Change history for Faust 1.5

This document contain change notes for bugfix releases in the Faust 1.5.x series. If you’re looking for changes in the latest series, please visit the latest Changes.

For even older releases you can visit the History section.



2019-04-9 2:09 P.M PST


Ask Solem (@ask)

  • New producer_api_version setting.

    This can be set to the value “0.10” to remove headers from all messages produced.

    Use this if you have downstream consumers that do not support the new Kafka protocol format yet.

  • The stream_recovery_delay setting has been disabled by default.

    After rebalancing the worker will sleep a bit before starting recovery, the idea being that another recovery may be waiting just behind it so we wait a bit, but this has shown to be not as effective as intended.

  • Web: Cache can now be configured to take headers into account.

    Create the cache manager for your blueprint with the include_headers argument:

    cache = blueprint.cache(timeout=300.0, include_headers=True)

    Contributed by Sanyam Satia (@ssatia).



2019-04-06 11:25 P.M PST


Ask Solem (@ask)

  • Requirements:

    • Now depends on robinhood-aiokafka 1.0.2

      This version disables the “LeaveGroup” timeout added in 1.0.0, as it was causing problems.

  • Documentation: Fixed spelling.

  • Tests: Fixed flaky regression test.



2019-03-28 11:00 A.M PST


Ask Solem (@ask)

  • Requirements

  • Timers: Prevent drift + add some tiny drift.

    Thanks to Bob Haddleton (@bobh66).

  • App: Autodiscovery now avoids importing (Issue #324).

    Added regression test.

  • The stream_ack_exceptions setting has been deprecated.

    It was not having any effect, and we have no current use for it.

  • The stream_ack_cancelled_tasks setting has been deprecated.

    It was not having any effect, and we have no current use for it.

  • App: Autodiscovery failed to load when using app.main() in some cases (Issue #323).

    Added regression test.

  • Worker: Fixed error during agent shutdown.

  • Monitor: Monitor assignment latency + assignments completed/failed.

    Implemented in the default monitor, but also for statsd and datadog.

  • CLI: The faust program had the wrong help description.

  • Docs: Fixes typo in web_cors_options example.

  • App: Do no wait for table recovery finished signal, if the app is not starting the recovery service.



2019-03-24 09:45 P.M PST


Ask Solem (@ask)

  • Fixed hanging in partition assignment introduced in Faust 1.5 (Issue #320).

    Contributed by Bob Haddleton (@bobh66).



2019-03-22 02:18 P.M PST


Ask Solem (@ask)

  • Requirements

  • Exactly-Once semantics: New processing_guarantee setting.

    Experimental support for “exactly-once” semantics.

    This mode ensures tables and counts in tables/windows are consistent even as nodes in the cluster are abruptly terminated.

    To enable this mode set the processing_guarantee setting:



    If you do enable “exactly_once” for an existing app, you must make sure all workers are running the latest version and possibly starting from a clean set of intermediate topics.

    You can accomplish this by bumping up the app version number:

    App(version=2, processing_guarantee='exactly_once')

    The new processing guarantee require a new version of the assignor protocol, for this reason a “exactly_once” worker will not work with older versions of Faust running in the same consumer group: so to roll out this change you will have to stop all the workers, deploy the new version and only then restart the workers.

  • New optimizations for stream processing and windows.

    If Cython is available during installation, Faust will be installed with compiled extensions.

    You can set the NO_CYTHON environment variable to disable the use of these extensions even if compiled.

  • New topic_allow_declare setting.

    If disabled your faust worker instances will never actually declare topics.

    Use this if your Kafka administrator does not allow you to create topics.

  • New ConsumerScheduler setting.

    This class can override how events are delivered to agents. The default will go round robin between both topics and partitions, to ensure all topic partitions get a chance of being processed.

    Contributed by Miha Troha (@miatroha).

  • Authentication: Support for GSSAPI authentication.

    See documentation for the broker_credentials setting.

    Contributed by Julien Surloppe (@jsurloppe).

  • Authentication: Support for SASL authentication.

    See documentation for the broker_credentials setting.

  • New broker_credentials setting can also be used to configure SSL authentication.

  • Models: Records can now use comparison operators.

    Comparison of models using the >, <, >= and <= operators now work similarly to dataclasses.

  • Models: Now raise an error if non-default fields follows default fields.

    The following model will now raise an error:

    class Account(faust.Record):
        name: str
        amount: int = 3
        userid: str

    This is because a non-default field is defined after a default field, and this would mess up argument ordering.

    To define the model without error, make sure you move default fields below any non-default fields:

    class Account(faust.Record):
        name: str
        userid: str
        amount: int = 3


    Remember that when adding fields to an already existing model you should always add new fields as optional fields.

    This will help your application stay backward compatible.

  • App: Sending messages API now supports a headers argument.

    When sending messages you can now attach arbitrary headers as a dict, or list of tuples; where the values are bytes:

    await topic.send(key=key, value=value, headers={'x': b'foo'})

    Supported transports

    Headers are currently only supported by the default aiokafka transport, and requires Kafka server 0.11 and later.

  • Agent: RPC operations can now take advantage of message headers.

    The default way to attach metadata to values, such as the reply-to address and the correlation id, is to wrap the value in an envelope.

