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Discussion thread:  [DISCUSS] KIP-354 Time-based log compaction policy

JIRA: KAFKA-7321

Motivation

Compaction enables Kafka to remove old messages that are flagged for deletion while other messages can be retained for a relatively longer time.  Today, a log segment may remain un-compacted for a long time since the eligibility for log compaction is determined based on compaction ratio (“min.cleanable.dirty.ratio”) and min compaction lag ("min.compaction.lag.ms") setting.  Ability to delete a log message through compaction in a timely manner has become an important requirement in some use cases (e.g., GDPR).  For example,  one use case is to delete PII (Personal Identifiable information) data within 7 days while keeping non-PII indefinitely in compacted format.  The goal of this change is to provide a time-based compaction policy that ensures the cleanable section is compacted after the specified time interval regardless of dirty ratio and “min compaction lag”.  However, dirty ratio and “min compaction lag” are still honored if the time based compaction rule is not violated. In other words, if Kafka receives a deletion request on a key (e..g, a key with null value), the corresponding log segment will be picked up for compaction after the configured time interval to remove the key.

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