Hermes: A Fast Fault-Tolerant Together With Linearizable Replication Protocol

This paper (from ASPLOS'20 which was held remotely) is past times Antonios Katsarakis, Vasilis Gavrielatos, M. R. Siavash Katebzadeh, Arpit Joshi, Aleksandar Dragojevic, Boris Grot, Vijay Nagarajan. The newspaper has its ain website, where y'all tin acquire to the video presentation, slides, as well as code.

Introduction

Hermes is a replication protocol that guarantees linearizability. It enables local reads: a customer tin execute a read locally on whatever of the replicas. Hermes enables whatever replica to coordinate a write to a key, as well as supports concurrent writes to unlike keys quickly.

Too goodness to hold out true? You possess got to read the protocol department below to run into how these are achieved.

But if y'all are a distributed systems expert, hither is my shortcut explanation of the protocol. Hermes is simply chain replication (with CRAQ optimization) deployed with the next "chain" topology:

1. The caput as well as tail node of the chain is colocated inwards 1 node, called the coordinator
2. The intermediate nodes of the "chain" are all parallel-connected (rather than serial) to the coordinator

In chain replication (or CRAQ) the replication goes inwards a series manner, but  the chain replication newspaper mentioned that parallel wiring is possible equally long equally y'all possess got 1 caput as well as 1 tail. And when y'all colocate the caput as well as the tail inwards the same node, Voila!, y'all possess got Hermes. To build clean the replicas for reads from whatever replica, the tail (i.e., the coordinator) sends an ACK message equally inwards CRAQ. (see Figure 2 below.)

In improver to solving the latency job inwards chain replication past times using parallel-wiring, Hermes too allows multiple coordinators to aid residual the coordinating charge across nodes. Any node tin hold out a coordinator for whatever key. Thanks to the logical clock timestamping (using node-ids equally tie-breakers), the writes are total-ordered inwards Hermes. Since higher timestamped writes invalidate lower timestamped writes, the number of concurrent writes volition hold out the same at each node, as well as linearizability is achieved fifty-fifty with local reads.

I summarize Hermes below, as well as and so hash out how Hermes compare with Paxos-based protocols.

Protocol

Writes tin hold out initiated past times whatever replica:

1. the replica initiating the write (called coordinator) broadcasts an Invalidation (INV) message to the ease of the replicas (called followers) as well as waits on acknowledgments (ACKs)
2. once all ACKs possess got been received; the write completes via a Validation (VAL) message broadcast past times the coordinator replica

As the get-go step, the coordinator invalidates replicas for this key, so that linearizability is non violated if a customer reads the key from a node that has an one-time value.

A read asking tin hold out served locally on whatever operational replica (i.e., 1 with a lease from the membership service). The replica returns the local value of the requested key exclusively if it is inwards the Valid state. When an INV message for a key is received, the replica is placed inwards an Invalid Blue Planet for that key, pregnant that reads to the key cannot hold out served past times the replica.

Membership service

In the write performance described above, the coordinator waits to listen an ACK from each replica. If a replica crashes, this results inwards the write to acquire stuck forever, right? To address this issue, at that topographic point is a scream for for detecting crashed nodes.

Instead of making each node possess got a failure detector ---which is difficult to maintain consistent---, Hermes (similar to chain replication) employs an external Paxos-backed configuration/membership service that decides on the wellness of the nodes. This service acts equally a unmarried consistent (but non necessarily perfect) failure detector for the replicas. It becomes the sole source of "truth/perspective": While it tin hold out false inwards its judgment, it keeps every replica inwards Hermes consistent with honour to their sentiment of which nodes are good for y'all as well as role of the protocol.

This Paxos-powered membership/configuration service changes configuration/view when needed, as well as at each view-change it increases the epoch number. This keeps Hermes condom (and eventually live) inwards a partially synchronous environs ---with bouts of asynchrony.

Well, at that topographic point is nevertheless the job with lease security at replication nodes. Each replica scream for a lease from the membership service for this to locomote (again equally inwards chain replication). See the fault-tolerance department below for how this is handled.

Concurrent writes

Hermes allows writes to unlike keys to drib dead along inwards parallel for impoving the throughput.

As for concurrent writes to the same key, invalidations addition logical timestamps impose a total social club on these writes. This prevents conflicts as well as aborts, as well as ensures that those are correctly linearized at the replicas.

A coordinator node issues a write to a key exclusively if it is inwards the Valid state; otherwise the write is stalled. This doesn't appear to hold out necessary for safety, because the higher timestamped writes volition preempt the lower timestamped writes. So why does Hermes practise this? I scream upwards they practise this, because it acquire replicas run into the writes concluded, fifty-fifty when at that topographic point is a deluge of writes to the same key. This may inwards plow aid alleviate the read starvation due to constant overflowing of writes to the same key. I constitute this inwards the slack channel for ASPLOS'20 from the get-go author:

  It is condom for a read that initially constitute the object invalidated with version timestamp 2 as well as and so after invalidated with a version timestamp iii to acquire serialized as well as furnish the version 2 value. Intuitively this is partly condom because a write with version iii could non possess got started unless the write with version 2 has been committed. 

This assumes no epoch change, I presume. A duad sections below, I volition hash out close our Paxos-Quorum-Reads technique which does a like thing, but without blocking the writes to aspect for before writes to finish, as well as without requiring leases or a configuration/membership service.

Read-modify-write updates

Linearizability is non the whole story. You tin acquire linearizability inwards Cassandra, using the ABD algorithm, which is non fifty-fifty bailiwick to FLP. But the job is ABD is non consensus, as well as it is non goodness lonely for maintaining Blue Planet machine replication. 

