Database / ValKey Interview questions

How does Valkey implement publish/subscribe messaging?

SUBSCRIBE and PSUBSCRIBE register a client's interest in specific channels or glob patterns, and PUBLISH broadcasts a message to every client currently subscribed at that moment.

Classic pub/sub is fire-and-forget: messages are never stored, so a client that wasn't subscribed when a message was published simply never sees it, which distinguishes it from the durable, replayable log semantics of Streams.

In Cluster mode, SSUBSCRIBE/SPUBLISH scope pub/sub traffic to a single shard instead of fanning it out across the entire cluster bus, which reduces unnecessary cross-node message traffic.

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A key characteristic of classic Valkey pub/sub is that messages... are always durably stored for later replay

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are not persisted and are lost if no client is subscribed at publish time

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are guaranteed exactly-once delivery

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are stored in an AOF-only channel

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SSUBSCRIBE/SPUBLISH were introduced mainly to... replace Streams entirely

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scope pub/sub traffic to a cluster shard instead of broadcasting cluster-wide

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remove the need for channels

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add persistence to pub/sub

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Database / ValKey Interview questions

How does Valkey implement publish/subscribe messaging?

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