Kafka Glossary

Canonical terms for owning a Kafka log: how it's partitioned, and what ordering and parallelism guarantee follows from that.

Terms

ACL (Access Control List):
A grant of a specific operation (Read, Write, Create, Describe, Alter, and others) on a specific resource (Topic, Group, or Cluster) to a specific principal. Once an authorizer is enabled, an operation with no matching ACL is denied, not silently allowed.
Avoid: permission (vague; "ACL" is the specific, checkable grant this workspace means)

Controller quorum:
The set of KRaft nodes (process.roles=controller) that replicate the cluster's metadata log via Raft; only quorum members are eligible to become the active controller, and only the active controller processes metadata writes.
Avoid: ZooKeeper ensemble (a different, removed external system; "controller quorum" is KRaft's own internal quorum)

In-sync replica set (ISR):
The subset of a partition's replicas, leader included, caught up closely enough (per replica.lag.time.max.ms) to be eligible for leader election without losing data. acks=all waits for the current ISR, not the topic's full configured replication factor.
Avoid: healthy replicas (vague; "ISR" is the specific, checkable set this workspace means)

KRaft:
Kafka's own Raft-based metadata consensus, replacing ZooKeeper. A controller quorum replicates a metadata log that every controller continuously replays, so failover promotes an already-caught-up standby instead of re-fetching a full state from an external store.
Avoid: ZooKeeper (the external system KRaft replaced; current Kafka no longer supports it)

Log compaction:
A cleanup policy retaining only the latest value per key, via a periodic log cleaner, rather than deleting by age. A key is fully removed only after a tombstone (a null-valued write for that key) has itself aged past delete.retention.ms.
Avoid: garbage collection (a different mechanism in other systems; say "log compaction" for this specific Kafka policy)

Partition:
One independent, append-only, ordered log that a topic is split into. A message lands in exactly one partition, chosen by its key's hash or, absent a key, spread across partitions with no ordering relationship.
Avoid: shard (use only when quoting a source that uses it)

Principal:
The identity a client establishes by authenticating (via mutual TLS or a SASL mechanism), and the subject an ACL actually grants an operation to. Authentication establishes a principal; authorization decides what that principal may do.
Avoid: user (this workspace's ACLs and quotas are keyed on the principal, which can also be a service identity, not only a human user)

Quota:
A per-client or per-principal limit on produce/consume byte rate or on request-handling percentage, enforced independently of ACLs. A fully authorized client can still be throttled by a quota; the two mechanisms protect against different failure modes.
Avoid: rate limit (use only when quoting a source that uses it; "quota" is this workspace's term)

Topic:
A named collection of one or more partitions. Ordering is guaranteed only within a partition, never across a topic's partitions as a whole.
Avoid: queue, channel

Under-replicated partition:
A partition whose ISR has shrunk below its configured replication factor, exposed per-partition as UnderReplicated and, cluster-wide, as the most watched single Kafka operational metric. Distinct from and earlier than UnderMinIsr, which marks the ISR falling to or below min.insync.replicas, the point produce requests actually start being rejected.
Avoid: replication lag (vague; "under-replicated" is the specific, checkable gauge this workspace means)