All 7 Pub/Sub questions
More in Pub/Sub
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 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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Redis › Pub/Sub

What is the Redis Pub/Sub feature, and how does it let different parts of an application communicate?

Beginner

Answer

Pub/Sub lets one client publish a message to a named channel, and any other clients currently subscribed to that same channel instantly receive the message, providing a simple way for different parts of a distributed application to communicate in real time without needing to know about each other directly.
-- Subscriber
SUBSCRIBE notifications

-- Publisher
PUBLISH notifications 'New order received'
Real-world example A live chat application uses Pub/Sub to instantly deliver a new message to every connected user in a chat room, with the message being published once and received by every subscribed client immediately.

Common follow-ups: Do messages published to a channel get stored anywhere if no one is currently subscribed?;Can a single client subscribe to multiple channels at once?

Streams;Keyspace Notifications
All 7 Pub/Sub questions