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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 acknowledge that a message read from a consumer group has been successfully processed? How do you limit the size of a Redis Stream to prevent it from growing indefinitely and consuming unlimited memory? How would you design a robust event sourcing system using Redis Streams as the primary event log?
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How would you handle a message that was delivered to a consumer but never acknowledged, perhaps because that consumer crashed while processing it?

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Answer

You use the XPENDING command to identify messages that have been delivered but not yet acknowledged, and XCLAIM to transfer ownership of a specific pending message to a different, healthy consumer after enough time has passed, ensuring that a crashed worker's unfinished work is not permanently lost and can be picked up by another available consumer.
XPENDING orders processing_group
XCLAIM orders processing_group worker2 60000 1725800000-0
Real-world example A monitoring process periodically checks for messages that have been pending for too long using XPENDING, automatically reclaiming them with XCLAIM and assigning them to a healthy worker whenever the original consumer appears to have crashed.

Common follow-ups: How long should you typically wait before claiming a pending message from another consumer?;What happens if the original consumer eventually does complete and try to acknowledge a message that has already been claimed?

Redis as a Message Queue;Distributed Locks with Redis
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