In-sync Replica

An ISR is a Broker who is not a Partition Leader put contains a replicate of a given Topic.

The number of ISR is dictated by the Topic Replication Factor being 2+.

References

A Broker who is not a Partition Leader but contains a replicate of a given Topics::: In-sync Replica

Partial ISR::: ISR Count < Topic Replication Factor

In-sync Replica is involved in leader election by::: being in the candidate pool for partition leader election