Change Commit to consist of just signatures

[ebuchman]

Currently, every block includes a LastCommit which is the set of +2/3 Precommit votes for the last block.

Note votes include validator address, index, blockID, as well as signature.

All of that information is redundant except for the signature - thus we could make blocks significantly smaller if we only included the signature in the Commit, instead of the whole vote.

[UnitylChaos]

I can work on this, but if we're back to doing vote based bft time, we're going to run into the same problem trying to do this as we would with BLS signatures. We can still do this, but we will need the LastCommit to include all the signatures and all the timestamps.

[ebuchman]

First draft in #2179, see review there. Needs an ADR, and some fixes to the PR, but ideally a more incremental implementation

[milosevic]

Note that the similar optimisation can be used at the gossip level to exchange votes. Instead of sending single vote, we can send set of the votes for the same value by sending data once and then signature for every vote.

[ebuchman]

Currently we have about ~100 redundant bytes per vote. In a block with 100 validators, that means up to 10kB in unnecessary space with every block. With blocks every 5s, that's over a MB of useless storage every 10 minutes, or a GB every week.

#3245 takes some first steps towards resolving this.

We currently rely on getting a full Vote out of the Commit in a few places:

• when rebuilding the last vote set in reconstructLastCommit upon starting the ConsensusState
• when gossiping commit votes in the ConsensusReactor (this is why Commit needs to implement VoteSetReader interface with the GetByIndex method).

For now, we can easily convert a CommitSig back to a Vote for this cases. In the future, we'll need to provide more info (eg the Commit and the ValidatorSet) in order to reconstruct votes.

[ebuchman]

Another step taken in #3275. Once the Commit can memoize the ValidatorSet, it shouldn't be hard to finish this up. Cool!

It would be super cool if we could make the change somehow based on a block version, so it could be done in a backwards compatible way (ie. would still work with blockchains from before the change, on the older block version). We should look into what that might take a bit more, but could be alot.

Another note, we should consider the naming. Is CommitSig the right name? What about in the commit, it's currently called Precommits []*CommitSig but maybe it should be something like PrecommitWitnesses []*CommitSig? Or should it just be CommitSigs []*CommitSig ? See #3245 (comment)

[ebuchman]

Note #3298 (comment) - we need to modify the commit structure to indicate which block a commit was for, since we currently include commits for different blocks (just requiring +2/3 is for the same block, namely "this" one).

[b00f]

probably can be linked with #2252

[melekes]

Or should it just be CommitSigs []*CommitSig ?

simpler the better