Anatomical Similarities As Well As Differences Betwixt Paxos As Well As Blockchain Consensus Protocols
Now that nosotros read through some blockchain papers inward our seminar, I started to develop unopen to understanding of the "blockchain way". Since I already know virtually the "Paxos way", I idea it would live instructive as well as fun to compare & contrast Paxos as well as blockchain consensus protocols. If you lot know virtually either of these protocols well, this postal service tin assist you lot acquire a headstart on learning the other one.
I volition live referring to the below Paxos figure for the protocol phases, as well as hence let's pivot it here.
A span days agone I tweeted that. That is how leader election is done inward blockchain consensus protocol. The showtime miner to solve the puzzle showtime becomes the leader for the electrical flow round. For blockchain it is par for the course of written report that for each circular unopen to other leader (the 1 who solves the adjacent puzzle first) serves. In other words, instead of Phase1a "propose leadership" as well as Phase1b "promise to follow" back-and-forth communication amid the nodes, blockchain uses a proof-of-work puzzle to elect a leader. What if multiple miners solve the puzzle at virtually the same time? Well, encounter the below department for that comparison.
For Paxos, the leader election is done via Phase 1. A leader candidate sends a stage 1a message amongst its ballot give away (essentially a counter or a logical clock if you lot like) as well as elbow grease to acquire OKs from a bulk give away of participants inward the quorum. Receiving 1 "Not OK" message is a bargain breaker, because it indicates in that place is unopen to other leader candidate amongst a higher ballot give away reaching to the participants.
In Paxos applications, typically nosotros similar to proceed amongst the same leader for many rounds, genuinely every bit long every bit nosotros can. So to avoid paying the terms of stage 1 repeatedly, MultiPaxos skips stage 1 as well as continues amongst iterating over stage 2 messages for the consecutive rounds. If an incumbent leader arises, the stage 1 leader election may remove to come about again. An orthogonal mechanism, such every bit a failure detection service, is used to ensure that in that place are non many candidates running to acquire a leader at a given time, every bit that may interfere amongst the progress of the protocol. That province of affairs is called the "dueling leaders" situation. But worry not. Even when in that place are multiple leader candidates, Paxos is prophylactic every bit nosotros utter over below.
In Paxos, a failure detector service (which relies on heartbeats as well as oft has unopen to randomized backoff implementation) oft helps amongst converging the give away of leaders/candidates to 1. However, Paxos has the built-in machinery (with the ballot numbers as well as bulk acknowledgment via Phase 1 as well as Phase 2) that ensures security of understanding fifty-fifty when in that place are multiple active leaders/candidates. If a player is aware of an choice candidate/leader amongst a higher ballot number, it sends a "Not OK" message to the incumbent leader. The incumbent leader waits for receiving an acknowledgment from a bulk of participants to commit a value. If it receives a "Not OK" message, it needs to give-up the ghost dorsum to Phase 1 to duel inward the leader election again. On the other hand, it is prophylactic for the incumbent leader to commit a value afterward a bulk acknowledgment is received because fifty-fifty when the incumbent leader is dethroned, the newcomer is saltation past times Paxos protocol rules to repropose as well as select the same value every bit the consensus decision. Therefore, fifty-fifty when in that place are multiple leaders, you lot won't remove hold forks inward Paxos, because alone 1 of them--if at all-- volition live able to consummate Phase 2 as well as commit a value.
Since the blockchain consensus protocol skips Phase 2b as well as Phase 3, it provides unavoidably a probabilistic consensus.(I had talked virtually why this is the illustration for leader based distributed consensus such every bit Paxos inward this previous post.) Since blockchain consensus is inherently probabilistic, you lot remove to expression half-dozen to a greater extent than blocks to encounter if that block sticks. At that dot the run required to rewrite history is as well as hence huge that you lot tin conclude the block is last committed.
For stage 2 inward Paxos, the leader has a two-way acknowledged communication. The two-way communication plant OK for the leader because the give away of the participants is typically less than one-half a dozen (for the reasons explained below).
On the other hand, if 1000s nodes participated inward a Paxos protocol, the leader would non live able to hold upwards receiving 1000s of ack messages for each round. (Well, I approximate it could have, exactly the phases would select several seconds than the typical 10s of milliseconds.) To avoid this problem, amongst 1000s of participants, the Blockchain has alone 1 means broadcast communication, which is propagated from the leader to the participants via a gradual gossip protocol.
