type: :class:`int`
:default: ``1000000``
:environment: :envvar:`PRODUCER_MAX_REQUEST_SIZE`

Producer maximum request size.

Maximum size of a request in bytes in the producer.

Should rarely have to change this.


.. setting:: producer_partitioner

``producer_partitioner``
------------------------

.. versionadded:: 1.2

:type: :class:`str` / :class:`~typing.Type`
:default: :const:`None`

Producer partitioning strategy.

The Kafka producer can be configured with a custom partitioner
to change how keys are partitioned when producing to topics.

The default partitioner for Kafka is implemented as follows,
and can be used as a template for your own partitioner:

.. sourcecode:: python

   import random
   from typing import List
   from aiokafka.partitioner.hashed import murmur2

   def partition(key: bytes,
               all_partitions: List[int],
               available: List[int]) -> int:
       '''Default partitioner.

       Hashes key to partition using murmur2 hashing
       (from java client) If key is None, selects partition
       randomly from available, or from all partitions if none
       are currently available

       Arguments:
           key: partitioning key
           all_partitions: list of all partitions sorted by
                           partition ID.
           available: list of available partitions
                      in no particular order
       Returns:
           int: one of the values from ``all_partitions``
                or ``available``.
       '''
       if key is None:
           source = available if available else all_paritions
           return random.choice(source)
       index: int = murmur2(key)
       index &= 0x7fffffff
       index %= len(all_partitions)
       return all_partitions[index]


.. setting:: producer_request_timeout

``producer_request_timeout``
----------------------------

.. versionadded:: 1.4

:type: :class:`float` / :class:`~datetime.timedelta`
:default: ``1200.0``
:environment: :envvar:`PRODUCER_REQUEST_TIMEOUT`

Producer request timeout.

Timeout for producer operations.
This is set high by default, as this is also the time when
producer batches expire and will no longer be retried.



.. _settings-rpc:

Agent RPC Settings
==================

.. setting:: reply_create_topic

``reply_create_topic``
----------------------

:type: :class:`bool`
:default: :const:`False`
:environment: :envvar:`APP_REPLY_CREATE_TOPIC`

Automatically create reply topics.

Set this to :const:`True` if you plan on using the RPC with agents.

This will create the internal topic used for RPC replies on that
instance at startup.


.. setting:: reply_expires

``reply_expires``
-----------------

:type: :class:`float` / :class:`~datetime.timedelta`
:default: ``86400.0``
:environment: :envvar:`APP_REPLY_EXPIRES`

RPC reply expiry time in seconds.

The expiry time (in seconds :class:`float`,
or :class:`~datetime.timedelta`), for how long replies will stay
in the instances local reply topic before being removed.


.. setting:: reply_to

``reply_to``
------------

:type: :class:`str`
:default: :const:`None`

Reply to address.

The name of the reply topic used by this instance.
If not set one will be automatically generated when the app
is created.


.. setting:: reply_to_prefix

``reply_to_prefix``
-------------------

:type: :class:`str`
:default: ``'f-reply-'``
:environment: :envvar:`APP_REPLY_TO_PREFIX`

Reply address topic name prefix.

The prefix used when generating reply topic names.



.. _setting-stream:

Advanced Stream Settings
========================

.. setting:: stream_buffer_maxsize

``stream_buffer_maxsize``
-------------------------

:type: :class:`int`
:default: ``4096``
:environment: :envvar:`STREAM_BUFFER_MAXSIZE`

Stream buffer maximum size.

This setting control back pressure to streams and agents reading
from streams.

If set to 4096 (default) this means that an agent can only keep
at most 4096 unprocessed items in the stream buffer.

Essentially this will limit the number of messages a stream
can "prefetch".

Higher numbers gives better throughput, but do note that if
your agent sends messages or update tables (which
sends changelog messages).

This means that if the buffer size is large, the
:setting:`broker_commit_interval` or :setting:`broker_commit_every`
settings must be set to commit frequently, avoiding back pressure
from building up.

A buffer size of 131_072 may let you process over 30,000 events
a second as a baseline, but be careful with a buffer size that large
when you also send messages or update tables.


.. setting:: stream_processing_timeout

``stream_processing_timeout``
-----------------------------

.. versionadded:: 1.10

:type: :class:`float` / :class:`~datetime.timedelta`
:default: ``300.0``
:environment: :envvar:`STREAM_PROCESSING_TIMEOUT`

Stream processing timeout.

Timeout (in seconds) for processing events in the stream.
If processing of a single event exceeds this time we log an error,
but do not stop processing.

