To use compaction filter, applications needs to implement the interface found in rocksdb/compaction_filter.h and set it to ColumnFamilyOptions. Alternatively, applications can implement the CompactionFilterFactory interface, which gives the flexibility to create different compaction filter instance per (sub)compaction. The compaction filter factory also gets to know some context from the compaction (whether it is a full compaction or whether it is a manual compaction) through the given param. The factory can choose to return different compaction filter based on the context.

Compaction filter will not be invoked during flush, despite arguably flush is a special type of compaction.

Each time a (sub)compaction sees a new key from its input and when the value is a normal value, it invokes the compaction filter. Based on the result of the compaction filter:

Before release 6.0, if there is a snapshot taken later than the key/value pair, RocksDB always try to prevent the key/value pair from being filtered by compaction filter so that users can preserve the same view from a snapshot, unless the compaction filter returns . However, this feature is deleted since 6.0, after realized that the feature has a bug which can't be easily fixed. Since release 6.0, with compaction filter enabled, RocksDB always invoke filtering for any key, even if it knows it will make a snapshot not repeatable.