Paper Review. Asynchronous Consensus Without Rounds
This newspaper past times Robbert van Renesse appeared on Arxiv two weeks ago. (Update: Huh, I missed this earlier, exactly the newspaper has a footnote that says it was written inwards 2010.) The newspaper looks real interesting. I solely got to skim the paper, exactly I volition give this a careful read later.
All published crash together with Byzantine error tolerant asynchronous consensus protocols role rounds. (Yes, indeed... Paxos, Viewstamped Replication, fifty-fifty Nakamoto consensus, together with Avalanche protocol all role rounds.) Rounds are such an inherent business office of consensus algorithms that it is tempting to conclude that solving error tolerant consensus requires ordered rounds. This newspaper shows that such a determination would travel incorrect past times showing an asynchronous consensus protocol that does non role rounds.
The protocol is named after Avalanche inwards that it is a decentralized binary consensus algorithm that industrial plant past times nodes polling other nodes. However, inwards contrast to Avalanche which uses rounds together with is probabilistic, Texel does non role rounds together with it is deterministic. Instead of rounds, nodes role querying of consistent cuts together with modify their vote. The consistent cuts are identified past times using vector clocks inwards the Texel algorithm.
Texel is non a real efficient algorithm, because each node makes upward its ain withdraw heed inwards a decentralized manner. In contrast, past times having leaders coordinate other nodes for which proposal to vote on for a given round, Paxos achieved efficiency together with economy.
What is more, inwards Texel, processes are non allowed to answer to queries if they are experimenting themselves. How is this supposed to terminate/decide? In this respect, Texel over again resembles the Avalanche algorithm. If 2 conflicting proposals exist, consensus is non guaranteed to move for Avalanche. The same affair holds for Texel every bit well.
Texel requires 3f+1 processes, where f is the issue of processes that tin crash. Byzantine fault-tolerant Texel requires 5f+1 nodes. (I postulate to cheque how Texel deals alongside vector clock integrity inwards the presence of Byzantine nodes. Maybe it is done through signing entries past times corresponding processes, because vector clock entries are solely maxed, non updated past times processes other than the originators.)
On the other hand, this is even together with then a super exciting paper, because Texel proves that distributed fault-tolerant consensus is possible without rounds. Texel breaks novel ground! It may travel possible to choose to a greater extent than useful instances of no-round consensus algorithms inwards the future. The Texel protocol is derived using stepwise refinement. (Robbert had every bit good used stepwise refinement inwards his function on chain replication. It is a technique that keeps on giving.) Starting from a high-level specificition of Consensus, an intermediate score specification called ProtoConsensus alongside no-rounds is shown to refine the Consensus specification, together with Texel is shown to refine ProtoConsensus. It may travel possible to search for option implementations refining ProtoConsensus.
I am happy that novel territory is beingness explored for decentralized consensus for the concluding couplet years. It is exciting times for distributed systems together with algorithms.
Texel is a single-instance consensus algorithm? Is it possible to extend Texel to multiple-instance consensus inwards an efficient means together with implement state machine replication using it? Is it possible to practise linearizable reads from that multi-instance algorithm? Given that in that location is no leader, together with commit fourth dimension of an update functioning is murky, this volition travel tricky.
2. Is it possible to role HLC instead of VC for finding consistent cuts?
I suppose it may travel possible to role HLC for the non-byzantine version of Texel together with practise goodness from loosely synchronized clocks, exactly I don't know if in that location would travel a big practical gain.
3. How practise nosotros implement this protocol inwards TLA+?
In PlusCal implementing Texel won't travel real hard. Modeling Texel inwards PlusCal may render value because it volition allow you lot exam unlike invariants together with temporal properties together with exploring variations on the protocol. If I tin larn the scaffold for this inwards place, I may fifty-fifty assign this every bit course of didactics projection this year.
