.. _broker-redis: ============= Using Redis ============= .. _broker-redis-installation: Installation ============ For the Redis support you have to install additional dependencies. You can install both Celery and these dependencies in one go using the ``celery[redis]`` :ref:`bundle `: .. code-block:: console $ pip install -U "celery[redis]" .. _broker-redis-configuration: Configuration ============= Configuration is easy, just configure the location of your Redis database: .. code-block:: python app.conf.broker_url = 'redis://localhost:6379/0' Where the URL is in the format of: .. code-block:: text redis://:password@hostname:port/db_number all fields after the scheme are optional, and will default to ``localhost`` on port 6379, using database 0. If a Unix socket connection should be used, the URL needs to be in the format: .. code-block:: text redis+socket:///path/to/redis.sock Specifying a different database number when using a Unix socket is possible by adding the ``virtual_host`` parameter to the URL: .. code-block:: text redis+socket:///path/to/redis.sock?virtual_host=db_number It is also easy to connect directly to a list of Redis Sentinel: .. code-block:: python app.conf.broker_url = 'sentinel://localhost:26379;sentinel://localhost:26380;sentinel://localhost:26381' app.conf.broker_transport_options = { 'master_name': "cluster1" } Additional options can be passed to the Sentinel client using ``sentinel_kwargs``: .. code-block:: python app.conf.broker_transport_options = { 'sentinel_kwargs': { 'password': "password" } } .. _redis-visibility_timeout: Visibility Timeout ------------------ The visibility timeout defines the number of seconds to wait for the worker to acknowledge the task before the message is redelivered to another worker. Be sure to see :ref:`redis-caveats` below. This option is set via the :setting:`broker_transport_options` setting: .. code-block:: python app.conf.broker_transport_options = {'visibility_timeout': 3600} # 1 hour. The default visibility timeout for Redis is 1 hour. .. _redis-results-configuration: Results ------- If you also want to store the state and return values of tasks in Redis, you should configure these settings:: app.conf.result_backend = 'redis://localhost:6379/0' For a complete list of options supported by the Redis result backend, see :ref:`conf-redis-result-backend`. If you are using Sentinel, you should specify the master_name using the :setting:`result_backend_transport_options` setting: .. code-block:: python app.conf.result_backend_transport_options = {'master_name': "mymaster"} .. _redis-result-backend-global-keyprefix: Global keyprefix ^^^^^^^^^^^^^^^^ The global key prefix will be prepended to all keys used for the result backend, which can be useful when a redis database is shared by different users. By default, no prefix is prepended. To configure the global keyprefix for the Redis result backend, use the ``global_keyprefix`` key under :setting:`result_backend_transport_options`: .. code-block:: python app.conf.result_backend_transport_options = { 'global_keyprefix': 'my_prefix_' } .. _redis-result-backend-timeout: Connection timeouts ^^^^^^^^^^^^^^^^^^^ To configure the connection timeouts for the Redis result backend, use the ``retry_policy`` key under :setting:`result_backend_transport_options`: .. code-block:: python app.conf.result_backend_transport_options = { 'retry_policy': { 'timeout': 5.0 } } See :func:`~kombu.utils.functional.retry_over_time` for the possible retry policy options. .. _redis-serverless: Serverless ========== Celery supports utilizing a remote serverless Redis, which can significantly reduce the operational overhead and cost, making it a favorable choice in microservice architectures or environments where minimizing operational expenses is crucial. Serverless Redis provides the necessary functionalities without the need for manual setup, configuration, and management, thus aligning well with the principles of automation and scalability that Celery promotes. Upstash ------- `Upstash `_ offers a serverless Redis database service, providing a seamless solution for Celery users looking to leverage serverless architectures. Upstash's serverless Redis service is designed with an eventual consistency model and durable storage, facilitated through a multi-tier storage architecture. Integration with Celery is straightforward as demonstrated in an `example provided by Upstash `_. .. _redis-caveats: Caveats ======= Visibility timeout ------------------ If a task isn't acknowledged within the :ref:`redis-visibility_timeout` the task will be redelivered to another worker and executed. This causes problems with ETA/countdown/retry tasks where the time to execute exceeds the visibility timeout; in fact if that happens it will be executed again, and again in a loop. To remediate that, you can increase the visibility timeout to match the time of the longest ETA you're planning to use. However, this is not recommended as it may have negative impact on the reliability. Celery will redeliver messages at worker shutdown, so having a long visibility timeout will only delay the redelivery of 'lost' tasks in the event of a power failure or forcefully terminated workers. Broker is not a database, so if you are in need of scheduling tasks for a more distant future, database-backed periodic task might be a better choice. Periodic tasks won't be affected by the visibility timeout, as this is a concept separate from ETA/countdown. You can increase this timeout by configuring several options with the same name: .. code-block:: python app.conf.broker_transport_options = {'visibility_timeout': 43200} app.conf.result_backend_transport_options = {'visibility_timeout': 43200} app.conf.visibility_timeout = 43200 The value must be an int describing the number of seconds. Key eviction ------------ Redis may evict keys from the database in some situations If you experience an error like: .. code-block:: text InconsistencyError: Probably the key ('_kombu.binding.celery') has been removed from the Redis database. then you may want to configure the :command:`redis-server` to not evict keys by setting in the redis configuration file: - the ``maxmemory`` option - the ``maxmemory-policy`` option to ``noeviction`` or ``allkeys-lru`` See Redis server documentation about Eviction Policies for details: https://redis.io/topics/lru-cache .. _redis-group-result-ordering: Group result ordering --------------------- Versions of Celery up to and including 4.4.6 used an unsorted list to store result objects for groups in the Redis backend. This can cause those results to be be returned in a different order to their associated tasks in the original group instantiation. Celery 4.4.7 introduced an opt-in behaviour which fixes this issue and ensures that group results are returned in the same order the tasks were defined, matching the behaviour of other backends. In Celery 5.0 this behaviour was changed to be opt-out. The behaviour is controlled by the `result_chord_ordered` configuration option which may be set like so: .. code-block:: python # Specifying this for workers running Celery 4.4.6 or earlier has no effect app.conf.result_backend_transport_options = { 'result_chord_ordered': True # or False } This is an incompatible change in the runtime behaviour of workers sharing the same Redis backend for result storage, so all workers must follow either the new or old behaviour to avoid breakage. For clusters with some workers running Celery 4.4.6 or earlier, this means that workers running 4.4.7 need no special configuration and workers running 5.0 or later must have `result_chord_ordered` set to `False`. For clusters with no workers running 4.4.6 or earlier but some workers running 4.4.7, it is recommended that `result_chord_ordered` be set to `True` for all workers to ease future migration. Migration between behaviours will disrupt results currently held in the Redis backend and cause breakage if downstream tasks are run by migrated workers - plan accordingly.