If you are seeing a warning like this you should either

1) increase this timeout to allow agents to spend more time
   on a single event, or
2) add a timeout to the operation in the agent, so stream processing
   always completes before the timeout.

The latter is preferred for network operations such as web requests.
If a network service you depend on is temporarily offline you should
consider doing retries (send to separate topic):

.. sourcecode:: python

   main_topic = app.topic('main')
   deadletter_topic = app.topic('main_deadletter')

   async def send_request(value, timeout: float = None) -> None:
       await app.http_client.get('http://foo.com', timeout=timeout)

   @app.agent(main_topic)
   async def main(stream):
       async for value in stream:
       try:
           await send_request(value, timeout=5)
       except asyncio.TimeoutError:
           await deadletter_topic.send(value)

   @app.agent(deadletter_topic)
       async def main_deadletter(stream):
           async for value in stream:
           # wait for 30 seconds before retrying.
           await stream.sleep(30)
           await send_request(value)


.. setting:: stream_publish_on_commit

``stream_publish_on_commit``
----------------------------

:type: :class:`bool`
:default: :const:`False`

Stream delay producing until commit time.

If enabled we buffer up sending messages until the
source topic offset related to that processing is committed.
This means when we do commit, we may have buffered up a LOT of messages
so commit needs to happen frequently (make sure to decrease
:setting:`broker_commit_every`).


.. setting:: stream_recovery_delay

``stream_recovery_delay``
-------------------------

.. versionadded:: 1.3

.. versionchanged:: 1.5.3

        Disabled by default.

:type: :class:`float` / :class:`~datetime.timedelta`
:default: ``0.0``
:environment: :envvar:`STREAM_RECOVERY_DELAY`

Stream recovery delayl

Number of seconds to sleep before continuing after rebalance.
We wait for a bit to allow for more nodes to join/leave before
starting recovery tables and then processing streams. This to minimize
the chance of errors rebalancing loops.


.. setting:: stream_wait_empty

``stream_wait_empty``
---------------------

:type: :class:`bool`
:default: :const:`True`
:environment: :envvar:`STREAM_WAIT_EMPTY`

Stream wait empty.

This setting controls whether the worker should wait for the
currently processing task in an agent to complete before
rebalancing or shutting down.

On rebalance/shut down we clear the stream buffers. Those events
will be reprocessed after the rebalance anyway, but we may have
already started processing one event in every agent, and if we
rebalance we will process that event again.

By default we will wait for the currently active tasks, but if your
streams are idempotent you can disable it using this setting.



.. _settings-table:

Advanced Table Settings
=======================

.. setting:: table_cleanup_interval

``table_cleanup_interval``
--------------------------

:type: :class:`float` / :class:`~datetime.timedelta`
:default: ``30.0``
:environment: :envvar:`TABLE_CLEANUP_INTERVAL`

Table cleanup interval.

How often we cleanup tables to remove expired entries.


.. setting:: table_key_index_size

``table_key_index_size``
------------------------

.. versionadded:: 1.7

:type: :class:`int`
:default: ``1000``
:environment: :envvar:`TABLE_KEY_INDEX_SIZE`

Table key index size.

Tables keep a cache of key to partition number to speed up
table lookups.

This setting configures the maximum size of that cache.


.. setting:: table_standby_replicas

``table_standby_replicas``
--------------------------

:type: :class:`int`
:default: ``1``
:environment: :envvar:`TABLE_STANDBY_REPLICAS`

Table standby replicas.

The number of standby replicas for each table.



.. _settings-topic:

Topic Settings
==============

.. setting:: topic_allow_declare

``topic_allow_declare``
-----------------------

.. versionadded:: 1.5

:type: :class:`bool`
:default: :const:`True`
:environment: :envvar:`TOPIC_ALLOW_DECLARE`

Allow creating new topics.

This setting disables the creation of internal topics.

Faust will only create topics that it considers to be fully
owned and managed, such as intermediate repartition topics,
table changelog topics etc.

Some Kafka managers does not allow services to create topics,
in that case you should set this to :const:`False`.


.. setting:: topic_disable_leader

``topic_disable_leader``
------------------------

.. versionadded:: 1.7

:type: :class:`bool`
:default: :const:`False`
:environment: :envvar:`TOPIC_DISABLE_LEADER`

Disable leader election topic.

This setting disables the creation of the leader election topic.

If you're not using the ``on_leader=True`` argument to task/timer/etc.,
decorators then use this setting to disable creation of the topic.


