📡 Kafka Playground

Produce messages and watch them hash by key into partitions. Add consumers to a group and watch partitions rebalance. Consume, and watch committed offsets advance and lag shrink. A live mental model of how Kafka actually moves data — all in your browser.

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pending (lag) consumed (committed) key ▪ hashes to a partition → same key, same partition, in order

Try this: produce a few messages with the same key → they all land in one partition, in order (Kafka only guarantees order within a partition). Now produce with no key → they spread round-robin. Add a 3rd consumer to a 3-partition topic → each consumer owns exactly one partition. Add a 4th → it sits idle (you can't have more active consumers than partitions).