In this post I'll present a short code snippet demonstrating how to use Redis streams to implement a multi-process task queue with Python. Task queues are commonly-used in web-based applications, as they allow decoupling time-consuming computation from the request/response cycle. For example when someone submits the "contact me" form, the webapp puts a message onto a task queue, so that the relatively time-consuming process of checking for spam and sending an email occurs outside the web request in a separate worker process.
# Calculate 100,000th fibonacci number in worker process.
# Block until the result becomes ready, then display last 6 digits.
When using Redis as a message broker, I've always favored using LPUSH/BRPOP (left-push, blocking right-pop) to enqueue and dequeue a message. Pushing items onto a list ensures that messages will not be lost if the queue is growing faster than it can be processed – messages just get added until the consumer(s) catch up. Blocking right-pop is an atomic operation, so Redis also guarantees that no matter how many consumers you've got listening for messages, each message is delivered to only one consumer.
There are some downsides to using lists, primarily the fact that blocking right-pop is a destructive read. Once a message is read, the application can no longer tell whether the message was processed successfully or has failed and needs to be retried. Similarly, there is no visibility into which consumer processed a given message.
Redis 5.0 includes a new streams data-type for modelling append-only, persistent message logging. Streams are identified by a key, like other data-types, and support append, read and delete operations. Streams provide a number of benefits over other data-types typically used for building distributed task queues using Redis, particularly when used with consumer groups.
Streams support fan-out message delivery to all interested readers (kinda like pub/sub), or you can use consumer groups to ensure that messages are distributed evenly among a pool of consumers (like lpush/brpop).
Messages are persistent and history is kept around, even after a message has been read by a consumer.
Message delivery information is tracked by Redis, making it easy to identify which tasks were completed successfully, and which failed and need to be retried (at the cost of an explicit ACK).
Messages are structured as any number of arbitrary key/value pairs, providing a bit more internal structure than an opaque blob stored in a list.
Consumer groups provide us with a unified interface for managing message delivery and querying the status of the task queue. These features make Redis a nice option if you need a message broker.
Redis 5.0 contains, among lots of fixes and improvements, a new data-type and set of commands for working with persistent, append-only streams.
Redis streams are a complex topic, so I won't be covering all aspects of the APIs, but hopefully after reading this post you'll have a feel for how they work and whether they might be useful in your own projects.
Once a message is read it's effectively "gone forever". If the worker crashes there's no way to tell if the message was processed or needs to be rescheduled. This pushes the responsibility of retrying failed operations onto the consumer.
Only one client can read a given message. There's no "fan-out".
No visibility into message state after the message is read.
Similarly, Redis pub/sub can be used to publish a stream of messages to any number of interested consumers. Pub/sub is limited by the fact that it is "fire and forget". There is no history, nor is there any indication that a message has been read.
streams allow messages to be fanned-out to multiple consumers or you can use stateful consumers ("consumer groups") to coordinate message processing among multiple workers.
message history is preserved and visible to other clients.
consumer groups support message acknowledgements, claiming stale unacknowledged messages, and introspecting pending messages, ensuring that messages are not lost in the event of an application crash.
streams support blocking read operations.
The rest of the post will show some examples of working with streams using the walrus Redis library. If you prefer to just read the code, this post is also available as an ipython notebook.
A couple weekends ago I got it into my head that I would build a thin Python wrapper for working with Redis. Andy McCurdy's redis-py is a fantastic low-level client library with built-in support for connection-pooling and pipelining, but it does little more than provide an interface to Redis' built-in commands (and rightly so). I decided to build a project on top of redis-py that exposed pythonic containers for the Redis data-types. I went on to add a few extras, including a cache and a declarative model layer. The result is walrus.