Pigpaxos: Devouring The Communication Bottlenecks Inwards Distributed Consensus
This is our most recent work, started in addition to led yesteryear Aleksey Charapko. (This is a articulation postal service amongst him.) You tin give the sack acquire the newspaper at arxiv.org. The newspaper is currently nether submission to a journal.
Aleksey, who is non known for existence an optimist, said that nosotros tin give the sack scale Paxos to several hundreds of nodes! He said this may hold upwardly possible yesteryear employing intermediate proxy nodes to relay the communication betwixt the leader in addition to followers, equally this would salvage the communication bottleneck at the leader.
I persuasion "yeah, it is a cracking trick, but perchance non that impressive, because it is really simple". Surely others must take away hold tried this, in addition to at that topographic point must hold upwardly a catch/drawback. We checked but couldn't discover whatever previous run on this. At the Sigmod'19 reception at Van Gogh museum, I mentioned this persuasion to Miguel Castro (of PBFT fame amidst others). He liked the persuasion in addition to said he couldn't intend of anyone that studied this before.
The cardinal persuasion inwards PigPaxos is to decouple the communication from the decision-making at the leader. PigPaxos revises the communication period of time to supplant the straight communication betwixt the leader in addition to followers inwards Paxos amongst a relay based communication flow. PigPaxos chooses relays randomly from follower clusters at each communication circular to trim contestation in addition to improve scalability of throughput.
When Aleksey started evaluating PigPaxos, nosotros were surprised yesteryear the effectiveness of this uncomplicated technique. This shouldn't take away hold been likewise much of a surprise because in our recent Sigmod paper nosotros showed that leader bottleneck is the culprit behind the scalability problems of Paxos theater unit of measurement of protocols in addition to quantified on this bottleneck amongst back-of-the-envelope formulas. Still, the results from PigPaxos were beyond our expectations. We repeated our experiments many times, in addition to double-checked everything earlier nosotros could allow ourselves to believe these results. Employing relay nodes for relieving the leader, and randomly rotating the relay nodes for communication bottleneck shedding did wonders for the performance. We found that PigPaxos improved the throughput boundary to a greater extent than than 3 folds over Paxos amongst negligible latency deterioration at 25 nodes. Even for equally depression equally nine nodes, nosotros were able to come across 1.5 folds throughput improvement amongst the same latency equally Paxos.
This was a really fun newspaper to write. The writing went tardily in addition to quick. The analytical results department at the halt of the newspaper added a lot to the paper. We pose that department after the evaluation department to exhibit that a uncomplicated back-of-the-envelope analysis formula tin give the sack explicate the evaluation results nicely.
I kept referring to the protocol equally PigPaxos inwards afterward conversations. With the submission deadline looming, nosotros were unable to fix for 100 node experiments in addition to add together the optimizations nosotros had inwards mind. I told Aleksey that nosotros should telephone yell upwardly this protocol PigPaxos officially, in addition to reserve the scream BigPaxos for the large scale protocol nosotros tin give the sack exhibit inwards the side yesteryear side paper.
Aleksey was non really receptive to the idea, thinking PigPaxos would aspect likewise informal inwards a interrogation paper. He also argued that nosotros don't take away hold a adept agency to connect the scream to the algorithm, all nosotros had was a dizzy typo. The connectedness occurred to me on Slack again. In the protocol, the relay nodes hold off for the followers’ responses from its cluster in addition to piggyback them together into a unmarried message. Well, this was a closed plenty association, hence nosotros went amongst the scream PigPaxos.
Another illustration is geo-replicated databases. A consensus grouping inwards a geo-replicated database may consist of dozens of nodes across many regions simply about the globe. As nosotros exhibit inwards our evaluation, PigPaxos increases throughput scalability significantly across WAN deployments amongst a large break of nodes. Even for Paxos clusters amongst a small-scale break of (say 5) nodes, large messages (such equally database replication messages equally inwards CockroachDB in addition to Spanner) trigger a communication bottleneck at the leader. PigPaxos's randomized relaying technique tin give the sack assist amongst those bottlenecks equally well.
