We have a use case within a Streams application that requires a persistent TTL key-value cache store. As Streams does not currently offer such a store, I've implemented it by abusing WindowStore, as it allows for a configurable retention period. Nonetheless, it is not an ideal solution, as you can only iterate forward on the iterator returned by fetch(), whereas the use case calls for a reverse iterator, as we are only interested in the latest value for an entry.
I am curious as to the appetite for a KIP to add such a TTL caching store. KAFKA-4212 is the issue I opened requesting such a store. Do others have a need for them? If there is interest in such a KIP, I can get one started. If there is interest, there are two ways such a store could be implemented. It could make use of RocksDB TTL feature or it could mirror WindowStore and make use multiple segmented RockDBs, possibly reusing the RocksDBSegmentedBytesStore from the latest refactoring of the stores. The former deletes most of the work to RocksDB compaction, although likely at the expense of greater write amplification. The later is more efficient at dropping expired entries, but potentially more space inefficient. Thoughts?