All published crash together with Byzantine error tolerant asynchronous consensus protocols role rounds. (Yes, indeed... Paxos, Viewstamped Replication, fifty-fifty Nakamoto consensus, together with Avalanche protocol all role rounds.) Rounds are such an inherent business office of consensus algorithms that it is tempting to conclude that solving error tolerant consensus requires ordered rounds. This newspaper shows that such a determination would travel incorrect past times showing an asynchronous consensus protocol that does non role rounds.
The protocol is named after Avalanche inwards that it is a decentralized binary consensus algorithm that industrial plant past times nodes polling other nodes. However, inwards contrast to Avalanche which uses rounds together with is probabilistic, Texel does non role rounds together with it is deterministic. Instead of rounds, nodes role querying of consistent cuts together with modify their vote. The consistent cuts are identified past times using vector clocks inwards the Texel algorithm.
Texel is non a real efficient algorithm, because each node makes upward its ain withdraw heed inwards a decentralized manner. In contrast, past times having leaders coordinate other nodes for which proposal to vote on for a given round, Paxos achieved efficiency together with economy.
What is more, inwards Texel, processes are non allowed to answer to queries if they are experimenting themselves. How is this supposed to terminate/decide? In this respect, Texel over again resembles the Avalanche algorithm. If 2 conflicting proposals exist, consensus is non guaranteed to move for Avalanche. The same affair holds for Texel every bit well.
Texel requires 3f+1 processes, where f is the issue of processes that tin crash. Byzantine fault-tolerant Texel requires 5f+1 nodes. (I postulate to cheque how Texel deals alongside vector clock integrity inwards the presence of Byzantine nodes. Maybe it is done through signing entries past times corresponding processes, because vector clock entries are solely maxed, non updated past times processes other than the originators.)
Conclusion
Ok, together with then what practise nosotros choose here? Texel is non real efficient. It may non terminate. It uses to a greater extent than processes to tolerate f faulty processes. This all makes me think, rounds are a slap-up abstraction for distributed systems. Ain't null incorrect alongside rounds. They are implementable via ballotnumbers every bit inwards PAxos together with you lot are done. The newspaper every bit good doesn't claim that in that location is something incorrect alongside rounds, together with neither does it claim that solving consensus without rounds brings whatsoever advantages.On the other hand, this is even together with then a super exciting paper, because Texel proves that distributed fault-tolerant consensus is possible without rounds. Texel breaks novel ground! It may travel possible to choose to a greater extent than useful instances of no-round consensus algorithms inwards the future. The Texel protocol is derived using stepwise refinement. (Robbert had every bit good used stepwise refinement inwards his function on chain replication. It is a technique that keeps on giving.) Starting from a high-level specificition of Consensus, an intermediate score specification called ProtoConsensus alongside no-rounds is shown to refine the Consensus specification, together with Texel is shown to refine ProtoConsensus. It may travel possible to search for option implementations refining ProtoConsensus.
I am happy that novel territory is beingness explored for decentralized consensus for the concluding couplet years. It is exciting times for distributed systems together with algorithms.
MAD questions
1. How could nosotros extend this to multi-decree consensus?Texel is a single-instance consensus algorithm? Is it possible to extend Texel to multiple-instance consensus inwards an efficient means together with implement state machine replication using it? Is it possible to practise linearizable reads from that multi-instance algorithm? Given that in that location is no leader, together with commit fourth dimension of an update functioning is murky, this volition travel tricky.
2. Is it possible to role HLC instead of VC for finding consistent cuts?
I suppose it may travel possible to role HLC for the non-byzantine version of Texel together with practise goodness from loosely synchronized clocks, exactly I don't know if in that location would travel a big practical gain.
3. How practise nosotros implement this protocol inwards TLA+?
In PlusCal implementing Texel won't travel real hard. Modeling Texel inwards PlusCal may render value because it volition allow you lot exam unlike invariants together with temporal properties together with exploring variations on the protocol. If I tin larn the scaffold for this inwards place, I may fifty-fifty assign this every bit course of didactics projection this year.
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