Constructing and deploying MySQL Raft at Meta

• We’re rolling out MySQL Raft with the purpose to ultimately exchange our present MySQL semisynchronous databases. 
• The most important win of MySQL Raft was simplification of the operation and making MySQL servers deal with promotions and membership. This gave the provable security of Raft and diminished important operational ache.
• Making MySQL server a real distributed system additionally has opened up potentialities in downstream techniques to leverage it. A few of these concepts are beginning to take form.

At Meta, we run one of many largest deployments of MySQL on the earth. The deployment powers the social graph together with many different providers, like Messaging, Adverts, and Feed. Over the previous couple of years, we now have carried out MySQL Raft, a Raft consensus engine that was built-in with MySQL to construct a replicated state machine. We’ve migrated a big portion of our deployment to MySQL Raft and plan to completely exchange the present MySQL semisynchronous databases with it. The mission has delivered important advantages to the MySQL deployment at Meta, together with increased reliability, provable security, important enhancements in failover time, and operational simplicity — all with equal or comparable write efficiency.

Background

To permit for prime availability, fault tolerance, and scaling reads, Meta’s MySQL datastore is a massively sharded, geo-replicated deployment with hundreds of thousands of shards, holding petabytes of knowledge. The deployment consists of hundreds of machines operating over a number of areas and knowledge facilities throughout a number of continents.

Beforehand, our replication resolution used the MySQL semisynchronous (semisync) replication protocol. This was a knowledge path–solely protocol. The MySQL main would use semisynchronous replication to 2 log-only replicas (logtailers) inside the main area however exterior of the first’s failure area. These two logtailers would act as semisynchronous ACKer (An ACK is an acknowledgment to the first that the transaction has been regionally written).  This is able to permit the info path to have very low latency (sub-millisecond) commits and would supply excessive availability/sturdiness for the writes. Common MySQL primary-to-replica asynchronous replication was used for wider distribution to different areas.

The management aircraft operations (e.g., promotions, failover, and membership change) could be the accountability of a set of Python daemons (henceforth known as automation). Automation would do the required orchestration to advertise a brand new MySQL server in a failover location as a main. The automation would additionally level the earlier main and the remaining replicas to duplicate from the brand new main. Membership change operations could be orchestrated by one other piece of automation known as MySQL pool scanner (MPS). So as to add a brand new member, MPS would level the brand new reproduction to the first and add it to the service discovery retailer. A failover could be a extra advanced operation by which the tailing threads of the logtailers (semisynchronous ACKers) could be shut right down to fence the earlier lifeless main.

Why was MySQL Raft essential?

Prior to now, to assist assure security and keep away from knowledge loss through the advanced promotion and failover operations, a number of automation daemons and scripts would use locking, orchestration steps, a fencing mechanism, and SMC,  a service discovery system. It was a distributed setup, and it was tough to perform this atomically. The automation turned extra advanced and more durable to keep up over time as increasingly more nook instances wanted to be patched.

We determined to take a totally totally different method. We enhanced MySQL and made it a real distributed system. Realizing that management aircraft operations like promotions and membership adjustments have been the set off of most points, we wished the management aircraft and knowledge aircraft operations to be a part of the identical replicated log. For this, we used the well-understood consensus protocol Raft. This additionally meant that the supply of fact of membership and management moved contained in the server (mysqld). This was the one largest contribution of bringing in Raft as a result of it enabled provable correctness (security property) throughout promotions and membership adjustments into the MySQL server.

The Raft library and the MySQL Raft plugin

Our implementation of Raft for MySQL relies on Apache Kudu. We enhanced it considerably for the wants of MySQL and our deployment. We printed this fork as an open supply mission, kuduraft.

A number of the key options that we added to kuduraft are:

• FlexiRaft — assist for 2 totally different intersecting quorums: the info quorum and the chief election quorum
• Proxying — the flexibility to make use of a proxy intermediate node to scale back community bandwidth
• Compression — the place we compress binary log (transaction) payloads as soon as earlier than distribution
• Log abstraction — to assist totally different bodily logfile implementations
• Major ban — the flexibility to stop some entities from being main quickly

We additionally needed to make comparatively large adjustments to MySQL replication to interface with Raft. For this, we created a brand new closed supply MySQL plugin known as MyRaft. MySQL would interface with MyRaft via the plugin APIs (related APIs had been used for semisync as effectively), whereas we created a separate API for MyRaft to interface again with MySQL server (callbacks).

MySQL Raft replication topologies

A Raft ring would include a number of MySQL situations (4 in the diagram) in numerous areas. The communication round-trip time (RTT) between these areas would vary from 10 to 100 milliseconds. A number of of those MySQLs (sometimes three) have been allowed to change into primaries, whereas the remainder of them have been solely allowed to be pure learn replicas (non-primary-capable). The MySQL deployment at Meta additionally has a long-standing requirement for terribly low latency commits. The providers that use MySQL as a retailer (e.g., the social graph) want or have been designed to such extraordinarily quick writes.

To fulfill this requirement, the configuration of FlexiRaft would use solely in-region commits (single area dynamic mode). To allow this, every main succesful area would have two further logtailers (witnesses or log-only entities). The info quorum for writes could be 2/3 (2 ACKs out of the 1 MySQL + 2 logtailers). Raft would nonetheless handle and run a replicated log throughout all of the entities (1 primary-capable MySQL + 2 logtailers ) * 3 areas + (non-primary-capable MySQL) * 3 areas = 12 entities.