Hermes is trying to practise to a greater extent than as well as accomplish Blue Planet machine replication. It enforces the replicas to possess got the same log inwards the same social club (for the same key). The newspaper too shows how Hermes tin back upwards read-modify-write (RMW) updates, an atomic execution of a read followed past times a write to a key (e.g., a compare-and- swap to acquire a lock).

  An RMW update inwards Hermes is executed similarly to a write, but it is conflicting. An RMW which is concurrently executed with about other update performance to the same key may acquire aborted. Hermes commits an RMW if as well as exclusively if the RMW has the highest timestamp alongside whatever concurrent updates to that key. Moreover, it purposefully assigns higher timestamps to writes compared to their concurrent RMWs. As a result, whatever write racing with an RMW to a given key is guaranteed to possess got a higher timestamp, thus safely aborting the RMW. Meanwhile, if exclusively RMW updates are racing, the RMW with the highest node id volition commit, as well as the ease volition abort.

A recent paper, Gryff inwards NSDI20, too investigates this problem. It uses the ABD algorithm for read-write registers, as well as EPaxos inwards conjunction with consensus-after-register timestamps (carstamps) for the RMW updates. In Gryff, the RMW operations practise non acquire aborted, they exactly acquire ordered correctly past times EPaxos fifty-fifty after a conflict.

While nosotros are on the theme of related work, I wonder how Hermes compares with RIFL:
Implementing Linearizability at Large Scale as well as Low Latency (SOSP'15). The newspaper does non cite RIFL, but it would hold out prissy to compare as well as contrast the ii protocols.

Fault-tolerance

Hermes seamlessly recovers from a gain of node as well as network faults thank y'all to its write replays, enabled past times early value propagation.

  Node as well as network faults during a write to a key may acquire out the key inwards a permanently Invalid Blue Planet inwards about or all of the nodes. To forestall this, Hermes allows whatever invalidated operational replica to replay the write to completion without violating linearizability. This is accomplished using ii mechanisms. First, the novel value for a key is propagated to the replicas inwards INV messages (see Figure 2). Such early on value propagation guarantees that every invalidated node is aware of the novel value. Secondly, logical timestamps enable a precise global ordering of writes inwards each of the replicas. By combining these ideas, a node that finds a key inwards an Invalid Blue Planet for an extended current tin safely replay a write past times taking on a coordinator role as well as retransmitting INV messages to the replica ensemble with the master copy timestamp (i.e., master copy version number as well as cid), therefore preserving the global write order.

For fault-tolerance, the membership service as well as leases at replicas play a cardinal role. If 1 replica is partitioned out, the coordinator cannot brand progress unless the membership service updates the membership to withdraw that replica. The membership service waits until the lease it granted to the partitioned replica expires. The lease expiration makes the replica invalid. The membership service as well as so increases epoch number, as well as disseminates the novel membership information to the replicas, as well as the coordinator (or whatever other replica node via the early on value propagation technique) tin brand progress.

Protocol-level comparing to Paxos based solutions, as well as PQR

The circular trip as well as a one-half protocol inwards Hermes has similarities (at to the lowest degree inwards damage of performance bottleneck characteristics) to the Phase-2 "accept" as well as Phase-3 "commit" the Paxos leader (via MultiPaxos optimization) performs with the followers.

The prissy affair close Paxos based solutions is that at that topographic point is no exterior membership/reconfiguration box needed inwards that solution. Below let's hash out how good Paxos-based solutions tin gibe upwards to Hermes's features.

Hermes distributes the coordination charge across replicas. EPaxos has the same characteristic due to opportunistic leaders approach. It is too possible to deploy Paxos with per-key sharding to the leaders (this was mentioned as well as compared with inwards EPaxos newspaper I think). In our WPaxos protocol, nosotros improved over the per-key sharding to the leaders approach, as well as showed how WPaxos tin outperform it past times stealing keys as well as assigning it to the closest leaders to improve performance based on the access designing inwards the workload.

Hermes does local reads from 1 replica. Megastore from Google too allows local reads from 1 replica with back upwards from coordinators.

In our recent work, nosotros introduced Paxos Quorum Reads (PQR) as well as showed how to perform linearizable reads from Paxos protocols without involving the leader as well as without using whatever leases. Since PQR does non require leases, it industrial plant inwards an asynchronous environment. PQR requires reading from bulk of the nodes, to select take hold of if at that topographic point has been a newer pending update to the key. If at that topographic point is a pending update, the clearing of the read tin hold out done past times exactly barriering on 1 replica. It is possible to relax the initial majority-quorum read as well as instead work fewer number of nodes past times using a larger write quorum. While reading from multiple nodes inwards parallel requires to a greater extent than messages, it does non increment the latency. See this brief summary of Paxos Quorum Reads to acquire more.

Evaluation

Hermes is evaluated over an RDMA-enabled reliable datastore with v replicas. The evaluation compares Hermes with ZAB as well as CRAQ. At 5% writes, the tail latency of Hermes is 3.6× lower than that of CRAQ as well as ZAB.

The performance improvement inwards Hermes, I scream upwards comes from using multiple coordinators, which was non made available to ZAB or CRAQ.

The figures demo that "fewer writes=more reads" is improve for Hermes because of the local reads inwards Hermes. On the other hand, observe that for uniform key distribution workloads, CRAQ is equally goodness equally Hermes for throughput fifty-fifty though the replicas at that topographic point are serially wired instead of parallel-wired.

For latency, the improvement due to parallel-wiring replicas is significant.

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