.. setting:: topic_partitions

``topic_partitions``
--------------------

:type: :class:`int`
:default: ``8``
:environment: :envvar:`TOPIC_PARTITIONS`

Topic partitions.

Default number of partitions for new topics.

.. note::

   This defines the maximum number of workers we could distribute the
   workload of the application (also sometimes referred as the
   sharding factor of the application).


.. setting:: topic_replication_factor

``topic_replication_factor``
----------------------------

:type: :class:`int`
:default: ``1``
:environment: :envvar:`TOPIC_REPLICATION_FACTOR`

Topic replication factor.

The default replication factor for topics created by the application.

.. note::

   Generally this should be the same as the configured
   replication factor for your Kafka cluster.



.. _setting-web:

Advanced Web Server Settings
============================

.. setting:: web

``web``
-------

.. versionadded:: 1.2

:type: :class:`str` / :class:`~yarl.URL`
:default: ``'aiohttp://'``

Web server driver to use.

.. setting:: web_bind

``web_bind``
------------

.. versionadded:: 1.2

:type: :class:`str`
:default: ``'0.0.0.0'``
:environment: :envvar:`WEB_BIND`
:related-command-options: :option:`faust worker --web-bind`

Web network interface binding mask.

The IP network address mask that decides what interfaces
the web server will bind to.

By default this will bind to all interfaces.

This option is usually set by :option:`faust worker --web-bind`,
not by passing it as a keyword argument to :class:`app`.


.. setting:: web_cors_options

``web_cors_options``
--------------------

.. versionadded:: 1.5

:type: :class:`dict`
:default: :const:`None`

Cross Origin Resource Sharing options.

Enable `Cross-Origin Resource Sharing`_ options for all web views
in the internal web server.

This should be specified as a dictionary of
URLs to :class:`~faust.web.ResourceOptions`:

.. sourcecode:: python

   app = App(..., web_cors_options={
       'http://foo.example.com': ResourceOptions(
           allow_credentials=True,
           allow_methods='*'k,
       )
   })

Individual views may override the CORS options used as
arguments to to ``@app.page`` and ``blueprint.route``.

.. seealso::

   :pypi:`aiohttp_cors`: https://github.com/aio-libs/aiohttp-cors

.. _`Cross-Origin Resource Sharing`:
   https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS


.. setting:: web_enabled

``web_enabled``
---------------

.. versionadded:: 1.2

:type: :class:`bool`
:default: :const:`True`
:environment: :envvar:`APP_WEB_ENABLED`
:related-command-options: :option:`faust worker --with-web`

Enable/disable internal web server.

Enable web server and other web components.

This option can also be set using :option:`faust worker --without-web`.


.. setting:: web_host

``web_host``
------------

.. versionadded:: 1.2

:type: :class:`str`
:default (template): ``'{conf.NODE_HOSTNAME}'``
:environment: :envvar:`WEB_HOST`
:related-command-options: :option:`faust worker --web-host`

Web server host name.

Hostname used to access this web server, used for generating
the :setting:`canonical_url` setting.

This option is usually set by :option:`faust worker --web-host`,
not by passing it as a keyword argument to :class:`app`.


.. setting:: web_in_thread

``web_in_thread``
-----------------

.. versionadded:: 1.5

:type: :class:`bool`
:default: :const:`False`

Run the web server in a separate thread.

Use this if you have a large value for
:setting:`stream_buffer_maxsize` and want the web server
to be responsive when the worker is otherwise busy processing streams.

.. note::

   Running the web server in a separate thread means web views
   and agents will not share the same event loop.


.. setting:: web_port

``web_port``
------------

.. versionadded:: 1.2

:type: :class:`int`
:default: ``6066``
:environment: :envvar:`WEB_PORT`
:related-command-options: :option:`faust worker --web-port`

Web server port.

A port number between 1024 and 65535 to use for the web server.

This option is usually set by :option:`faust worker --web-port`,
not by passing it as a keyword argument to :class:`app`.


.. setting:: web_transport

``web_transport``
-----------------

.. versionadded:: 1.2

:type: :class:`str` / :class:`~yarl.URL`
:default: ``URL('tcp:')``
:related-command-options: :option:`faust worker --web-transport`

Network transport used for the web server.

Default is to use TCP, but this setting also enables you to use
Unix domainN sockets.  To use domain sockets specify an URL including
the path to the file you want to create like this:

.. sourcecode:: text

   unix:///tmp/server.sock

This will create a new domain socket available
in :file:`/tmp/server.sock`.


