Glossary

Terms coined or leaned on along the journey.

Producer side

• Base sequence — the sequence number stamped on the first record of a batch; per-partition it must ascend without gaps.
• Epoch — the producer epoch, bumped to fence stale idempotent state after an ambiguous failure.
• Ambiguous loss — a failure with no broker response, where the client cannot know if the write landed; resolved by an epoch bump.
• EnqueueSequencer — kacrab’s ticket-based ordering that reconstructs Java’s single-Sender-thread enqueue order across concurrent dispatch tasks.
• inflightBatchesBySequence — the per-partition map of sent-but-unacked batches, ordered by base sequence.
• firstInFlightSequence — the oldest unacked sequence on a partition; the retry gate.
• Sticky partitioning — batching-friendly null-key partitioning: stick to one partition until the batch fills, then rotate (adaptively weighted by queue depth).

Consumer side

• Position — a partition’s monotonic fetch cursor; advances only past records actually delivered to the caller.
• Committed offset — the group’s durable “read up to here” mark at the coordinator; where a member resumes after restart or rebalance.
• Coordinator — the broker owning a group’s slice of __consumer_offsets; target of joins, heartbeats, and commits. It can move; clients must re-discover it.
• Assignor — the algorithm dividing partitions among group members (range, roundrobin, sticky, cooperative-sticky).
• Cooperative rebalance (KIP-429) — the two-round incremental handoff that moves only the partitions that change hands, never double-owning one.
• Member epoch (KIP-848) — the server-side protocol’s generation counter, advanced by the coordinator on each assignment change.
• Fetch session (KIP-227) — per-broker state letting incremental fetches send only changed partitions instead of the full set.
• Static membership — a group.instance.id identity that survives restarts, so a quick bounce triggers no rebalance.

Wire & protocol

• Record batch v2 — the Kafka record-batch format (magic byte 2) that wraps compressed records with a CRC32C over the compressed payload.
• Topic id (KIP-516) — the UUID that keys newer Fetch versions in place of the topic name.
• Oracle matrix — the Java-client-checked test fixtures that prove Rust’s generated wire encoding matches Kafka byte-for-byte.
• KRaft — Kafka’s ZooKeeper-less consensus mode, used by every test cluster here.