State Management

[jspenger]

The Portals runtime currently only supports in-memory state, and does not support checkpointing/recovering state. We should fix this, and implement a suitable persistent state backend. RocksDB is a good choice for this, as it is widely used by other stream processing systems.

The tasks for this issue are the following:

• [ ] Create a state backend, which can checkpoint and recover.
• [ ] Test the state backend.
• [x] #97
• [ ] Make runtime checkpoint state at regular intervals.
• [ ] Recover from previous state after a restart.
• [x] #100
• [ ] #146

Notes:

• The new state backend should implement the StateBackend trait, a good example of this is the MapStateBackendImpl.
• I have previously had issues with RocksDB on the new M1 Macbook, this may again be an issue, just something to be aware of.
• Adding checkpointing and recovery, and enabling the state backend, should be made available through some config settings in the runtime.
• The new state backend should be put in a new sub-project, as to keep the core pure Scala (so it compiles with ScalaJS). This can be done as a last step, and we should probably discuss together how to do it best, once we have all the pieces; still, I would start of working directly form the core directory.