Effect of duplicate Redis subscription to same channel name

Question

To subscribe to an instance of StackExchange.Redis.ISubscriber one needs to call the following API:

void Subscribe(RedisChannel channel, Action<RedisChannel, RedisValue> handler, CommandFlags flags = CommandFlags.None);

Question is, what happens if one calls this same line of code with the same channel name as a simple string, say "TestChannel"?

Does ISubscriber check for string equality or it just does not care and therefore we will have two subscriptions?

Answer 1

I am making an assumption that your question is targeted at the Redis API itself. Please let me know if it isn't.

The answer is also based on the assumption that you are using a single redis client connection.

The pubsub map is a hashtable.

To answer your question: If you subscribe multiple times with the same string, you will continue to have only one subscription(you can see that the subscribe happens based on the hashtable here: https://github.com/antirez/redis/blob/3.2.6/src/pubsub.c#L64.

Conversely, calling a single unsubscribe will unsubscribe your other subscriptions for that channel/pattern as well.

If it helps, here is a simple example in Go (I have used the go-redis library) that illustrates the unsubscribe and hashtable storage parts of the answer.

package main

import (
    "fmt"
    "log"
    "time"

    "github.com/go-redis/redis"
)

func main() {
    cl := redis.NewClient((&redis.Options{
        Addr:     "127.0.0.1:6379",
        PoolSize: 1,
    }))

    ps := cl.Subscribe()

    err := ps.Subscribe("testchannel")
    if err != nil {
        log.Fatal(err)
    }

    err = ps.Subscribe("testchannel")
    if err != nil {
        log.Fatal(err)
    }

    err = ps.Unsubscribe("testchannel")
    if err != nil {
        log.Fatal(err)
    }

    go func() {
        msg, err := ps.ReceiveMessage()
        if err != nil {
            log.Fatal(err)
        }
        fmt.Println(msg.Payload)
    }()

    err = cl.Publish("testchannel", "some value").Err()
    if err != nil {
        log.Fatal(err)
    }

    time.Sleep(10 * time.Second)
}