Sdpaxos: Edifice Efficient Semi-Decentralized Geo-Replicated Blue Planet Machines

In the in conclusion decade, the Paxos protocol menage unit of measurement grew amongst the improver of novel categories.

• Rotating leader: Mencius
• Leaderless: EPaxos, Fast Paxos
• Paxos federations: Spanner, vertical Paxos 
• Dynamic key-leader: WPaxos 

This paper, which appeared inwards SOCC 18, proposes SDPaxos which prescribes separating the command airplane (single leader) from the replication airplane (multiple leaders). SD inwards SDPaxos stands for "semi-decentralized".

The motivation for this stems from the next observation. Single leader Paxos approach has a centralized leader together with runs into performance bottleneck problems. On the other hand, the leaderless (or opportunistic multileader) approach is fully decentralized but suffers from the conflicting command problems. Taking a hybrid approach to capture the best of both worlds, SDPaxos makes the command-leaders to hold upward decentralized (the closest replica tin mail away atomic number 82 the command), but the ordering-leader (i.e., the sequencer) is soundless centralized/unique inwards the system.

Below I give a brief explanation of the Paxos protocol categories earlier I speak over how SDPaxos compares together with contrasts amongst those.

Plain vanilla Paxos

Paxos provides error tolerant consensus amid a laid of nodes.

• Agreement: No ii right nodes tin mail away create upward one's heed on different values.
• Validity: If all initial values are same, nodes must create upward one's heed that value.
• Termination: Correct nodes create upward one's heed eventually.

Paxos runs inwards 3 phases: suggest (phase-1), have got (phase-2), together with commit (phase-3).

1. A node tries to exceed the leader past times proposing a unique ballot number b to its followers amongst a phase-1a message. The followers admit a leader amongst the highest ballot seen so far, or refuse it amongst a ballot seen amongst a number greater than b. Receiving whatsoever rejection fails the candidate. 
2. In the absence of a rejection, a node becomes leader together with advances to phase-2 after receiving a bulk quorum of acknowledgments. In this phase, the leader chooses a suitable value v for its ballot. The value would hold upward some uncommitted value associated amongst the highest ballot learned inwards previous phase, or a novel value if no pending value exists. The leader commands its followers to have got the value v together with waits for acknowledgement messages. Once the bulk of followers admit the value, it becomes anchored together with cannot hold upward revoked. Again a unmarried rejection message (carrying an observed higher ballot number) received inwards phase-2b nullifies the leadership of the node, together with sends it dorsum to phase-1 to elbow grease amongst a higher ballot number. 
3. Finally, the leader sends a commit message inwards phase-3 that allows the followers to commit together with apply the value to their respected nation machines.

It's of import to run into that after phase-2, an anchored value cannot hold upward overridden after equally it is guaranteed that whatsoever leader amongst higher ballot number would larn it equally role of its phase-1 earlier proposing a value inwards its phase-2.

You tin mail away reveal Modeling Paxos inwards TLA+ could hold upward of involvement to you.

Single leader approach

The traditional single-leader Paxos protocol employs a centralized leader to procedure all customer requests together with suggest commands. The unmarried leader takes a significantly heavier charge than the other replicas, together with becomes a performance bottleneck. Moreover, inwards geo-replication, clients non co-located amongst the leader involve to mail the requests to the remote leader, which incurs significantly higher wide-area network latency.

Mencius approach

Mencius is a multileader version of Paxos that aims to eliminate the unmarried leader bottleneck inwards Paxos. Mencius achieves charge balancing past times partitioning the consensus instances amid multiple servers. E.g., if nosotros have got 3 servers, server 0 is responsible for acting equally a leader for consensus instances numbered 0,3,6, server 1 for 1,4,7, together with server 2 for 2,5,8, etc. Mencius tries to avoid the straggler work past times making the replicas skip their turns when they autumn behind, however, it cannot fully eliminate the slow-down. Since it uses multiple leaders, Mencius besides loses out on the "serve reads locally at the leader" optimization possible inwards Paxos.

Optimistic leaders approach

EPaxos is a  solution, where every node tin mail away opportunistically exceed a leader for some command together with commit it. When a command does non interfere amongst other concurrent commands, it is committed inwards a unmarried circular after receiving the acks from a fast quorum (which is some 3/4ths of all nodes). In a sense, EPaxos compresses the phase-2 to hold upward a role of phase-1 when in that place are no conflicts. However, if the fast quorum detects a conflict betwixt the commands, EPaxos defaults dorsum to the traditional Paxos mode together with proceeds amongst a minute stage to flora social club on the conflicting commands.

Unfortunately, services similar E-commerce together with social network tin mail away generate high-contention workload, amongst many interfering commands on the same object from multiple clients. This work is aggrevated inwards broad expanse network deployments: since requests convey much longer fourth dimension to finish, the probability of controversy rises.

Multileader approach

Spanner together with CockroachDB are examples of databases that uses a federation of Paxos groups to orbit different partitions/shards. These partitioned consensus systems employ some other solution on top (such equally vertical Paxos) for relocating/assigning information from 1 Paxos grouping to another.

