Can a parachain experience a fork?

Question

Under what circumstances will a parachain experience a fork where blocks later can be rearranged?

Blocks are created by the collators and if one of the collators are offline and misses its turn there will be a prolonged blocktime until the next collator is ready to produce its block. Finality is generated by the relaychain which will only receive one stream of blocks, where there cannot be any reorganization.

Are any of above statements incorrect and are there under a set of conditions a possibility of forks and reorganization of the blocks?

Answer 1

There is no feasible situation under which a parachain has finalized blocks rearranged. For all practical purposes, finalized blocks are final and will not change.

However, there are some situations where something comparable can happen.

If there are issues in the relay chain, and the validators do not come to a consensus on the best chain for a prolonged period, no new blocks will be finalized, although many are produced. If that goes on long enough, users of parachains will see no value in doing off-chain or cross-chain operations based on finalized blocks, because no forward progress is made. They would then have to use a probabilistic finality mechanism like in other chains, which is susceptible to reorganization.

Similarly, it is technically possible that >2/3 of the validators could collude and decide the state is now something different, which would be able to bypass consensus mechanisms. However, this is exceedingly unlikely.