All 7 Pub/Sub questions
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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 would you design a real time notification system using Pub/Sub while working around its lack of guaranteed message delivery?
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How does Redis Pub/Sub behave differently in a Redis Cluster environment compared to a single instance setup?

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Answer

In a standard Redis Cluster, published messages are automatically forwarded to all nodes in the cluster so that a subscriber connected to any node receives messages published from any other node, though this cluster wide message propagation adds some additional overhead, and Redis also offers sharded Pub/Sub as an alternative that restricts message propagation to reduce this overhead for cluster deployments with heavy Pub/Sub usage.
-- Sharded Pub/Sub commands, introduced for cluster efficiency
SSUBSCRIBE mychannel
SPUBLISH mychannel 'message'
Real-world example A large scale application running Redis Cluster switches from regular Pub/Sub to sharded Pub/Sub commands, reducing the cluster wide message propagation overhead that had been affecting performance under their heavy notification traffic.

Common follow-ups: What is the specific difference between regular and sharded Pub/Sub commands?;When does the overhead of cluster wide Pub/Sub propagation actually become a meaningful problem?

Clustering;Redis Performance Tuning & Benchmarking
All 7 Pub/Sub questions