Liveliness and finalizing a notorized round after broadcasting a nullify

[matthias-wright]

Hi all,

I ran into an issue while testing. A network of 4 validators wasn't able to finalize a round, and hence lost liveliness.

It's very possible that this issue is on my end. We just implemented state sync, and maybe something got out of whack.
I'm just curious if it is possible that this happened because of a user error.

Here is an excerpt from the logs from one of the validators:

generated proposal proposal=Proposal { round: Round(21, 651), parent: 650
broadcasting notarize round=Round(21, 651) proposal=Proposal { round: Round(21, 651), parent: 650
broadcasting nullify round=Round(21, 651)
leader elected round=Round(21, 652)
broadcasting notarization proposal=Proposal { round: Round(21, 651), parent: 650
cached round=Round(21, 651) name="notarization"

After looking at the logs, here is my theory for what happened:

• Round(21, 651) was entered.
• The validator sent a notorize.
• Constructing a notorize doesn't clear the deadlines, and maybe there was a networking delay, which lead to a timeout, and a subsequent nullify broadcast.
• Enough validators broadcasted a notorize, so the round was notorized, but at least 2f+1 validators also broadcasted a nullify.
• Once a validator broadcasts a nullify, it can no longer construct a finalize.
• The round was notorized, but never finalized.

Also note that I was bringing in commonware from this commit hash.

I can't find anything about this in the papers, so I'm curious if anyone else experienced this, or if I'm missing something here.

Thanks!

[patrick-ogrady]

What you are observing is the correct behavior 👍

Once a validator broadcasts a nullify, it cannot broadcast a finalize. If 2f+1 broadcast nullify (before seeing the notarization before the timeout), that view cannot be directly finalized. A future view, however, may indirectly finalize the view if it builds on the block in the constructed notarization.

You should not assume every view will have a direct finalization (not a guarantee of Simplex).

[patrick-ogrady]

This slide deck does a good job of visualizing these outcomes: https://simplex.blog/slides-cornell23.pdf

This video is a great walkthrough of the slides: https://www.youtube.com/watch?v=fQl9cnIjs7E

[matthias-wright]

Thank you! 🙏