While the persuasion inwards PigPaxos is uncomplicated in addition to similar aggregation-based approaches take away hold been employed inwards the context of weak-consistency replication protocols, PigPaxos is novel because it shows how these aggregation-based approaches tin give the sack hold upwardly effectively in addition to safely integrated into the potent consistency distributed consensus protocols. The PigPaxos technique, existence a uncomplicated full general technique, is applicable to many Paxos-variant protocols, including Raft, Zab, WPaxos, etc.
There are many optimizations possible over the basic system nosotros outline below, but we relegate that give-and-take to the paper.
In the most generic form, Paxos runs inwards 3 phases. Phase-1 establishes some node equally the leader, phase-2 lets the leader to impose its volition onto the followers yesteryear telling what command to accept, in addition to phase-3 finalizes the commitment yesteryear informing the followers that consensus has been reached.
The basic protocol is rather inefficient amongst these iii communication phases, in addition to Multi-Paxos optimization is ofttimes adopted to cutting downwards the unnecessary pleasantries. Multi-Paxos elects ane node equally a stable leader for some prolonged time, in addition to repeats the phase-2 nonetheless many times possible nether the same leader, without needing to perform some other phase-1. Phase-3 also gets piggybacked to some futurity phase-2 to trim communication fifty-fifty more.
It is evident that the leader bears the brunt of the charge inwards Paxos in addition to MultiPaxos. In previous work, Mencius relieved leaders' workload yesteryear rotating them. Recent blockchain consensus protocol LibraBFT from Facebook Libra also used pipelining to improve throughput (the master ground for pipelining was to trim the effects of a Byzantine leader on the protocol). In contrast, the pipelining inwards PigPaxos employs random rotation of relay nodes, rather than leader rotation, in addition to improves the throughput scalability significantly without whatever side effects. Since this is a really uncomplicated technique, it is to a greater extent than easily applicable in addition to implementable.
For simplicity sake, PigPaxos divides the entire cluster into a small-scale static break of relay groups, in addition to a unmarried relay node is chosen from each relay grouping randomly for every circular trip communication round. We usage PigPaxos amongst the MultiPaxos optimization hence entirely the phase-2 communication is performed inwards the normal case.
The randomized rotation of the relay nodes provide a large relief from communication bottlenecks. True, a relay node has its run cutting out if it needs to aggregate responses from 10 nodes inwards its cluster. But since the relay nodes randomly rotate, a item relay node volition hold upwardly off the claw for the several consequent rounds, in addition to volition hold upwardly able to procedure these messages in addition to ship to the leader without getting overwhelmed.
The randomized rotation of the relay nodes also assist for improving liveness when a relay crash occurs. Moreover, to guard against crashed or sluggish follower nodes, a timeout is used for setting a fourth dimension threshold for followers inwards the grouping to reply. When the timeout occurs, the relay node acks to the leader amongst a partial aggregation.
We banknote that the security in addition to liveness proofs of Paxos create non depend on the communication implementation betwixt the leader in addition to follower nodes. In Paxos, maintaining correctness inwards spite of failures is guaranteed yesteryear quorum size in addition to the information exchanged inside the quorums, in addition to the proofs are oblivious to how communication of this information is achieved. Therefore, PigPaxos preserves the security in addition to liveness properties of Paxos, equally it entirely modifies the communication implementation. For reasoning nearly liveness, the message period of time designing in addition to the usage of relay nodes requires particular attention, equally failure of a relay node has disproportionate impact compared to the failure of a regular follower. Since PigPaxos uses random choice of relay/aggregator nodes at each round, it circumvents this occupation in addition to retains liveness.
The PigPaxos leader communicates amongst entirely a handful of nodes for each consensus instance. Naturally, nosotros wanted to come across if relay groups acquire overwhelmed yesteryear the extra communication burden. To our surprise, PigPaxos amongst simply 2 relay groups performed the best inwards a 25 node deployment. This suggests that the leader soundless remains a bottleneck in addition to the relay groups soundless take away hold resources to spare. As shown inwards Figure 7, for a cluster of “25 nodes divided into 2 relay groups”, PigPaxos had nearly 25% payoff inwards maximum throughput compared to a 5-relay grouping setup.
