What are the pros and cons of each? Please advice when to use one and not the other.
Pub/Sub is a Publisher/Subscriber platform, it's not data storage. Published messages evaporate, regardless if there was any subscriber.
In Redis/Valkey Streams, stream is a data type, a data structure on its own right. Messages or entries are stored in memory and stay there until commanded to be deleted.
Pub/Sub is synchronous communication (push protocol). All parties need to be active at the same time to be able to communicate. Here Redis/Valkey is a pure synchronous messaging broker.
Streams allow for both synchronous (XREAD with BLOCK and the special $ ID is a push protocol) and asynchronous communication (regular XREAD is a pull protocol). XREAD with BLOCK is like Pub/Sub, but with the ability to resume on disconnection without losing messages.
Pub/Sub is At-most-once, i.e. "fire and forget".
Streams allows for both At-most-once or At-least-once (explicit acknowledgement sent by the receiver)
Pub/Sub is blocking-mode only. Once subscribed to a channel, the client is put into subscriber mode and it cannot issue commands (except for [P]SUBSCRIBE, [P]UNSUBSCRIBE, PING and QUIT), it has become read-only.
Streams allows consumers to read messages in blocking mode or not.
Pub/Sub is fan-out only. All active clients get all messages.
Streams allows fan-out (with XREAD), but also to provide a different subset of messages from the same stream to many clients. This allows scaling message processing, by routing different messages to different workers, in a way that it is not possible that the same message is delivered to multiple consumers. This last scenario is achieved with consumer groups.
Streams provide many more features, like time-stamps, field-value pairs, ranges, etc. It doesn't mean you should always go for Streams. If your use-case can be achieved with Pub/Sub, it is better for you to use Pub/Sub then. With Streams, you have to care for memory usage.
https://valkey.io/topics/pubsub/ https://valkey.io/topics/streams-intro/