In Redis Pub/Sub, we create different Redis channels as per the application requirement. Each application using Redis Pub/Sub will use these channels for either publishing purposes or subscribing purposes.
If you are not using Redis in cluster mode or master-slave mode & still getting this error CLUSTERDOWN Hash slot not served then you must try the below solution. That should fix the issue.
I came across this issue when I was checking Redis log file on server. When I looked at the Redis Log File, I wondered after seeing its size. It was about 4GB. When I tried to open that file, I got a prompt saying File is too large to open in notepad.