The functioning relative to Multi-Paxos on a 25-node cluster is astonishing (Figure 8). PigPaxos shows nearly 3.5 flexure improvement inwards throughput compared to Paxos in addition to 10 folds improvement over EPaxos.
The benefits of PigPaxos create non halt at the large clusters. To our surprise, nosotros observed improve throughput from PigPaxos on simply a five node cluster. Yes, PigPaxos has slightly higher latency due to the additional network hop, but it is soundless able to border out some throughput improvements.
Computing the relative message charge on the follower is to a greater extent than involved, equally nosotros take away to concern human relationship the roles the follower tin give the sack bring on in addition to the probability of each role:
Plugging our experimental configuration into these uncomplicated formulas shows us that the relay nodes are never a bottleneck (regardless of both the break of nodes due north in addition to break of relay groups r), in addition to keeping the break of relay nodes small-scale tin give the sack motion the entire organisation closer to the charge parity betwixt the leader in addition to followers. The ground the relay nodes don’t travel a bottleneck is because the random alternation of the relay nodes shields them from becoming hotspots: the extra traffic charge a relay node incurs inwards ane circular is offset inwards consecutive rounds when the node no longer serves equally relay.
The drawback amongst using a really small-scale break of relay nodes (say entirely 1 relay grouping inwards the extreme) is that the functioning becomes likewise fragile: few sluggish or crushed nodes may strength the organisation to hold off the timeouts at the relay groups. Using to a greater extent than relay groups hateful the leader volition have bulk confirmation fifty-fifty inwards the presence of sluggish/crashed nodes belongings dorsum the response from some relay groups. Finally the charge that randomly rotated relays tin give the sack acquire by is probable to hold upwardly express inwards exercise due to hw/sw limitations at the node, and that is also a factor to hold upwardly explored.
Here is a link to our newspaper again, if yous are looking for some quarantine reading.
The story
One solar daytime I challenged Aleksey to give me a ballpark break on how much he thinks nosotros tin give the sack scale Paxos vertically. While sharding --as inwards CockroachDB in addition to Spanner-- helps for scaling Paxos deployments horizontally, vertical scaling is nearly how many nodes yous tin give the sack cram inwards a unmarried Paxos cluster, amongst a unmarried conflict domain.Aleksey, who is non known for existence an optimist, said that nosotros tin give the sack scale Paxos to several hundreds of nodes! He said this may hold upwardly possible yesteryear employing intermediate proxy nodes to relay the communication betwixt the leader in addition to followers, equally this would salvage the communication bottleneck at the leader.
I persuasion "yeah, it is a cracking trick, but perchance non that impressive, because it is really simple". Surely others must take away hold tried this, in addition to at that topographic point must hold upwardly a catch/drawback. We checked but couldn't discover whatever previous run on this. At the Sigmod'19 reception at Van Gogh museum, I mentioned this persuasion to Miguel Castro (of PBFT fame amidst others). He liked the persuasion in addition to said he couldn't intend of anyone that studied this before.
The cardinal persuasion inwards PigPaxos is to decouple the communication from the decision-making at the leader. PigPaxos revises the communication period of time to supplant the straight communication betwixt the leader in addition to followers inwards Paxos amongst a relay based communication flow. PigPaxos chooses relays randomly from follower clusters at each communication circular to trim contestation in addition to improve scalability of throughput.
When Aleksey started evaluating PigPaxos, nosotros were surprised yesteryear the effectiveness of this uncomplicated technique. This shouldn't take away hold been likewise much of a surprise because in our recent Sigmod paper nosotros showed that leader bottleneck is the culprit behind the scalability problems of Paxos theater unit of measurement of protocols in addition to quantified on this bottleneck amongst back-of-the-envelope formulas. Still, the results from PigPaxos were beyond our expectations. We repeated our experiments many times, in addition to double-checked everything earlier nosotros could allow ourselves to believe these results. Employing relay nodes for relieving the leader, and randomly rotating the relay nodes for communication bottleneck shedding did wonders for the performance. We found that PigPaxos improved the throughput boundary to a greater extent than than 3 folds over Paxos amongst negligible latency deterioration at 25 nodes. Even for equally depression equally nine nodes, nosotros were able to come across 1.5 folds throughput improvement amongst the same latency equally Paxos.