.. setting:: canonical_url

``canonical_url``
-----------------

:type: :class:`str` / :class:`~yarl.URL`
:default (template): ``'http://{conf.web_host}:{conf.web_port}'``
:environment: :envvar:`NODE_CANONICAL_URL`
:related-command-options: :option:`faust worker --web-host`, :option:`faust worker --web-port`
:related-settings: :setting:`web_host`, :setting:`web_port`

Node specific canonical URL.

You shouldn't have to set this manually.

The canonical URL defines how to reach the web server on a running
worker node, and is usually set by combining the
:setting:`web_host` and :setting:`web_port` settings.



.. _settings-worker:

Advanced Worker Settings
========================

.. setting:: worker_redirect_stdouts

``worker_redirect_stdouts``
---------------------------

:type: :class:`bool`
:default: :const:`True`
:environment: :envvar:`WORKER_REDIRECT_STDOUTS`

Redirecting standard outputs.

Enable to have the worker redirect output to :data:`sys.stdout` and
:data:`sys.stderr` to the Python logging system.

Enabled by default.


.. setting:: worker_redirect_stdouts_level

``worker_redirect_stdouts_level``
---------------------------------

:type: :class:`str` / :class:`int`
:default: ``'WARN'``
:environment: :envvar:`WORKER_REDIRECT_STDOUTS_LEVEL`

Level used when redirecting standard outputs.

The logging level to use when redirect STDOUT/STDERR to logging.



.. _settings-extending:

Extension Settings
==================

.. setting:: Agent

``Agent``
---------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust:Agent'``

Agent class type.

The :class:`~faust.Agent` class to use for agents, or the
fully-qualified path to one (supported by
:func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   class MyAgent(faust.Agent):
       ...

   app = App(..., Agent=MyAgent)

Example using the string path to a class::

   app = App(..., Agent='myproj.agents.Agent')


.. setting:: Event

``Event``
---------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust:Event'``

Event class type.

The :class:`~faust.Event` class to use for creating new event objects,
or the fully-qualified path to one (supported by
:func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   class MyBaseEvent(faust.Event):
       ...

   app = App(..., Event=MyBaseEvent)

Example using the string path to a class::

   app = App(..., Event='myproj.events.Event')


.. setting:: Schema

``Schema``
----------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust:Schema'``

Schema class type.

The :class:`~faust.Schema` class to use as the default
schema type when no schema specified. or the fully-qualified
path to one (supported by :func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   class MyBaseSchema(faust.Schema):
       ...

   app = App(..., Schema=MyBaseSchema)

Example using the string path to a class::

   app = App(..., Schema='myproj.schemas.Schema')


.. setting:: Stream

``Stream``
----------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust:Stream'``

Stream class type.

The :class:`~faust.Stream` class to use for streams, or the
fully-qualified path to one (supported by
:func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   class MyBaseStream(faust.Stream):
       ...

   app = App(..., Stream=MyBaseStream)

Example using the string path to a class::

   app = App(..., Stream='myproj.streams.Stream')


.. setting:: Table

``Table``
---------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust:Table'``

Table class type.

The :class:`~faust.Table` class to use for tables, or the
fully-qualified path to one (supported by
:func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   class MyBaseTable(faust.Table):
       ...

   app = App(..., Table=MyBaseTable)

Example using the string path to a class::

   app = App(..., Table='myproj.tables.Table')


.. setting:: SetTable

``SetTable``
------------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust:SetTable'``

SetTable extension table.

The :class:`~faust.SetTable` class to use for table-of-set tables,
or the fully-qualified path to one (supported
by :func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   class MySetTable(faust.SetTable):
       ...

   app = App(..., Table=MySetTable)

Example using the string path to a class::

   app = App(..., Table='myproj.tables.MySetTable')


.. setting:: GlobalTable

``GlobalTable``
---------------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust:GlobalTable'``

GlobalTable class type.

The :class:`~faust.GlobalTable` class to use for tables,
or the fully-qualified path to one (supported by
:func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   class MyBaseGlobalTable(faust.GlobalTable):
       ...

   app = App(..., GlobalTable=MyBaseGlobalTable)

Example using the string path to a class::

   app = App(..., GlobalTable='myproj.tables.GlobalTable')


.. setting:: SetGlobalTable

``SetGlobalTable``
------------------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust:SetGlobalTable'``

SetGlobalTable class type.

The :class:`~faust.SetGlobalTable` class to use for tables, or the
fully-qualified path to one (supported by
:func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   class MyBaseSetGlobalTable(faust.SetGlobalTable):
       ...

   app = App(..., SetGlobalTable=MyBaseGlobalSetTable)

Example using the string path to a class::

   app = App(..., SetGlobalTable='myproj.tables.SetGlobalTable')


.. setting:: TableManager

``TableManager``
----------------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust.tables:TableManager'``

Table manager class type.