WPaxos uses sharding of the key infinite together with takes wages of flexible quorums thought to better WAN performance, peculiarly inwards the presence of access locality. In WPaxos, every node tin mail away ain some objects/microshards together with operate on these independently. Unlike Vertical Paxos, WPaxos does non consider changing the object/microshard ownership equally a reconfiguration performance together with does non require an external consensus group. Instead WPaxos performs object/microshard migration betwixt leaders past times carrying out a phase-1 across the WAN amongst a higher ballot number, together with commands are committed via phase-2 inside the portion or neighboring regions.

The SDPaxos approach

The origin of the inefficiency of leaderless together with multileader protocols is the decentralized coordination pattern. Although decentralization addresses the single-leader bottleneck equally every replica tin mail away suggest commands, the replicas soundless involve to concur on a total social club on conflicting commands proposed past times different replicas to avoid inconsistent state.

To address this issue, SDPaxos divides the consensus protocol inwards 2 parts: durably replicating each command across replicas without global social club (via C-instance Paxos), together with ordering all commands to enforce the consistency guarantee (via O-instance Paxos). Replicating via C-instance Paxos is completely decentralized where every replica tin mail away freely suggest commands together with replicate them to other replicas. This evenly distributes the charge amid replicas, together with enables clients to ever contact the nearest one. On the other hand, equally role of O-instance Paxos, 1 replica is elected equally the sequencer together with handles the ordering inwards a centralized manner: the global persuasion enables this replica to ever social club commands appropriately. Provided that the ordering messages are smaller than replication messages, the charge on the sequencer volition non hold upward equally severe equally that on the unmarried leader inwards Paxos.

Fault tolerance is provided equally both the replicating together with ordering instances are conducted based on Paxos. Each replica proposes commands inwards a series of C-instances of its ain to create its partial log. The sequencer proposes replicas' IDs inwards O-instances to create an assignment log. Based on the assignment log, all replicas' partial logs are finally merged into a global log.

Comparison amongst other protocols

The separation betwixt C-instances together with O-instances is the source of SDPaxos's advantages over existing protocols. The decentralization distributes charge evenly across replicas, together with allows clients to ever contact the nearest replica inwards geo-replication to serve equally the command leader. The O-instance leader, i.e., the sequencer, provides conflict complimentary operation.

So, SDPaxos is similar Paxos, but it has local leader inwards each region. This agency it avoids the terms of going to the leader, together with back.

Also SDPaxos is similar EPaxos but amongst no conflicts, ever!

In SDPaxos, the sequencer is 1 node, but is backed past times O-instances. A novel sequencer tin mail away hold upward chosen easily inwards a fault-tolerant agency using Phase-1 of Paxos over O-instances. This alleviates the availability problems due to the serializer failure inwards systems that utilisation Paxos for serializing the log inwards a fundamental region. In such systems (e.g., Calvin) if the unmarried log serializer is Paxos-replicated inside a region, together with so the availability suffers on portion failure. Instead, if the serializer is Paxos-replicated across regions together with so the performance suffers.

The protocol

In this example, upon receiving a customer asking for a command, replica R0 becomes the command leader of this command, picks 1 of its ain C-instance together with replicates the command to others (using the C-accept, i.e., Accept stage message of the C-instance). In the meantime, this C-accept besides informs the sequencer (R2) to commencement an O-instance for this command. Then R2 proposes R0’s ID inwards the side past times side (e.g., the jth) O-instance together with sends O-accepts to others, to assign this command to the jth global slot. Replicas volition together with so have got these instances together with mail C-ACKs together with O-ACKs to R0; R2 besides sends an O-ACK equally it has sent an O-accept to itself. The algorithm denotes the ith C-instance of Rn equally Cni, together with the jth O-instance equally Oj.

A C-instance tin mail away come upward from whatsoever node without a Paxos phase-1a, because each replica has its ain distinct replication log for C-instance. The C-instance messages do non conflict amongst each other together with gets accepted immediately. The C-instance messages do non fifty-fifty involve a ballotnum; the ballotnum used is that of the O-instance to announce epoch (i.e., which sequencer the sender thinks is soundless in-charge).

A command beingness create requires the C-instance together with plenty number of O-instances hold upward committed. The weather condition of an instance beingness committed together with a command beingness create are defined inwards lines eighteen through 31, which nosotros speak over next. There are ii questions here.

• The security enquiry is: How do nosotros ensure that the replication is anchored (performed at the bulk quorum) from the command leader perspective?
• The performance enquiry is: How do nosotros attain consensus inwards 1 circular piece satisfying the security concern? 

The 1-round feat

In the best instance when O-instance of the sequencer overlaps perfectly amongst the C-instance of the command replication leader, consensus is achieved inwards 1 round-trip fourth dimension ---the optimal possible. But, since the O-instance starts one-half a circular trip after than the C-instance for non-sequencer replicas, it is non ever possible to optimize the O-instance completion to precisely one-half a circular trip to attain the one-round-trip latency. But the newspaper shows how this tin mail away hold upward achieved for N=3 together with N=5 replicas. In groups amongst to a greater extent than than five replicas, the O-instances soundless involve 1 circular trip, so the overall latency remains at 1.5 round-trips.