This was a really fun newspaper to write. The writing went tardily in addition to quick. The analytical results department at the halt of the newspaper added a lot to the paper. We pose that department after the evaluation department to exhibit that a uncomplicated back-of-the-envelope analysis formula tin give the sack explicate the evaluation results nicely.
What is upwardly amongst the name?
When nosotros started working on the idea, nosotros were calling it BigPaxos, because our ultimate finish was to scale to hundreds of nodes. One solar daytime piece shooting the shit nearly BigPaxos on Slack, Aleksey made a typo in addition to wrote "PigPaxos". He didn't realize he made the typo (even though it is really difficult to confuse b in addition to p on the keyboard). I teased Aleksey for a duad of minutes yesteryear putting diverse Sus scrofa emojis inwards my responses to his comments. He soundless didn't acquire the hint, in addition to hence I pointed the typo to him. We got a adept express joy out of it.I kept referring to the protocol equally PigPaxos inwards afterward conversations. With the submission deadline looming, nosotros were unable to fix for 100 node experiments in addition to add together the optimizations nosotros had inwards mind. I told Aleksey that nosotros should telephone yell upwardly this protocol PigPaxos officially, in addition to reserve the scream BigPaxos for the large scale protocol nosotros tin give the sack exhibit inwards the side yesteryear side paper.
Aleksey was non really receptive to the idea, thinking PigPaxos would aspect likewise informal inwards a interrogation paper. He also argued that nosotros don't take away hold a adept agency to connect the scream to the algorithm, all nosotros had was a dizzy typo. The connectedness occurred to me on Slack again. In the protocol, the relay nodes hold off for the followers’ responses from its cluster in addition to piggyback them together into a unmarried message. Well, this was a closed plenty association, hence nosotros went amongst the scream PigPaxos.
Where tin give the sack nosotros usage PigPaxos?
Paxos protocols are most usually deployed amongst 3 in addition to five nodes. But at that topographic point are also several applications that require vertically scaling Paxos to run on a large break of nodes, all inside the same conflict domain. One illustration is consistent cloud configuration management. Configuration administration is required for gating novel production features, conducting experiments (A/B tests), performing application-level traffic control, performing topology setup in addition to charge balancing, monitoring in addition to remediation, updating automobile learning models (which vary from KBs to GBs), controlling applications’ behaviors (related to caching, batching, prefetching etc), in addition to controlling chain replication topologies equally inwards Physalia.Another illustration is geo-replicated databases. A consensus grouping inwards a geo-replicated database may consist of dozens of nodes across many regions simply about the globe. As nosotros exhibit inwards our evaluation, PigPaxos increases throughput scalability significantly across WAN deployments amongst a large break of nodes. Even for Paxos clusters amongst a small-scale break of (say 5) nodes, large messages (such equally database replication messages equally inwards CockroachDB in addition to Spanner) trigger a communication bottleneck at the leader. PigPaxos's randomized relaying technique tin give the sack assist amongst those bottlenecks equally well.
While the persuasion inwards PigPaxos is uncomplicated in addition to similar aggregation-based approaches take away hold been employed inwards the context of weak-consistency replication protocols, PigPaxos is novel because it shows how these aggregation-based approaches tin give the sack hold upwardly effectively in addition to safely integrated into the potent consistency distributed consensus protocols. The PigPaxos technique, existence a uncomplicated full general technique, is applicable to many Paxos-variant protocols, including Raft, Zab, WPaxos, etc.
Summary of the paper
OK, hither is where the to a greater extent than technical content starts. Ready? I hope this volition hold upwardly good. You take away a pause from reading Covid-19 word anyways.There are many optimizations possible over the basic system nosotros outline below, but we relegate that give-and-take to the paper.
