All 7 Pub/Sub questions
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How do you subscribe to and publish messages on a specific Redis channel? What is pattern based subscription using PSUBSCRIBE, and how does it differ from subscribing to specific individual channels? What happens to a message published on a channel if there are currently no subscribers listening to that channel? How do you check how many subscribers are currently listening to a specific channel in Redis? How does Redis Pub/Sub behave differently in a Redis Cluster environment compared to a single instance setup?
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How would you design a real time notification system using Pub/Sub while working around its lack of guaranteed message delivery?

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Answer

You would use Pub/Sub for the fast, real time delivery portion of the system, but also persist important notifications to a database or a Redis Stream as the source of truth, so if a client happens to be disconnected when a Pub/Sub message is sent, it can still retrieve any missed notifications by querying the persistent storage once it reconnects.
-- Publish for real time delivery
PUBLISH notifications:user123 'New message'
-- Also persist for reliability
XADD notifications_log '*' user 'user123' message 'New message'
Real-world example A messaging application delivers new message notifications instantly through Pub/Sub for currently connected users, while also logging every notification to a Redis Stream so a user who was briefly offline can catch up on anything they missed upon reconnecting.

Common follow-ups: How do you efficiently let a reconnecting client catch up on missed messages?;What is the tradeoff of maintaining both a Pub/Sub channel and a persistent log for the same events?

Streams;Redis as a Message Queue
All 7 Pub/Sub questions