Some jobs needs to maintain as state a large set of key-values for some period of time. I.e. they need to maintain a TTL cache of values potentially larger than memory.
Currently Kafka Streams provides non-windowed and windowed key-value stores. Neither is an exact fit to this use case.
The RocksDBStore, a KeyValueStore, stores one value per key as required, but does not support expiration. The TTL option of RocksDB is explicitly not used.
The RocksDBWindowsStore, a WindowsStore, can expire items via segment dropping, but it stores multiple items per key, based on their timestamp. But this store can be repurposed as a cache by fetching the items in reverse chronological order and returning the first item found.
KAFKA-2594 introduced a fixed-capacity in-memory LRU caching store, but here we desire a variable-capacity memory-overflowing TTL caching store.
Although RocksDBWindowsStore can be repurposed as a cache, it would be useful to have an official and proper TTL cache API and implementation.