Skip to content

CONCEPT Cited by 1 source

In-sync replica set (ISR)

Definition

For each Kafka partition, the in-sync replica set (ISR) is the subset of the partition's replicas that are caught up with the leader's log. Replicas outside the ISR are considered out-of-sync — they do not have the latest data for the partition. The leader is always a member of the ISR (by definition).

Why ISR matters — durability depends on it

The ISR is what producer acks are computed against.

Kozlovski's Kafka-101 framing:

"Every partition has a set of replicas (called the 'replica set'). A replica can be in two states — in-sync or out-of-sync. As the name suggests, out-of-sync replicas are ones that don't have the latest data for the partition." (Source: sources/2024-05-09-highscalability-kafka-101)

For a producer writing with acks=all:

  • The leader waits until all in-sync replicas persist the record before acknowledging.
  • If the ISR shrinks (a follower falls behind), the effective durability guarantee silently weakens — in the degenerate case where only the leader is in-sync, acks=all collapses to acks=1.

The min.insync.replicas configuration prevents this collapse:

"To further control the acks=all property and ensure it doesn't regress to an acks=1 property when there is only one in-sync replica, the min.insync.replicas setting exists to denote the minimum number of in-sync replicas required to acknowledge a write that's configured with acks=all." (Source: sources/2024-05-09-highscalability-kafka-101)

If the ISR drops below min.insync.replicas, the leader rejects acks=all writes rather than silently delivering weaker durability — a fail-closed posture on durability.

ISR membership — state transitions

Replicas transition in and out of the ISR based on replication lag:

  • A follower that falls behind the leader by more than a configured amount (time or offset) is evicted from the ISR (becomes out-of-sync).
  • A follower that catches up rejoins the ISR.
  • Leader election only considers ISR members — an out-of-sync replica cannot become leader (Kafka's default posture; modes for unclean-leader-election exist as an availability-over- durability trade-off).

Seen in

Last updated · 319 distilled / 1,201 read