title: Using a load balancer to dispatch messages from Redis pub sub
body: <p>I have several Python application that all connect to a Redis server and consume messages using the pubsub mechanism. I have containerized the applications with Docker and I would like to scale each application by replicating the number of container instances. The challenge is that I don’t want each container to act as an independent subscriber to Redis, meaning I would essentially like to load balance the network traffic so that, when a message is published, only one container receives it <strong>per service</strong>.</p>
  <p>Let’s take the simple example of two services, Service A and Service B. Both services need to be subscribed to the same topic so that each is notified upon a message published to that topic. Each service will process the message differently; in other words the same message will trigger two different outcomes, one executed by Service A and one by Service B. Now, I am trying to imagine an architecture in which these services consist of replicated containers, let’s call them workers. Say Service A consists of two workers A1 and A2, and Service B consists of three workers B1, B2, and B3 (maybe it requires more processing power per message than Service A, so it requires more workers for the same message load). So my use case requires that both Service A and Service B  need to subscribe to the same topic so that they both receive updates as they come in, but I only want one worker to handle the message per service. Imagine that a message comes in and worker A1 handles it for Service A while B3 handles it for Service B.</p>
  <p>Overall this feels like it should be pretty straightforward, I essentially have multiple applications, each of which needs to scale horizontally and should handle network traffic as if they were sitting behind a load balancer.</p>
  <p>I am intending to deploy these applications with something like Amazon ECS, where each application is essentially a service with task replication and all services connect to a centralized Redis cache acting as a message broker. In a situation like this, from the limited research I’ve done, it would be nice to just put a network load balancer up in front of each service so that published messages would be directed to what looks like a single subscriber, but behind the scenes is a collection of workers acting like they’re pulling off a task queue.</p>
  <p>I haven’t had much luck finding examples of this kind of architecture, or for that matter any examples of tasks that use something like Redis in the way I’m imagining. This is an architecture I’ve more or less dreamed up, so I could just be thinking about this all wrong, but at the same time it doesn’t seem like a crazy use case to me. I’m looking for any advice about how this could be accomplished and/or if what I’m talking about just sounds insane and there’s a better way.</p>