All 7 Pub/Sub questions
More in Pub/Sub
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 is pattern based subscription using PSUBSCRIBE, and how does it differ from subscribing to specific individual channels? 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 happens to a message published on a channel if there are currently no subscribers listening to that channel?

Intermediate

Answer

If no client is currently subscribed to a channel when a message is published, that message is simply lost forever and never delivered to anyone, since Redis Pub/Sub does not store or queue messages for later delivery, which is an important limitation to understand before relying on Pub/Sub for anything that requires guaranteed delivery.
PUBLISH orders 'New order 456' -- lost if no subscribers are currently listening
Real-world example A team initially used Pub/Sub for order notifications, then discovered orders were being silently missed whenever their notification service briefly restarted, prompting them to switch to Redis Streams for guaranteed delivery instead.

Common follow-ups: What alternative Redis feature provides message persistence and guaranteed delivery?;How do you detect if messages are being missed due to no active subscribers?

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