key-space-notifications and pub/sub are 2 different concepts.

PUB/SUB: It's usual method publishing data to a channel and other clients can subscribe to the same channel by it's name. published messages are characterised into channels, without knowledge of what (if any) subscribers there may be.

This is enabled by default. messages are not persisted here, and once delivered/lost, messages cannot be retrieved.

key-space-notifications: This also a way of subscribing to Pub/Sub channels in order to receive events by the clients.

This we need to enable manually as this consumes little more CPU. Use below code to enable this

redisClient.configSet("notify-keyspace-events", "Ex");

we can subscribe to 2 different channels

1. Key-space channel: receives the name of the event as message.
2. Key-event channel: receives the name of the key as message.

example: To subscribe to key expired events, use the below code

export const subscribeForExpiry = () => {
  //.: Subscribe to the "notify-keyspace-events" 
  // channel used for expired type events
  client.configSet("notify-keyspace-events", "Ex");
  const sub = client.duplicate();
  sub.connect();

  sub.subscribe(
    `__keyevent@${process.env.REDIS_DATABASE_INDEX}__:expired`,
    (key) => {
      console.log("key=> ", key);
      // do something with key, can't retrieve value here
    }
  );
};