So to recap, as well as to parent dorsum to the figure, hither is what nosotros have. Blockchain does a still Phase 1 (via proof-of-work) as well as follows that amongst alone Phase 2a.
The blockchain applications select novel constraints as well as requirements to the consensus problem. In blockchains, the participants tin forthwith live Byzantine, motivated past times fiscal gains. So it is non sufficient to confine the consensus participants to live 3-5 nodes, because the residuum of the network does non necessarily trust these nodes. In Blockchains, for reasons of attestability as well as tolerating colluding groups of Byzantine participants, it is preferred to maintain the participants at 1000s to 10,000s. Thus the occupation becomes: How exercise you lot pattern byzantine tolerant consensus algorithm that scales to 1000s of participants?
Moreover, the consensus problems solved are likewise slightly different. The Bitcoin-NG newspaper provided a overnice formulation of the blockchain consensus occupation inward its model section.
Comparing the Nakamoto consensus occupation amongst the classic Byzantine consensus occupation is real instructional. Nakamoto consensus has probabilistic guarantees for Termination, Agreement, as well as Validity, whereas the classic Byzantine Consensus (which a Byzantized version of Paxos solves) has deterministic guarantees for them. (To maintain the give-and-take uncomplicated inward this post, I didn't move inward a byzantine tolerant Paxos protocol, which is to a greater extent than complicated. By utilizing randomization/probabilities, blockchain has a genuinely uncomplicated protocol --I remove hold to give it to it.)
1. Well, this entire postal service started every bit a effect of a MAD question: "What if I elbow grease to map Blockchain protocol components to that of Paxos protocol components?" So I chalk that upwards every bit my showtime MAD enquiry :-)
2. Now that nosotros remove hold this mapping, is in that place a means to leverage on this to synthesize a novel insight? What is the adjacent step? Sometimes providing such a mapping tin assist give someone a novel insight, which tin atomic number 82 to a novel protocol.
I volition live referring to the below Paxos figure for the protocol phases, as well as hence let's pivot it here.
Leader election: still vs loud
A span days agone I tweeted that. That is how leader election is done inward blockchain consensus protocol. The showtime miner to solve the puzzle showtime becomes the leader for the electrical flow round. For blockchain it is par for the course of written report that for each circular unopen to other leader (the 1 who solves the adjacent puzzle first) serves. In other words, instead of Phase1a "propose leadership" as well as Phase1b "promise to follow" back-and-forth communication amid the nodes, blockchain uses a proof-of-work puzzle to elect a leader. What if multiple miners solve the puzzle at virtually the same time? Well, encounter the below department for that comparison.
For Paxos, the leader election is done via Phase 1. A leader candidate sends a stage 1a message amongst its ballot give away (essentially a counter or a logical clock if you lot like) as well as elbow grease to acquire OKs from a bulk give away of participants inward the quorum. Receiving 1 "Not OK" message is a bargain breaker, because it indicates in that place is unopen to other leader candidate amongst a higher ballot give away reaching to the participants.
In Paxos applications, typically nosotros similar to proceed amongst the same leader for many rounds, genuinely every bit long every bit nosotros can. So to avoid paying the terms of stage 1 repeatedly, MultiPaxos skips stage 1 as well as continues amongst iterating over stage 2 messages for the consecutive rounds. If an incumbent leader arises, the stage 1 leader election may remove to come about again. An orthogonal mechanism, such every bit a failure detection service, is used to ensure that in that place are non many candidates running to acquire a leader at a given time, every bit that may interfere amongst the progress of the protocol. That province of affairs is called the "dueling leaders" situation. But worry not. Even when in that place are multiple leader candidates, Paxos is prophylactic every bit nosotros utter over below.
