Partitions and Consumer Groups

Placeholder content. Written to exercise the standalone .puml embed and a table. Replace the prose before publishing.

Kafka is an append-only distributed log. Most questions reduce to two facts: ordering is per-partition, and consumers track their own offsets.

Partitions decide everything

• A topic is split into partitions; each partition is an ordered, immutable sequence
• Ordering is guaranteed within a partition, never across a topic
• The partition is chosen by hash(key) % partitionCount, so the key is what buys you ordering
• Partition count can grow but never shrink, and growing it rehashes future keys

Consumer groups

Each partition is consumed by exactly one member of a group, which is why adding consumers beyond the partition count buys nothing.