Background in addition to related work
Paxos theater unit of measurement of protocols are employed yesteryear many cloud computing services in addition to distributed databases due to their splendid fault-tolerance properties. Unfortunately, electrical flow Paxos deployments create non scale for to a greater extent than than a dozen nodes due to the communication bottleneck at the leader.In the most generic form, Paxos runs inwards 3 phases. Phase-1 establishes some node equally the leader, phase-2 lets the leader to impose its volition onto the followers yesteryear telling what command to accept, in addition to phase-3 finalizes the commitment yesteryear informing the followers that consensus has been reached.
The basic protocol is rather inefficient amongst these iii communication phases, in addition to Multi-Paxos optimization is ofttimes adopted to cutting downwards the unnecessary pleasantries. Multi-Paxos elects ane node equally a stable leader for some prolonged time, in addition to repeats the phase-2 nonetheless many times possible nether the same leader, without needing to perform some other phase-1. Phase-3 also gets piggybacked to some futurity phase-2 to trim communication fifty-fifty more.
It is evident that the leader bears the brunt of the charge inwards Paxos in addition to MultiPaxos. In previous work, Mencius relieved leaders' workload yesteryear rotating them. Recent blockchain consensus protocol LibraBFT from Facebook Libra also used pipelining to improve throughput (the master ground for pipelining was to trim the effects of a Byzantine leader on the protocol). In contrast, the pipelining inwards PigPaxos employs random rotation of relay nodes, rather than leader rotation, in addition to improves the throughput scalability significantly without whatever side effects. Since this is a really uncomplicated technique, it is to a greater extent than easily applicable in addition to implementable.
PigPaxos communication flow
As shown inwards Figure 1&2, the communication inwards Paxos is straight betwixt the leader in addition to the followers amongst a fan-out to ship messages in addition to fan-in to collect the replies. PigPaxos observes that it is possible to employ intermediate nodes/proxies to assist relay in addition to aggregate the messages inwards this communication pattern. Instead of sending the fan-out messages to all of the followers, the leader transmits these to a small-scale laid of relay nodes, which propagate the messages to the remaining followers. The relay nodes also human activity equally aggregators for the fan-in communication of the followers’ responses, in addition to exceed the combined results to the leader.
For simplicity sake, PigPaxos divides the entire cluster into a small-scale static break of relay groups, in addition to a unmarried relay node is chosen from each relay grouping randomly for every circular trip communication round. We usage PigPaxos amongst the MultiPaxos optimization hence entirely the phase-2 communication is performed inwards the normal case.
The randomized rotation of the relay nodes provide a large relief from communication bottlenecks. True, a relay node has its run cutting out if it needs to aggregate responses from 10 nodes inwards its cluster. But since the relay nodes randomly rotate, a item relay node volition hold upwardly off the claw for the several consequent rounds, in addition to volition hold upwardly able to procedure these messages in addition to ship to the leader without getting overwhelmed.
The randomized rotation of the relay nodes also assist for improving liveness when a relay crash occurs. Moreover, to guard against crashed or sluggish follower nodes, a timeout is used for setting a fourth dimension threshold for followers inwards the grouping to reply. When the timeout occurs, the relay node acks to the leader amongst a partial aggregation.
Paxos to PigPaxos mapping
PigPaxos generalizes the communication implementation of the Paxos protocol. Paxos has $N-1$ groups, where each grouping has ane chemical component in addition to the groups create non intersect amongst each other. In contrast inwards PigPaxos at that topographic point are $p$ groups where $p \in \{ 1..N-1\}$.We banknote that the security in addition to liveness proofs of Paxos create non depend on the communication implementation betwixt the leader in addition to follower nodes. In Paxos, maintaining correctness inwards spite of failures is guaranteed yesteryear quorum size in addition to the information exchanged inside the quorums, in addition to the proofs are oblivious to how communication of this information is achieved. Therefore, PigPaxos preserves the security in addition to liveness properties of Paxos, equally it entirely modifies the communication implementation. For reasoning nearly liveness, the message period of time designing in addition to the usage of relay nodes requires particular attention, equally failure of a relay node has disproportionate impact compared to the failure of a regular follower. Since PigPaxos uses random choice of relay/aggregator nodes at each round, it circumvents this occupation in addition to retains liveness.
