Redis publish/subscribe: see what channels are currently subscribed to

Question

I am currently interested in seeing what channels are subscribed to in a Redis pub/sub application I have. When a client connects to our server, we register them to a channel that looks like:

user:user_id

The reason for this is I want to be able to see who's "online". I currently blindly fire off messages to a channel without knowing if a client is online since it's not critical that they receive these types of messages.

In an effort to make my application smarter, I'd like to be able to discover if a client is online or not using the pub/sub API, and if they are offline, cache their messages to a separate redis queue which I can push to them when they get back online.

This does not have to be 100% accurate, but the more accurate it is, the better. I'm assuming a generic key does not get created when a channel gets subscribed to, so I cannot do something as trivial as:

redis-cli keys user* to find all online users.

The other strategy I've thought of is just maintaining my own Redis Set whenever a user published or removes themselves from a channel (which the client automatically handles when they hop online and close the app). That would be an additional layer of complexity that I need to manage and I'm hoping there is a more trivial approach with the data that's already available.

Answer 1

As of Redis 2.8 you can do:

PUBSUB CHANNELS [pattern]

The PUBSUB CHANNELS command has O(N) complexity, where N is the number of active channels.

So in your case:

redis-cli PUBSUB CHANNELS user*

would give you want you want.

Answer 2

There is currently no command for showing what channels "exist" by way of being subscribed to, but there is and "approved" issue and a pull request that implements this.

https://github.com/antirez/redis/issues/221
https://github.com/antirez/redis/pull/412

Due to the nature of this call, it is not something that can scale, and is thus a "DEBUG" command.

There are a few other ways to solve your problem, however. If you have reason to believe that a channel may be subscribed to, you can send it a message and look at the result. The result is the number of subscribers that got the message. If you got 0, you know that they're not there.

Assuming that your user_ids are incremental, you might be interested in using SETBIT to set a 1 or 0 to a user's offset bit to track presence. You can then do cool things like the new BITCOUNT to see how many users are online, and GETBIT to determine if a specific user is online.

The way I have solved your problem more specifically in the past is by signaling a subscription manager that I have subscribed to a channel. The manager then "pings" the channel by sending a blank message to confirm that there is a subscriber, and occasionally pings the channel thereafter to determine if the user is still online. Not ideal, but better than using DEBUG CHANNELS in production.