Add a note about multiple Pub/Sub channel listeners

[jamesw6811]

Clarify that multiple subscriptions create multiple listeners.

Description

This change informs the developer that multiple subscriptions will create multiple listeners. This is important because redis-cli SUBSCRIBE commands do not behave this way, so it may be counter-intuitive.
I personally spent some time debugging my code and my misunderstanding of this was the root cause.

Describe your pull request here
This changes the pub/sub docs to clarify that multiple subscriptions create multiple listeners.

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Checklist

• Does npm test pass with this change (including linting)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

[leibale]

@jamesw6811 Thanks for contributing, but... that's not true... when you SUBSCRIBE to the same channel more than once in redis-cli you will get the same message twice over the network, with node-redis you'll get the message once over the network, but both callbacks will be called:

// `SUBSCRIBE a`
await client.subscribe('a', message => {
  console.log(message);
});

// no command will be executed on the server
// but the callback will be added to the callback set
await client.subscribe('a', message => {
  console.log(message);
});

await anotherClient.publish('a', 'b');
// `client` will get the message once over the network, and then call both callbacks with the message.

[jamesw6811]

redis-cli stops accepting new commands after one SUBSCRIBE. I didn't realize this wasn't also the behavior of the api. It might confuse others, so I still think it's a helpful addition.

127.0.0.1:6379> subscribe hello
Reading messages... (press Ctrl-C to quit)
1) "subscribe"
2) "hello"
3) (integer) 1
subscribe hello     
subscribe hello

Redis docs:

Please note that when using redis-cli, in subscribed mode commands such as [UNSUBSCRIBE](https://redis.io/commands/unsubscribe) and [PUNSUBSCRIBE](https://redis.io/commands/punsubscribe) cannot be used because redis-cli will not accept any commands and can only quit the mode with Ctrl-C.

[jamesw6811]

Maybe I'm not understanding the redis source correctly, but my reading is that the subscriptions are based on a dictionary of clients, so if the same client tried to subscribe twice it would not duplicate:
https://github.com/redis/redis/blob/7.0/src/pubsub.c#L228

Am I missing something? I'm newer to Redis so please excuse all the things I'm probably getting wrong...

[leibale]

@jamesw6811 You can use telnet to do whatever you want...
Anyway, you are correct, even if you issue 2 SUBSCRIBE on the same channel you'll get the message once, I guess this made you expect the second listener to override the first one (and not just added on top)?

[leibale]

@guyroyse :)

[jamesw6811]

@jamesw6811 You can use telnet to do whatever you want... Anyway, you are correct, even if you issue 2 SUBSCRIBE on the same channel you'll get the message once, I guess this made you expect the second listener to override the first one (and not just added on top)?

Exactly. Expecting either functionality is reasonable, but that documentation one-liner would have saved me some time. Thought it might save some others' time too.

Thanks for the telnet tip!