What happens if a validator goes offline and it's part of the session?

Question

I've been using the validator-set pallet to add new validators on a permissioned network, and although it solves the issue related to block production slowing down when a validator is missing, it has a 'recovery' period where the block production slows down until the next session happens.

I was wondering how does Polkadot or any other chain deal with this problem when a validator goes offline and it is supposed to author a block. Does it also slow for some time and later on the missing validator is removed from the session to maintain the block production constant?

Answer 1

First, bear in mind that Polkadot uses a block authoring system completely different to what you saw in the validator-set pallet you mentioned in your question.

In Polkadot, BABE algorithm assigns validators per slot in a way that helps to greatly minimize the probability of a slot being missed. This is done by running a secondary, round-robin style validator selection algorithm in the background in parallel with the primary BABE election process. You can read the general description about this process in this doc link.

Moreover, if a validator is assigned to author a block and missed their slot, they will be reported as "unresponsive". If they are unresponsive for too long, they may be slashed. You will find the details of this protocol in the Unresponsiveness section of this documentation link.