How to use noteStalled to recover from a finality lagging chain?

Question

From the documentation:

Grandpa:

noteStalled (delay: u32, best_finalized_block_number: u32) interface: api.tx.grandpa.noteStalled summary: Note that the current authority set of the GRANDPA finality gadget has stalled. This will trigger a forced authority set change at the beginning of the next session, to be enacted delay blocks after that. The delay should be high enough to safely assume that the block signalling the forced change will not be re-orged (e.g. 1000 blocks). The GRANDPA voters will start the new authority set using the given finalized block as base. Only callable by root.

But what does it mean should I give the last generated block as the best finalized block or the one already finalized or the one I would like to continue from? In this case 57281 or 57354?

I tried to use the block 57281 and 100 as delay, nothing happened at block 57382

Answer 1

You should pass #57281 as the second parameter to the noteStalled call since that is the last block that was finalized, this means that when the GRANDPA validator set is forcibly changed it will resume finalizing from that block on. The delay only counts after the current session ends so the total delay until you see the change happens will be more than 100 blocks. Additionally, this will only rotate the validator set with whatever is selected for the next session, hence if the same validators are there (which presumably aren't finalizing for some reason), then the problem will remain. You should also use the grandpa_roundState RPC method to debug which validators aren't voting.

(Similar question here: Recovering from stalled finality (babe/grandpa))