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What is the Redis Pub/Sub feature, and how does it let different parts of an application communicate? How do you subscribe to and publish messages on a specific Redis channel? 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 would you design a real time notification system using Pub/Sub while working around its lack of guaranteed message delivery? How does Redis Pub/Sub behave differently in a Redis Cluster environment compared to a single instance setup?
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What is pattern based subscription using PSUBSCRIBE, and how does it differ from subscribing to specific individual channels?

Intermediate

Answer

PSUBSCRIBE lets you subscribe to multiple channels at once that match a specified wildcard pattern, rather than needing to individually subscribe to each specific channel name, which is useful when you want to receive messages from a whole category of related channels without knowing their exact names in advance.
PSUBSCRIBE news.*
-- Receives messages published to news.sports,
-- news.weather, news.technology, and any other matching channel
Real-world example A news aggregation application subscribes to all channels matching the news wildcard pattern, automatically receiving updates from any new category channel that gets created later without needing code changes.

Common follow-ups: What wildcard characters are supported in a pattern subscription?;Does pattern subscription have a performance cost compared to specific channel subscriptions?

Pub/Sub;Keyspace Notifications
All 7 Pub/Sub questions