In 3-replica groups, when the command leader receives O-ACK from the sequencer, the bulk (2 out of 3) is readily established for O-instance completion. This provides the one-round-trip consensus. (For the case, the command leader is besides the sequencer, sequencing tin mail away hold upward done inwards advance together with 1 round-trip is satisfied equally well.)


In 5-replica groups, a command tin mail away besides hold upward create inwards 1 circular trip, but dissimilar the instance of iii replicas, an O-instance cannot hold upward accepted past times a bulk inwards one-half a circular trip. Instead, SDPaxos lets each non-sequencer replica commit an O-instance, upon receiving the O-accept from the sequencer (line 29). Here, the O-instance does non rigorously follow Paxos, which raises a complication for recovery: if this non-sequencer replica together with the sequencer fail, nosotros cannot recover this O-instance only past times Paxos because the other hold upward replicas may have got non seen the O-accept yet. However, the newspaper discusses a agency for SDPaxos to correctly recover the O-instances of all replicas' create commands fifty-fifty inwards such cases (omitted inwards my review), together with is able to allow for 1-round-trip commits for N=5.


Note that the dynamic per key leader approach inwards WPaxos soundless has an border over SDPaxos when in that place is proficient locality inwards the workload (which is frequently the instance inwards practice) and/or when the number of replicas is greater than five (which is frequently the instance for geo-replicated databases). It may hold upward possible to utilisation WPaxos for coordination across regions together with integrate SDPaxos for coordination inside the portion upto five replicas.

Optimizations

As an optimization for reads, SDPaxos uses sequencer leases to authorize the sequencer to straight reply to the read requests. In contrast, such an optimization is non possible for leaderless approaches, equally in that place is no single/dedicated leader to lease together with read from.

As some other optimization, inwards some cases, it is possible to split the responsibleness of sequencer to all replicas for to a greater extent than charge balancing. For example, inwards a key-value store, nosotros tin mail away sectionalization the key infinite using approaches similar consistent hashing, together with so brand each replica social club the commands on 1 sectionalization (commands on different keys tin mail away hold upward out-of-order). Again, inwards this case, it would hold upward possible to utilisation the WPaxos approach for prophylactic key-stealing amid the sequencers, together with dynamically conform to the access pattern, rather than beingness confined to static partitioning.

Evaluation

They implemented a epitome of SDPaxos, together with compared its performance amongst typical single-leader (Multi-Paxos) together with multileader (Mencius, EPaxos) protocols. The experiment results demonstrate that SDPaxos achieves: (1) 1.6× the throughput of Mencius amongst a straggler, (2) stable performance nether different controversy degrees together with 1.7× the throughput of EPaxos fifty-fifty amongst a depression controversy charge per unit of measurement of 5%, (3) 6.1× the throughput of Multi-Paxos without straggler or contention, (4) 4.6× the throughput of writes when performing reads, together with (5) upward to 61% together with 99% lower wide-area latency of writes together with reads than other protocols.

MAD questions

1. Does SDPaxos assistance amongst the leader bottleneck significantly?
I wrote higher upward that "Provided that the ordering messages are smaller than the replication messages, the charge on the sequencer volition non hold upward equally severe equally that on the unmarried leader inwards Paxos." But on unopen inspection I don't recall I believe that judgement anymore.  Ailidani, Aleksey, together with I have got done a detailed bottleneck analysis of Paxos protocol categories (under submission), together with nosotros found that the outcast messages are non the biggest source of bottleneck for the leader, equally they are serialized 1 time earlier beingness sent out to the replicas. The incast messages contribute almost to the bottleneck, equally the CPU needs to procedure them 1 past times 1 together with they queue up. Moreover, the incast messages are ACK messages, which are already small, together with SDPaxos does non brand them smaller. So, mayhap SDPaxos does non better significantly on the single-leader bottleneck inwards Paxos. On the other hand, it is truthful that SDPaxos helps distribute the customer asking charge to the C-instance replicas relieving that bottleneck, together with it definitely helps amongst lowering the latency inwards WAN.

2. How would yous create reconfiguration of participants for this protocol? 
Reconfiguration of participants is of import to re-establish fault-tolerance capability past times replacing failed replicas past times accepting fresh novel replicas to the system. How would reconfiguration travel for SDPaxos? Would Raft's 2-phase reconfiguration method apply readily for SDPaxos?


3. What are additional challenges for efficient strongly-consistent geo-replication implementation at scale?
I am at Microsoft Azure Cosmos DB for my sabbatical. Cosmos DB late introduced general-availability of multiple write regions. While providing strong-consistency amongst multi-master writes allowed from whatsoever portion has terms across the globe, could SDPaxos ideas assistance better efficiency further?

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