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Under-minimum-isr partition count metric

As explained in Apache Kafka in Action

An under-minimum ISR is a metric indicating that a partition's in-sync replica set is below its configured minimum size.

The under-minimum ISR partition count metric identifies partitions whose in-sync replica set has fallen below the configured minimum. A nonzero value signals degraded replication and requires prompt investigation. section 15.2.1 section 15.6

Interpret the metric alongside replica and producer behavior:

  • When the minimum ISR is merely met, the partition is already at a critical reliability boundary. section 15.2.1
  • When the ISR falls below the minimum, producers using acks=all cannot produce to the affected partitions. section 15.2.1
  • If the partition leader is offline, reading and writing are impossible until a new leader is appointed. section 15.2.1
  • Treat under-minimum ISRs as a broker-health alert; the desired value is zero. section 15.6

Reading the signal

A monitoring dashboard reports a nonzero under-minimum ISR count. Check affected partitions and producer behavior first: acks=all producers may be unable to publish, while an offline leader can block both reads and writes. Replication-factor-one topics are an expected special case because every partition necessarily has the minimum ISR count. section 15.2.1

How Apache Kafka in Action covers it

How deep it goes
Defines the threshold and consequences for acks=all production, but does not specify alert thresholds or remediation commands.
Examples
Uses replication-factor-one behavior and nonzero under-minimum ISRs as concrete monitoring cases.
What it assumes
Assumes Kafka partitions and replicas, while clarifying in-sync replicas and minimum ISR behavior.
What you can do after
Diagnose partition reliability problems by correlating under-minimum ISRs with producer failures and offline replicas.
How it connects
Connects minimum ISR to in-sync replicas, acks=all, offline replicas, leader election, and under-replicated partitions.
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