The :class:`~faust.tables.TableManager` used for managing tables,
or the fully-qualified path to one (supported by
:func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   from faust.tables import TableManager

   class MyTableManager(TableManager):
       ...

   app = App(..., TableManager=MyTableManager)

Example using the string path to a class::

   app = App(..., TableManager='myproj.tables.TableManager')


.. setting:: Serializers

``Serializers``
---------------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust.serializers:Registry'``

Serializer registry class type.

The :class:`~faust.serializers.Registry` class used for
serializing/deserializing messages; or the fully-qualified path
to one (supported by :func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   from faust.serialiers import Registry

   class MyRegistry(Registry):
       ...

   app = App(..., Serializers=MyRegistry)

Example using the string path to a class::

   app = App(..., Serializers='myproj.serializers.Registry')


.. setting:: Worker

``Worker``
----------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust.worker:Worker'``

Worker class type.

The :class:`~faust.Worker` class used for starting a worker
for this app; or the fully-qualified path
to one (supported by :func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   import faust

   class MyWorker(faust.Worker):
       ...

   app = faust.App(..., Worker=Worker)

Example using the string path to a class::

   app = faust.App(..., Worker='myproj.workers.Worker')


.. setting:: PartitionAssignor

``PartitionAssignor``
---------------------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust.assignor:PartitionAssignor'``

Partition assignor class type.

The :class:`~faust.assignor.PartitionAssignor` class used for assigning
topic partitions to worker instances; or the fully-qualified path
to one (supported by :func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   from faust.assignor import PartitionAssignor

   class MyPartitionAssignor(PartitionAssignor):
       ...

   app = App(..., PartitionAssignor=PartitionAssignor)

Example using the string path to a class::

   app = App(..., Worker='myproj.assignor.PartitionAssignor')


.. setting:: LeaderAssignor

``LeaderAssignor``
------------------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust.assignor:LeaderAssignor'``

Leader assignor class type.

The :class:`~faust.assignor.LeaderAssignor` class used for assigning
a master Faust instance for the app; or the fully-qualified path
to one (supported by :func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   from faust.assignor import LeaderAssignor

   class MyLeaderAssignor(LeaderAssignor):
       ...

   app = App(..., LeaderAssignor=LeaderAssignor)

Example using the string path to a class::

   app = App(..., Worker='myproj.assignor.LeaderAssignor')


.. setting:: Router

``Router``
----------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust.app.router:Router'``

Router class type.

The :class:`~faust.router.Router` class used for routing requests
to a worker instance having the partition for a specific
key (e.g. table key); or the fully-qualified path to one
(supported by :func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   from faust.router import Router

   class MyRouter(Router):
       ...

   app = App(..., Router=Router)

Example using the string path to a class::

   app = App(..., Router='myproj.routers.Router')


.. setting:: Topic

``Topic``
---------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust:Topic'``

Topic class type.

The :class:`~faust.Topic` class used for defining new topics; or the
fully-qualified path to one (supported by
:func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   import faust

   class MyTopic(faust.Topic):
       ...

   app = faust.App(..., Topic=MyTopic)

Example using the string path to a class::

   app = faust.App(..., Topic='myproj.topics.Topic')


.. setting:: HttpClient

``HttpClient``
--------------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'aiohttp.client:ClientSession'``

Http client class type

The :class:`aiohttp.client.ClientSession` class used as
a HTTP client; or the fully-qualified path to one (supported by
:func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   import faust
   from aiohttp.client import ClientSession

   class HttpClient(ClientSession):
       ...

   app = faust.App(..., HttpClient=HttpClient)

Example using the string path to a class::

   app = faust.App(..., HttpClient='myproj.http.HttpClient')


.. setting:: Monitor

``Monitor``
-----------

:type: :class:`str` / :class:`~typing.Type`
:default: ``'faust.sensors:Monitor'``

Monitor sensor class type.

The :class:`~faust.sensors.Monitor` class as the main sensor
gathering statistics for the application; or the
fully-qualified path to one (supported by
:func:`~mode.utils.imports.symbol_by_name`).

Example using a class::

   import faust
   from faust.sensors import Monitor

   class MyMonitor(Monitor):
       ...

   app = faust.App(..., Monitor=MyMonitor)

Example using the string path to a class::

   app = faust.App(..., Monitor='myproj.monitors.Monitor')



