Redis Pub/Sub
Pub/Sub is a means of collecting data from any number of data publishers and allowing 1 or more subscribers to consume the feed in real time, without having to connect to the publishers directly. This pattern allows systems to scale more easily and allows publishers and subscribers to be added or removed at any time.
Redis allows the creation of Pub/Sub channels to which publishers and subscribers interface. Messages on a channel are sent at most once to each subscriber making them ideal for trigger actions which must not be duplicated, but beware because once delivered, a channel message is not persisted.
Use-cases:
- Real-time message.
- Ephemeral multi-user chat.
- Log streaming.
Subscribing to a channel
A subscriber simply issues a "SUBSCRIBE
> SUBSCRIBE mychannel
If a subscriber needs to subscribe to more than one channel, then this is possible too:
> SUBSCRIBE mychannel1 mychannel2 mychannel3
Subscribing to a channel means that the subscriber will receieve any messages that are published to that channel after they have subscribed. Older messages are not persisted, so the subscriber must be connected and subscribed to get messages as they happen.
If the channel does not exist then the subscriber will see no errors, nor will they see any messages.
Publishing to a channel
A publisher can publish to a channel with the PUBLISH <channel> <message> command:
> PUBLISH mychannel "my first message"
and any subscribers will receive the message as follows:
1) "message"
2) "mychannel"
3) "my first message"
The subscription message includes the channel name and the message itself.
A common pattern is to send JSON objects as the message text:
> PUBLISH mychannel '{"id":"1234","name":"db14","msg":"Out of Memory"}'
The return value from PUBLISH is an integer that tells the caller how many subscribers received the message.
Selective subscription
Instead of subscribing to whole channels, it is possible to subscribe to wildcard channels. By prudent naming of channels, this can be helpful to see, for example, all of an application's logs, just its production logs, or just its production database's logs.
> PSUBSCRIBE *
> PSUBSCRIBE prod_*
> PSUBSCRIBE prod_database_*
> PSUBSCRIBE prod_database_db57_activity_*
Unsubscribing
A subscriber can unsubscribe to all of its subscribed channels with:
> UNSUBSCRIBE