---
title: "Kafka Simulator v1.3 — durability and replication"
date: 2026-08-04T00:00:00.000Z
author: "michal"
excerpt: "The replication module arrives: ISR mechanics, the high watermark, acks=all and min.insync.replicas, leader election and what an unclean election actually loses. Twelve scenarios on the guarantees Kafka is famous for."
---
v1.3 opens the **replication** module in the [Kafka Simulator](/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](/kafka-simulator/), set `RF=3` and `min.insync.replicas=2`, and start killing brokers.