Homework 3 forum

Contention Tests 11, 14 and 14 (integration)

 
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Contention Tests 11, 14 and 14 (integration)
by Stratos Triantafyllou - Saturday, 12 December 2020, 22:57
 

In Tests 11, 14 and its integration version, how does a node learn that its proposal has been rejected so that it knows to retry? My understanding was that it just does not reach PROMISE (or PROPOSE) majority before the paxosRetry timeout, therefore it makes another attempt with a new ID (and, since TLC majority has been observed in the meantime, a higher block number).

However, in these tests, paxosRetry is set to 1000 seconds, which means it does not fire during the test. What am I missing?

Picture of Stratos Triantafyllou
Re: Contention Tests 11, 14 and 14 (integration)
by Stratos Triantafyllou - Saturday, 12 December 2020, 23:55
 
The paxosRetry issue I also come across in Test 10. Considering that Node B advertises a higher ID than Node A, its proposal is going to be the one accepted. The test concludes properly when A is not able to secure majority in the PREPARE/PROMISE round.

However, if A's PREPARE messages reach at least two peers before B's, A proceeds to the PROPOSE/ACCEPT round. It is not able to reach majority in that round, and it keeps waiting for paxosRetry seconds, which is set to 1000. Because of this, the test never finishes.
Picture of Bastien Beuchat
Re: Contention Tests 11, 14 and 14 (integration)
by Bastien Beuchat - Sunday, 13 December 2020, 01:53
 

Hi, I had kind of the same issue. I assumed that when we reach a consensus in a Paxos box (receiving a TLC message), the proposer can just stop waiting for promise or accept .

I hope that the approach is correct and that can help you!

Picture of Stratos Triantafyllou
Re: Contention Tests 11, 14 and 14 (integration)
by Stratos Triantafyllou - Monday, 14 December 2020, 10:47
 
Thanks for sharing! But may I request an answer from the teaching assistants on the "official" specification? :)
Picture of Cristina Basescu
Re: Contention Tests 11, 14 and 14 (integration)
by Cristina Basescu - Monday, 14 December 2020, 16:26
 

Hi Stratos,

Thanks for your question. What Bastien said makes sense (thanks, Bastien!). To summarize: say that node A proposes value v1 for block 5, but some time after sending out this proposal, A observes consensus on block 5 for a different value v2. Then node A learned the consensus value v2 for block 5 and can move to block 6.

To add to Bastien's answer: node A still wants to propose value v1, despite his value not being agreed on for block 5. A could directly propose value v2 for block 6. However, if some value in the blockchain conflicts with v2, e.g., v1 or some other value contains a different file name for the same metahash that v2 contains, A's proposal will be rejected. How does A learn about it? Because every node is a proposer, learner, and acceptor, A's proposer could first ask A's learner whether v2, the value to be proposed, conflicts with any of the values already in the blockchain. If it conflicts, A shouldn't propose v2 because it'll trivially be rejected by the other acceptors (and should be rejected by A's acceptor as well).

Does this make sense? If not, please write back.

Cristina

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Re: Contention Tests 11, 14 and 14 (integration)
by Stratos Triantafyllou - Monday, 14 December 2020, 16:38
 

Perfect, yes! Thanks, both Bastien and Cristina!