Currently, we only keep 1) the most recent promise we've made and 2) the last update we've accepted. But we don't keep the ballot at which that last update was accepted. And because a node always promise to newer ballot, this means an already committed update can be replayed even after another update has been committed. Re-committing a value is fine, but only as long as we've not start a new round yet.
Concretely, we can have the following case (with 3 nodes A, B and C) with the current implementation:
- A proposer P1 prepare and propose a value X at ballot t1. It is accepted by all nodes.
- A proposer P2 propose at t2 (wanting to commit a new value Y). If say A and B receive the commit of P1 before the propose of P2 but C receives those in the reverse order, we'll current have the following states:
Because C has received the t1 commit after promising t2, it won't have removed X during t1 commit (but note that the problem is not during commit, that example still stand if C never receive any commit message).
- Now, based on the promise of A and B, P2 will propose Y at t2 (C don't see this propose in particular, not before he promise on t3 below at least). A and B accepts, P2 will send a commit for Y.
- In the meantime a proposer P3 submit a prepare at t3 (for some other irrelevant value) which reaches C before it receives P2 propose&commit. That prepare reaches A and B too, but after the P2 commit. At that point the state will be:
In particular, C still has X as update because each time it got a commit, it has promised to a more recent ballot and thus skipped the delete. The value is still X because it has received the P2 propose after having promised t3 and has thus refused it.
- P3 gets back the promise of say C and A. Both response has t3 as in-progress ballot (and it is more recent than any mrc) but C comes with value X. So P3 will replay X. Assuming no more contention this replay will succeed and X will be committed at t3.
At the end of that example, we've comitted X, Y and then X again, even though only P1 has ever proposed X.
I believe the correct fix is to keep the ballot of when an update is accepted (instead of using the most recent promised ballot). That way, in the example above, P3 would receive from C a promise on t3, but would know that X was accepted at t1. And so P3 would be able to ignore X since the mrc of A will tell him it's an obsolete value.