Evaluation
We implemented PigPaxos inwards our Paxi framework amongst almost no changes to the center Paxos code, equally nosotros focused entirely on the message passing layer in addition to relay grouping orchestration. The entire protocol was implemented inwards simply 1200 lines of code. For evaluation nosotros deployed the protocols on a cluster of upwardly to 25 AWS EC2 m5a nodes amongst 2 vCPUs in addition to 8 GB of RAM.The PigPaxos leader communicates amongst entirely a handful of nodes for each consensus instance. Naturally, nosotros wanted to come across if relay groups acquire overwhelmed yesteryear the extra communication burden. To our surprise, PigPaxos amongst simply 2 relay groups performed the best inwards a 25 node deployment. This suggests that the leader soundless remains a bottleneck in addition to the relay groups soundless take away hold resources to spare. As shown inwards Figure 7, for a cluster of “25 nodes divided into 2 relay groups”, PigPaxos had nearly 25% payoff inwards maximum throughput compared to a 5-relay grouping setup.
The functioning relative to Multi-Paxos on a 25-node cluster is astonishing (Figure 8). PigPaxos shows nearly 3.5 flexure improvement inwards throughput compared to Paxos in addition to 10 folds improvement over EPaxos.
The benefits of PigPaxos create non halt at the large clusters. To our surprise, nosotros observed improve throughput from PigPaxos on simply a five node cluster. Yes, PigPaxos has slightly higher latency due to the additional network hop, but it is soundless able to border out some throughput improvements.
Analysis
We dorsum our empirical results amongst uncomplicated back-of-the-envelope charge formulas. Based on our run in the SIGMOD paper, nosotros usage a uncomplicated count of messages processed yesteryear each node equally a heuristic for the node’s relative load. To showtime amongst the leader, its communication is no longer subject on N, or the break of nodes inwards the cluster, in addition to instead it is a linear component of r, the break of relay groups, plus 2 messages (incoming in addition to outgoing) talking to the client:
Computing the relative message charge on the follower is to a greater extent than involved, equally nosotros take away to concern human relationship the roles the follower tin give the sack bring on in addition to the probability of each role:
Plugging our experimental configuration into these uncomplicated formulas shows us that the relay nodes are never a bottleneck (regardless of both the break of nodes due north in addition to break of relay groups r), in addition to keeping the break of relay nodes small-scale tin give the sack motion the entire organisation closer to the charge parity betwixt the leader in addition to followers. The ground the relay nodes don’t travel a bottleneck is because the random alternation of the relay nodes shields them from becoming hotspots: the extra traffic charge a relay node incurs inwards ane circular is offset inwards consecutive rounds when the node no longer serves equally relay.
The drawback amongst using a really small-scale break of relay nodes (say entirely 1 relay grouping inwards the extreme) is that the functioning becomes likewise fragile: few sluggish or crushed nodes may strength the organisation to hold off the timeouts at the relay groups. Using to a greater extent than relay groups hateful the leader volition have bulk confirmation fifty-fifty inwards the presence of sluggish/crashed nodes belongings dorsum the response from some relay groups. Finally the charge that randomly rotated relays tin give the sack acquire by is probable to hold upwardly express inwards exercise due to hw/sw limitations at the node, and that is also a factor to hold upwardly explored.
Future work
There are several optimizations to assist the Pig travel Big. In our implementation, the relay grouping partitioning was static to tumble out things simple. Using dynamic relay groups nosotros may add together fifty-fifty to a greater extent than randomness to the communication process, in addition to tin give the sack bargain amongst failures in addition to sluggish nodes to a greater extent than smoothly. Another optimization is to brand the relay nodes response to the leader earlier collecting all the responses from peers: collecting ane fewer response is non probable to impact the might to compass bulk quorum at the leader, but it prevents incurring a timeout due to ane sluggish node inwards the relay group. More experiments are underway to come across how this Pig squeals.Here is a link to our newspaper again, if yous are looking for some quarantine reading.
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