API support matrix

52 of the 87 api keys this build names, listed in wire-code order. Every row is negotiated per connection — the version column is deliberately absent, because there is no fixed version to report. See Version negotiation.

Reading the table

Routing is the class from crates/kafka-meta/src/routing.rs. Sending to the wrong class does not produce an error — it produces a NOT_CONTROLLER or NOT_COORDINATOR retry loop that presents as a flaky cluster. See Metadata, routing and the pool.

Mutating is ApiKey::is_mutating, which is what a read-only client refuses before opening a socket. Note the entries that look surprising in both directions: OffsetCommit and OffsetDelete are mutating because they write group state, while SaslHandshake, SaslAuthenticate and FindCoordinator are not, for reasons the read-only gate chapter explains.

DeleteRecords, CreateAcls, IncrementalAlterConfigs and the quota and SCRAM alters route to any broker rather than to the controller, which is correct — these are not controller-only APIs even though they mutate.

The other 35

Not sent, and they fall into four groups.

Broker-internal and KRaft APIs, which a client has no business sending: WriteTxnMarkers (27), Vote (52), BeginQuorumEpoch (53), EndQuorumEpoch (54), AlterPartition (56), Envelope (58), FetchSnapshot (59), BrokerRegistration (62), BrokerHeartbeat (63), UnregisterBroker (64), AllocateProducerIds (67), ControllerRegistration (70), AssignReplicasToDirs (73), the Raft voter APIs (80–82), and the share-group state APIs (83–87) that live between broker and coordinator. DescribeQuorum (55) is the one in this group a UI might plausibly want.

Share consumption, which is a scope decision. Share groups are observed, not joined: ShareGroupDescribe (77) is in the table, while ShareGroupHeartbeat (76), ShareFetch (78), ShareAcknowledge (79) and the share-group offset admin APIs (90–92) are not. KIP-932 consumption is a second consumption model rather than a variation on the first, and kafka-consume implements the classic and KIP-848 protocols.

Superseded. AlterConfigs (33) is deliberately absent — IncrementalAlterConfigs (44) replaces it, and using the older API silently resets every config you did not mention. There is no reason to offer it.

Not yet needed. OffsetForLeaderEpoch (23), AlterReplicaLogDirs (34), UpdateFeatures (57), the telemetry APIs (71–72) and ListConfigResources (74). These are ordinary gaps — nothing structural stops them being added.

And two that cannot be named at all. Api keys 88 and 89 are KIP-1071's StreamsGroupHeartbeat and StreamsGroupDescribe, which have no schema in kafka-protocol 0.17. They are not in the 87 this build names, they arrive in a version table as Unknown(88) and Unknown(89), and a Kafka Streams application's group is reported as GroupDescription::Unrecognized rather than failing the call. See The upstream schema gap.

Keeping this page honest

The table is generated from two sources of truth and must be regenerated when either changes:

• crates/kafka-conn/src/api_key.rs — the wire codes and is_mutating
• crates/kafka-meta/src/routing.rs — the routing class

The docs CI job asserts that every crates/… path cited anywhere in this book exists, so a refactor that moves a file fails the build rather than leaving a citation that is confidently wrong. It does not yet verify the rows of this table against the source.

It has now gone stale once, and worth recording how. Every code on this page from 8 upwards was low by exactly four, in the table as well as in the prose — OffsetCommit was listed as 4 when it is 8, delegation tokens as 34–37 when they are 38–41. Four is the number of api keys Kafka 4.0 removed (LeaderAndIsr, StopReplica, UpdateMetadata, ControlledShutdown, codes 4–7), so the page had been built by enumerating what remained and numbering it sequentially rather than by reading ApiKey::code. It looked entirely plausible, which is why it survived: every number was wrong and every number was consistent with its neighbours. That is the argument for generating the rows in xtask and asserting them, rather than for being more careful.