Multiple leaders case: fork vs no-fork
In blockchain, the difficulty of the puzzle makes certain that the probability of having 2 leaders at the same fourth dimension is low. When in that place are multiple leaders, you lot volition remove hold a fork. The fork is resolved eventually, inside a span to a greater extent than block additions because most probable 1 fork volition grow faster (the chances of both forks growing at the same speed becomes negligible afterward a span of blocks) as well as the participants prefer to mine on the longest chain for having a hazard to have incentive: the mining fee if they acquire the leader for that round. In other words blockchain provides a eventually-consistent/stabilizing consensus protocol.In Paxos, a failure detector service (which relies on heartbeats as well as oft has unopen to randomized backoff implementation) oft helps amongst converging the give away of leaders/candidates to 1. However, Paxos has the built-in machinery (with the ballot numbers as well as bulk acknowledgment via Phase 1 as well as Phase 2) that ensures security of understanding fifty-fifty when in that place are multiple active leaders/candidates. If a player is aware of an choice candidate/leader amongst a higher ballot number, it sends a "Not OK" message to the incumbent leader. The incumbent leader waits for receiving an acknowledgment from a bulk of participants to commit a value. If it receives a "Not OK" message, it needs to give-up the ghost dorsum to Phase 1 to duel inward the leader election again. On the other hand, it is prophylactic for the incumbent leader to commit a value afterward a bulk acknowledgment is received because fifty-fifty when the incumbent leader is dethroned, the newcomer is saltation past times Paxos protocol rules to repropose as well as select the same value every bit the consensus decision. Therefore, fifty-fifty when in that place are multiple leaders, you lot won't remove hold forks inward Paxos, because alone 1 of them--if at all-- volition live able to consummate Phase 2 as well as commit a value.
Communication inward Accept phase: 1-way vs 2-way
The blockchain protocol employs communication alone 1 time as well as alone inward 1 way, from the leader to the participants, as well as this corresponds to the stage 2a of Paxos: the Accept message Phase.Since the blockchain consensus protocol skips Phase 2b as well as Phase 3, it provides unavoidably a probabilistic consensus.(I had talked virtually why this is the illustration for leader based distributed consensus such every bit Paxos inward this previous post.) Since blockchain consensus is inherently probabilistic, you lot remove to expression half-dozen to a greater extent than blocks to encounter if that block sticks. At that dot the run required to rewrite history is as well as hence huge that you lot tin conclude the block is last committed.
For stage 2 inward Paxos, the leader has a two-way acknowledged communication. The two-way communication plant OK for the leader because the give away of the participants is typically less than one-half a dozen (for the reasons explained below).
On the other hand, if 1000s nodes participated inward a Paxos protocol, the leader would non live able to hold upwards receiving 1000s of ack messages for each round. (Well, I approximate it could have, exactly the phases would select several seconds than the typical 10s of milliseconds.) To avoid this problem, amongst 1000s of participants, the Blockchain has alone 1 means broadcast communication, which is propagated from the leader to the participants via a gradual gossip protocol.
So to recap, as well as to parent dorsum to the figure, hither is what nosotros have. Blockchain does a still Phase 1 (via proof-of-work) as well as follows that amongst alone Phase 2a.
The environment: populace vs private
The typical purpose illustration of Paxos is to replicate Earth to acquire inward consistently as well as safely hold upwards crash failures inward a person environment. So keeping the give away of participants minor (around five nodes) is OK for this purpose case.The blockchain applications select novel constraints as well as requirements to the consensus problem. In blockchains, the participants tin forthwith live Byzantine, motivated past times fiscal gains. So it is non sufficient to confine the consensus participants to live 3-5 nodes, because the residuum of the network does non necessarily trust these nodes. In Blockchains, for reasons of attestability as well as tolerating colluding groups of Byzantine participants, it is preferred to maintain the participants at 1000s to 10,000s. Thus the occupation becomes: How exercise you lot pattern byzantine tolerant consensus algorithm that scales to 1000s of participants?
Moreover, the consensus problems solved are likewise slightly different. The Bitcoin-NG newspaper provided a overnice formulation of the blockchain consensus occupation inward its model section.
Comparing the Nakamoto consensus occupation amongst the classic Byzantine consensus occupation is real instructional. Nakamoto consensus has probabilistic guarantees for Termination, Agreement, as well as Validity, whereas the classic Byzantine Consensus (which a Byzantized version of Paxos solves) has deterministic guarantees for them. (To maintain the give-and-take uncomplicated inward this post, I didn't move inward a byzantine tolerant Paxos protocol, which is to a greater extent than complicated. By utilizing randomization/probabilities, blockchain has a genuinely uncomplicated protocol --I remove hold to give it to it.)
MAD questions
(This department is hither due to my New Year's resolution.)1. Well, this entire postal service started every bit a effect of a MAD question: "What if I elbow grease to map Blockchain protocol components to that of Paxos protocol components?" So I chalk that upwards every bit my showtime MAD enquiry :-)
2. Now that nosotros remove hold this mapping, is in that place a means to leverage on this to synthesize a novel insight? What is the adjacent step? Sometimes providing such a mapping tin assist give someone a novel insight, which tin atomic number 82 to a novel protocol.
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