RocksDBStateBackend in Flink: how does it works exactly?

Question

I have read the official Flink's documentation about the State Backends, here. In particular, I was interested in the RocksDBStateBackend.

I don't understand, if I enable this kind of backend, RocksDB will be accessible from TaskManagers through another node inside the Flink's cluster?

What I have understood so far about the RocksDBStateBackend is that Task Managers will store the states inside their memory, i.e. the memory of the JVM process. After that, will they send the states to store inside RocksDB? If yes, where is RocksDB inside the Flink's cluster? Where is it phisically?

Answer 1

RocksDB is an embedded database. If you are using RocksDB as your state backend for Flink, then each task manager has a local instance of RocksDB, which runs as a native (JNI) library inside the JVM. When using RocksDB, your state lives as serialized bytes on the local disk, with an in-memory (off-heap) cache.

During checkpointing, the SST files from RocksDB are copied from the local disk to the distributed file system where the checkpoint is stored. If the local recovery option is enabled, then a local copy is retained as well, to speed up recovery. But it wouldn't be safe to rely only on the local copy, as the local disk might be lost if the node fails. This is why checkpoints are always stored on a distributed file system.

The alternative to RocksDB is to use one of the heap-based state backends, in which case your state will live as objects on the JVM heap.