Overview
The replication factor (RF) is how many servers replicate each message: with RF=3 every partition is stored on three brokers. The in-sync replica set (ISR) holds the replicas that are keeping up with the leader; a follower is removed if it has not sent fetch requests, or has not consumed up to the leader's log end offset, within replica.lag.time.max.ms. min.insync.replicas is the minimum number of in-sync replicas, including the leader, required for a write to succeed when the producer uses acks=all.
Analogy
Key configuration
--replication-factor 3 — argument of kafka-topics.sh --create (there is no topic config named replication.factor)
min.insync.replicas=2 — topic or broker level, default 1
default.replication.factor — broker default for automatically created topics, default 1Production tip
Check yourself
With replication factor 3, min.insync.replicas=2 and acks=all, how many broker failures can you survive while still accepting writes?
Sources
- Broker configs (4.2 documentation) (opens in a new tab)
- Topic configs (4.2 documentation) (opens in a new tab)
- Design (4.2 documentation) (opens in a new tab)
- Basic Kafka operations (4.2 documentation) (opens in a new tab)
- Producer configs (4.2 documentation) (opens in a new tab)
Checked against Kafka 4.2 documentation on Oct 2026. Draft: not yet reviewed by a Kafka specialist. Releases move on, so confirm against the current documentation before you act on a number.