v1.3 opens the replication module in the Kafka Simulator — the machinery behind every durability guarantee Kafka makes. It is the largest pack so far at 12 new scenarios, bringing the curriculum to 45 of 125.

What’s in v1.3

Replication is where Kafka’s guarantees become concrete, and where they can be lost:

  • ISR mechanics — how the in-sync replica set shrinks when a follower lags and expands when it catches up, and why membership of that set is the whole game.
  • The high watermark — why a consumer can only read up to the HW, and why the HW sometimes stops advancing even though the leader keeps accepting writes.
  • acks=all and min.insync.replicas — the exact relationship between them: why a producer can still write after one broker dies but starts receiving NotEnoughReplicas after the second.
  • Leader election — which replica takes over after a failure, and why it has to come from the ISR.
  • Unclean leader election — what you actually lose when you allow an out-of-sync replica to become leader, watched offset by offset.

This is the module the whole simulator was built to make visible. The interesting behavior lives entirely in the transitions — before and after a broker dies, before and after the ISR changes — and those transitions are exactly what a whiteboard cannot show.

New in free play: the active/passive topology

v1.3 also opens the first multi-datacenter shape in the free-play sandbox: active/passive — two regions, a one-way mirror, and a failover you can trigger on demand. It is the simplest way to see replication stop being a within-cluster concern.

The DR curriculum — MirrorMaker 2, offset translation, failback — comes later, in v1.8. This is the sandbox, unlocked early on purpose: the shape is worth playing with long before the scenarios that explain it arrive. Each of the next three packs adds one more topology.

Why replication is the anchor

Producers and consumers sit on top of these guarantees; storage, transactions and multi-DC all extend them. Once you can see the ISR move and the high watermark hold, the rest of the curriculum is a series of variations on a system you already understand.

Open the simulator, set RF=3 and min.insync.replicas=2, and start killing brokers.