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What is a Redis Stream, and how does it differ from a regular Redis list when used as a messaging system? How do you add a new entry to a Redis Stream, and how do you read entries back from it? What is a consumer group in Redis Streams, and how does it enable multiple workers to process the same stream cooperatively? How do you limit the size of a Redis Stream to prevent it from growing indefinitely and consuming unlimited memory? How would you handle a message that was delivered to a consumer but never acknowledged, perhaps because that consumer crashed while processing it? How would you design a robust event sourcing system using Redis Streams as the primary event log?
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Redis › Streams

How do you acknowledge that a message read from a consumer group has been successfully processed?

Intermediate

Answer

You use the XACK command, specifying the stream, the consumer group, and the specific message id, which marks that message as successfully processed for that group, removing it from the group's pending entries list, letting Redis know it does not need to be redelivered to another consumer if something goes wrong.
XACK orders processing_group 1725800000-0
Real-world example A worker processing an order event calls XACK immediately after successfully completing its work, confirming to Redis that this specific message does not need to be redelivered to another worker in the group.

Common follow-ups: What happens to a message that is never acknowledged?;How do you view the list of currently pending, unacknowledged messages for a consumer group?

Redis as a Message Queue;Distributed Locks with Redis
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