RocksDB (as state store)
RocksDB is the embedded LSM store behind Flink's big-state backend, plus the tuning tax that rides along.
RocksDB (as state store) - the embedded LSM key-value store behind the RocksDB Flink state backend, where big jobs keep state that will not fit on the heap. Every value is stored as serialised bytes on local disk, which buys state far larger than memory and costs a serialisation round-trip on every read and write, plus an operational surface transplanted straight from a storage engine: block cache, write buffers, compaction styles, bloom filters, file handles.
To be fair, it works, which is why Flink and Kafka Streams both lean on it. But "works" arrives with homework. The defaults suit nobody in particular, memory accounting spans the JVM heap and native allocations so container sizing gets interesting, and a badly timed compaction turns up in your p99 as a stall nobody can explain.
Do you need it? You need what it provides, state bigger than RAM. You do not need to adopt a storage engine to get it. Ubik holds state in memory up to a budget and spills to disk past it: one setting, no compaction to babysit. The knob farm stays on the farm.