Compaction

A phone keeps two lists of the people you talk to. The call log records every call in order, and old entries disappear after a while. The contact list keeps one entry per person, and when a friend changes number, the new number replaces the old one instead of sitting beside it.

Kafka can keep a topic either way. Time-based retention, from the previous lesson, works like the call log: every event stays for a fixed period and then goes. Compaction works more like the contact list, because Kafka keeps the latest event for each key for as long as the topic exists.