In-sync replica
An in-sync replica is a Kafka partition replica sufficiently caught up with its leader to remain eligible for failover.
Apache Kafka in Action
An in-sync replica is a Kafka partition replica whose data is up to date with the leader.
- How it is used
- Identifies current partition copies for broker-failure diagnosis and producer acknowledgement decisions.
- How deep it goes
- Explains follower fetching, lag and heartbeat removal, configurable timing, and the effect of ISR size on acknowledgements.
- Examples
- Uses topic-description output and a two-broker shutdown to show assigned replicas remaining listed while ISR shrinks and acks=all fails.
- What you can do after
- Diagnose/debug ISR shrinkage with kafka-topics.sh --describe and configure min.insync.replicas with acks=all to require replicated acknowledgements.
Kafka for Architects
An in-sync replica is a Kafka partition replica that has caught up with its leader and is not substantially lagging.
- How it is used
- Connects replica health to producer acknowledgments, write admission, failover safety, and durability–availability decisions.
- How deep it goes
- Defines ISR membership and explains configuration consequences, but defers deeper client-application behavior and replication internals.
- Examples
- Uses a replication factor of 3 with min.insync.replicas set to 2, plus durability- and availability-oriented Kafka settings.
- What you can do after
- Operate/configure Kafka replication by selecting ISR thresholds and acknowledgment behavior for resilience or uptime.
Kafka Streams in Action, Second Edition
An in-sync replica set is the set of Kafka partition replicas that are sufficiently caught up with the partition leader to remain eligible for failover.
- How it is used
- Explains Kafka replication, failover eligibility, and consumer-safe record visibility.
- How deep it goes
- Defines follower catch-up, lag-based removal, leader election, and high-water-mark behavior without implementing broker internals.
- Examples
- Uses leader-follower log copying, temporary post-producer lag, and configured follower delays.
- What you can do after
- Evaluate which replicas can replace a failed Kafka partition leader and why consumers avoid unreplicated records.
Read how Kafka Streams in Action, Second Edition explains in-sync replica →
Related concepts
- acknowledgment strategyused by · An acknowledgment strategy is a producer-side policy determining when a published message is acknowledged and what delivery guarantee applies.
- replicakind of · A replica is a copy of data or state, or an equivalent running instance of a component, maintained alongside others in a replicated system.
- controllerused by · A Kafka controller is a server role that manages cluster metadata and coordinates partition, replica, and leader state.
- all acknowledgmentused by · All acknowledgment is a Kafka producer setting requiring confirmation from all in-sync replicas before a record write is considered successful.
- min.insync.replicasconfigured by · min.insync.replicas is a Kafka setting specifying the minimum number of in-sync replicas that must acknowledge a write for it to succeed.
- primary nodeused by · A primary node is a designated authoritative or coordinating member of a distributed system, typically responsible for accepting operations or coordinating replicated state.
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