    With headers support now landed we can use message headers for this:

    async def x(stream):
        async for item in stream:
            yield item ** 2

    Faust will be using headers by default in version 2.0.

  • App: Sending messages API now supports a timestamp argument (Issue #276).

    When sending messages you can now specify the timestamp of the message:

    await topic.send(key=key, value=value, timestamp=custom_timestamp)

    If no timestamp is provided the current time will be used (time.time()).

    Contributed by Miha Troha (@mihatroha).

  • App: New consumer_auto_offset_reset setting (Issue #267).

    Contributed by Ryan Whitten (@rwhitten577).

  • Stream: group_by repartitioned topic name now includes the agent name (Issue #284).

  • App: Web server is no longer running in a separate thread by default.

    Running the web server in a separate thread is beneficial as it will not be affected by back pressure in the main thread event loop, but it also makes programming harder when it cannot share the loop of the parent.

    If you want to run the web server in a separate thread, use the new web_in_thread setting.

  • App: New web_in_thread controls separate thread for web server.

  • App: New logging_config setting.

  • App: Autodiscovery now ignores modules matching “test” (Issue #242).

    Contributed by Chris Seto (@chrisseto).

  • Transport: aiokafka transport now supports headers when using Kafka server versions 0.11 and later.

  • Tables: New flags can be used to check if actives/standbys are up to date.

    • app.tables.actives_ready

      Set to True when tables have synced all active partitions.

    • app.tables.standbys_ready

      Set to True when standby partitions are up-to-date.

  • RocksDB: Now crash with ConsistencyError if the persisted offset is greater than the current highwater.

    This means the changelog topic has been modified in Kafka and the recorded offset no longer exists. We crash as we believe this require human intervention, but should some projects have less strict durability requirements we may make this an option.

  • RocksDB: len(table) now only counts databases for active partitions (Issue #270).

  • Agent: Fixes crash when worker assigned no partitions and having the isolated_partitions flag enabled (Issue #181).

  • Table: Fixes KeyError crash for already removed key.

  • Table: WindowRange is no longer a NamedTuple.

    This will make it easier to avoid hashing mistakes such that window ranges are never represented as both normal tuple and named tuple variants in the table.

  • Transports: Adds experimental confluent:// transport.

    This transport uses the confluent-kafka client.

    It is not feature complete, and notably is missing sticky partition assignment so you should not use this transport for tables.


    The confluent:// transport is not recommended for production use at this time as it has several limitations.

  • Stream: Fixed deadlock when using Stream.take to buffer events (Issue #262).

    Contributed by Nimi Wariboko Jr (@nemosupremo).

  • Web: Views can now define options method to implement a handler for the HTTP OPTIONS method. (Issue #304)

    Contributed by Perk Lim (@perklun).

  • Stream: Fixed acking behavior of Stream.take (Issue #266).

    When take is buffering the events should be acked after processing the buffer is complete, instead it was acking when adding into the buffer.

    Fix contributed by Amit Ripshtos (@amitripshtos).

  • Transport: Aiokafka was not limiting how many messages to read in

    a fetch request (Issue #292).

    Fix contributed by Miha Troha (@mihatroha).

  • Typing: Added type stubs for faust.web.Request.

  • Typing: Fixed type stubs for @app.agent decorator.

  • Web: Added support for Cross-Resource Origin Sharing headers (CORS).

    See new web_cors_options setting.

  • Debugging: Added OpenTracing hooks to streams/tasks/timers/Crontabs

    and rebalancing process.

    To enable you have to define a custom Tracer class that will record and publish the traces to systems such as Jeager or Zipkin.

    This class needs to have a .trace(name, **extra_context) context manager:

    from typing import Any, Dict,
    import opentracing
    from opentracing.ext.tags import SAMPLING_PRIORITY
    class FaustTracer:
        _tracers: Dict[str, opentracing.Tracer]
        _default_tracer: opentracing.Tracer = None
        def __init__(self) -> None:
            self._tracers = {}
        def default_tracer(self) -> opentracing.Tracer:
            if self._default_tracer is None:
                self._default_tracer = self.get_tracer('APP_NAME')
        def trace(self, name: str,
                  sample_rate: float = None,
                  **extra_context: Any) -> opentracing.Span:
                span = self.default_tracer.start_span(
            if sample_rate is not None:
                priority = 1 if random.uniform(0, 1) < sample_rate else 0
                span.set_tag(SAMPLING_PRIORITY, priority)
            return span
        def get_tracer(self, service_name: str) -> opentracing.Tracer:
            tracer = self._tracers.get(service_name)
            if tracer is None:
                tracer = self._tracers[service_name] = CREATE_TRACER(service_name)
            return tracer._tracer

    After implementing the interface you need to set the app.tracer attribute:

    app = faust.App(...)
    app.tracer = FaustTracer()

    That’s it! Now traces will go through your custom tracing implementation.

  • CLI: Commands --help output now always show the default for every parameter.

  • Channels: Fixed bug in channel.send that caused a memory leak.

    This bug was not present when using app.topic().

  • Documentation: Improvements by:

  • Testing:

    • 99% total unit test coverage

    • New script to verify documentation defaults are up to date are run for every git commit.