parachain/disputes: dispute coordinator backend

[kanishkatn]

Changes

Implements backend for disputes-coordinator.

Parity's implementation:
https://github.com/paritytech/polkadot/blob/master/node/core/dispute-coordinator/src/backend.rs
https://github.com/paritytech/polkadot/tree/master/node/core/dispute-coordinator/src/db

Issues

#3302

Primary Reviewer

@kishansagathiya

[EclesioMeloJunior]

It looks like pebbleDb doesn't support transactions. It would require a bit of effort to implement the interface. Here's where it is mostly used https://github.com/paritytech/polkadot/blob/master/node/core/dispute-coordinator/src/db/v1.rs#L139
Do you think it would be an easy switch or could we continue to use badger in the subsystem?

You're right, but AFAIK you can use the pebble's Batcher which could replace the current badger transactions, more exactly the IndexedBatcher (https://github.com/cockroachdb/pebble/blob/master/batch.go), basically you can aggregate operations and commit or reset them

[kanishkatn]

It looks like pebbleDb doesn't support transactions. It would require a bit of effort to implement the interface. Here's where it is mostly used https://github.com/paritytech/polkadot/blob/master/node/core/dispute-coordinator/src/db/v1.rs#L139
Do you think it would be an easy switch or could we continue to use badger in the subsystem?

You're right, but AFAIK you can use the pebble's Batcher which could replace the current badger transactions, more exactly the IndexedBatcher (https://github.com/cockroachdb/pebble/blob/master/batch.go), basically you can aggregate operations and commit or reset them

hmm, that's interesting. I had come across this old issue cockroachdb/pebble#581. In their readme, they suggest rocksDB for transactions as well. I'm gonna have to go through it, but for now I'd like to continue with the integration tests. How about we revisit this at a later stage?

[jimjbrettj]

some remaining comments, but lgtm

[kanishkatn]

We still need to do the pebbleDB implementation. I'll pick it up in the end.