Scaling with Redis

As you can read before – it's possible to run multiple nodes of Centrifugo server and load balance clients between them. In this chapter I'll show how to do it. We will start 3 Centrifugo nodes and all those nodes will be connected over Redis.

For this purpose we must use Redis engine described in previous chapter.

First, you should have Redis running. As soon as it's running - we can launch 3 Centrifugo instances. Open your terminal and start first one:

centrifugo --config=config.json --port=8000 --engine=redis --redis_host= --redis_port=6379

If your Redis on the same machine and runs on its default port you can omit --redis_host and --redis_port options in command above.

Then open another terminal and launch another Centrifugo instance:

centrifugo --config=config.json --port=8001 --engine=redis --redis_host= --redis_port=6379

Note that we use another port number (8001) as port 8000 already busy by our first Centrifugo instance. If you are starting Centrifugo instances on different machines then you most probably can use the same port number for all instances.

And let's start third instance:

centrifugo --config=config.json --port=8002 --engine=redis --redis_host= --redis_port=6379

Now you have 3 Centrifugo instances running on ports 8000, 8001, 8002 and clients can connect to any of them. You can also send API request to any of those nodes - as all nodes connected over Redis PUB/SUB message will be delivered to all of them.

To load balance clients between nodes we use Nginx - you can find its configuration here in documentation. Note that it's important to route clients that use SockJS transports (except websocket) to the same node as that node keeps client's session information.

Redis sharding

Starting from Centrifugo v1.6.0 there is a builtin Redis sharding support.

This resolves some fears about Redis being bottleneck on some large Centrifugo setups. Redis is single-threaded server, it's insanely fast but if your Redis approaches 100% CPU usage then this sharding feature is what can help your application to scale.

At moment Centrifugo supports simple comma-based approach to configuring Redis shards. Let's just look on examples.

To start Centrifugo with 2 Redis shards on localhost running on port 6379 and port 6380:

centrifugo --config=config.json --engine=redis --redis_port=6379,6380

To start Centrifugo with Redis instances on different hosts:

centrifugo --config=config.json --engine=redis --redis_host=,

If you also need to customize AUTH password, Redis DB number then you can use --redis_url option.

Note, that due to how Redis PUB/SUB work it's not possible (and it's pretty useless anyway) to run shards in one Redis instances using different Redis DB numbers.

When sharding enabled Centrifugo will spread channels and history/presence keys over configured Redis instances using consistent hashing algorithm. At moment we use Jump consistent hash algorithm (see paper and implementation)

If you have any feedback on sharding feature - let us know.