The regular subscriber API should work fine - there is no assumption on use-cases, and this should work fine.

However, I do kinda agree that this is inbuilt functionality that could perhaps benefit from helper methods on the API, and perhaps a different delegate signature - to encapsulate the syntax of the keyapace notifications so that people don't need to duplicate it. For that: I suggest you log an issue so that it doesn't get forgotten.

Simple sample of how to subscribe to a keyspace event

First of all, it's important to check that Redis keyspace events are enabled. For example, events should be enabled on keys of type Set. This can be done using CONFIG SET command:

CONFIG SET notify-keyspace-events KEs

Once keyspace events are enabled, it's just about subscribing to the pub-sub channel:

using (ConnectionMultiplexer connection = ConnectionMultiplexer.Connect("localhost"))
{
    IDatabase db = connection.GetDatabase();
    ISubscriber subscriber = connection.GetSubscriber();

    subscriber.Subscribe("__keyspace@0__:*", (channel, value) =>
        {
            if ((string)channel == "__keyspace@0__:users" && (string)value == "sadd")
            {
                // Do stuff if some item is added to a hypothethical "users" set in Redis
            }
        }
    );
}

Learn more about keyspace events here.

From the surface, it looks like what you got should work.

Try setting notify-keyspace-events to AKE, the g is redundant, as noted in Notifications Config:

A Alias for g$lshztxe, so that the "AKE" string means all the events.

Try using SubscribeToChannels instead of SubscribeToChannelsMatching. The latter is for pattern subscription.

You can test how many subscribers you have with the PUBSUB NUMSUB __keyevent@0__:del command from redis-cli.

Try testing your events are being triggered with SUBSCRIBE __keyevent@0__:del from redis-cli. This will help you determine if the problem is on redis-server or the app code.

Please update the question with results if you can't get it to work after trying the above.

Just as the other answer mentioned, there's no such notification.

After all, Keyspace Notification is a notification for events on a single key. Each notification is associated with a key. For keyspace event, the key name is part of the channel name. For keyevent event, the key name is the message.

PUBLISH __keyspace@0__:key_name comamnd
PUBLISH __keyevent@0__:command key_name

Each command that sending a notification must have a key as argument. e.g. del key, set key val. However, the flushdb command has no key as argument. The command doesn't affect a single key. Instead, it removes all keys in the database. So there's no such notification for it. Otherwise, what do you expect from the channel? All keys that have been removed? It's not a good idea.

However, you can simulate an event for flushdb

1. set a special key, e.g. flushdb-event: set flushdb-event 0
2. subscribe on the corresponding channel: subscribe __keyspace@0__:flushdb-event
3. set the special key before you call flushdb: set flushdb-event 1

In this way, you can get the simulated flushdb notification.

The correct command that allows me to do the filtering is keyspace.

With the configuration AKE and the following code, I can get the notifications only for the keys my service is interested in:

subscriber.Subscribe("__keyspace@0__:my.key*", (_, key) =>
{
   _logger.LogWarning("Received '{CacheKey}'", key);            
});

From Redis Keyspace Notifications

Keyspace notifications allow clients to subscribe to Pub/Sub channels in order to receive events affecting the Redis data set in some way.

Examples of events that can be received are:

All the commands affecting a given key.
All the keys receiving an LPUSH operation.
All the keys expiring in the database 0.

Events are delivered using the normal Pub/Sub layer of Redis, so clients implementing Pub/Sub are able to use this feature without modifications.

So, if you need just pub/sub, there is no need of extra configuration regarding Keyspace Notifications