The following is an example for a structure that could be used to enable fast shard chain collations.
- Proof of Activity: hybrid proof of work and proof of stake where for a block to be valid to build on top of, >=M out of a randomly selected N PoS validators need to vote on it.
- Sequential proof of work, specifically this latest protocol by Bram Cohen
Consider a PoA-like model where a collation can be made by a proposer, and then for it to be eligible for the next proposer to build on top of, the collation needs to be approved by at least 4 of a random sample of 7 notaries. The randomness is sourced from (i) a recent main chain block hash, and (ii) a hash preimage revealed by the proposer. For any collation, there is an infinite sequence of proposers that can make collations on top of it; that is, for every integer x >= 0, there is a proposer P[x]. For a proposal by proposer P[x] to be valid, it must contain sequential proof of work with difficulty factor D * x; D is adjusted via an on-chain game, targeting toward five seconds.
The intention is that collations on shards would normally come as fast as network latency, in a graph like this:
And if at any point proposer 0 for the next collation is missing, then the chain would stop for ~5 seconds, at which point proposer 1 would be able to make a collation.
The notarizations would serve three purposes:
- Directly notarizing the collations they are building on top of
- Being a de-facto committee approving the shard chain (the main chain meta-committee would listen to the longest chain in this mechanism)
- Being Casper votes in the main-chain Casper FFG cycle