Raft roles: The chief, because the identify suggests, is the chief in a time period of the replicated log. A pacesetter in Raft would even be the first in MySQL and the one accepting consumer writes. The follower is a voting member of the ring and passively receives messages (AppendEntries) from the chief. A follower could be a duplicate in MySQL’s perspective and could be making use of the transactions to its engine. It could not permit direct writes from consumer connections (read_only=1 is ready). A learner could be a non-voting member of the ring, e.g., the three MySQLs in non-primary-capable areas (above). It could be a duplicate in MySQL’s perspective.

Replicated log

For replication, MySQL has traditionally used the binary log format. This format is central to MySQL’s replication, and we determined to protect this. From the Raft perspective, the binary log turned the replicated log. This was accomplished by way of the log abstraction enchancment to kuduraft. The MySQL transactions could be encoded as a sequence of occasions (e.g., Replace Rows occasion) with a begin and finish for every transaction. The binary log would even have acceptable headers and would sometimes finish with an ending occasion (Rotate occasion). 

We needed to tweak how MySQL manages its logs internally. On a main, Raft would write to a binlog. That is no totally different from what occurs in customary MySQL. In a duplicate, Raft would additionally write to a binlog as an alternative of to a separate relay log in customary MySQL. This created simplicity for Raft as there was just one namespace of log information that Raft could be involved about. If a follower have been promoted to chief, it may seamlessly return into its historical past of logs to ship transactions to lagging members. The reproduction’s applier threads would choose up transactions from the binlog after which apply them to the engine. Throughout this course of, a brand new log file, the apply log, could be created. This apply log would play an vital function in crash restoration of replicas however is in any other case a nonreplicated log file.

So, in abstract:

In customary MySQL:

• Major writes to binlog and sends binlog to replicas.
• Replicas obtain in relay log and apply the transactions to the engine. Throughout apply, a brand new replica-only binlog is created.

In MySQL Raft:

• Major writes to binlog by way of Raft, and Raft sends binlog to followers/replicas.
• Replicas/followers obtain in binlog and apply the transactions to the engine. An apply log is created throughout apply.
• Binlog is the replicated log from the Raft perspective.

Write transaction on MySQL main utilizing Raft

The transaction would first be ready within the engine. This is able to occur within the thread of the consumer connection. The act of making ready the transaction would contain interactions with the storage engine (e.g., InnoDB or MyRocks) and generate an in-memory binlog payload for the transaction. On the time of commit, the write would move via group commit/ordered_commit move. GTIDs could be assigned, after which Raft would assign an OpId (time period:index) to the transaction. At this level, Raft would compress the transaction, retailer it in its LogCache, and write via the transaction to a binlog file. It could asynchronously begin transport the transaction to different followers to get ACKs and attain consensus.

The consumer thread, which is in “commit” of the transaction, could be blocked, ready for consensus from Raft. When Raft would get two out of three in-region votes, consensus commit could be reached. Raft would additionally ship the transaction to all out-of-region members however would ignore their votes due to an algorithm known as FlexiRaft (described under). On consensus commit, the consumer thread could be unblocked, and the transaction would proceed and decide to the engine. After engine commit, the write question would end and return to the consumer. Quickly after, Raft would additionally asynchronously ship a commit marker (OpId of present commit) to downstream followers in order that they will additionally apply the transactions to their database. 

Crash restoration

Modifications needed to be made to crash restoration to make it work seamlessly with Raft. Crashes can occur at any time within the lifetime of a transaction and therefore the protocol has to make sure consistency of members. Listed below are some key insights on how we made it work.

1. Transaction was not flushed to binlog: On this case, the in-memory transaction payload (nonetheless in mysqld course of reminiscence as an in-memory buffer) could be misplaced and the ready transaction in engine could be rolled again on course of restart. Since there was no additional uncommitted transaction within the Raft log, no reconciliation with different members must be accomplished.
2. Transaction was flushed to binlog however by no means reached different members: Mysqld acts as a transaction coordinator and runs a two-phase commit protocol between the engine and the replicated binlog because the members. On crash restoration, the ready transaction in engine (e.g., InnoDB or MyRocks) could be rolled again (engine had not reached commit). Raft would undergo failover, and a brand new chief could be elected. This chief wouldn’t have this transaction in its binlog and henceforth would truncate this transaction from the erstwhile chief’s binlog due to to the next time period (by pushing a No-Op message), when the erstwhile chief joins again the ring.
3. Transaction was flushed to binlog and reached to subsequent chief. Present chief died earlier than committing to the engine: Just like no. 2 above, the ready transaction within the engine could be rolled again. The erstwhile chief would be part of the Raft ring as a follower. On this case, the brand new chief would have this transaction in its binlog and therefore no truncation would occur, for the reason that logs would match. When the commit marker is distributed by the brand new chief, the transaction could be reapplied once more from scratch.

Raft-initiated state machine transitions

Failover and common upkeep operations can set off management adjustments in Raft. After a pacesetter is elected, the MyRaft plugin would attempt to to transition the accompanying MySQL into main mode. For this, the plugin would orchestrate a set of steps. These callbacks from Raft → MySQL would abort in-flight transactions, roll again in-use GTIDs, transition the engine aspect log from apply-log to binlog, and ultimately set the correct read_only settings. This mechanism is advanced and at present not open sourced.

FlexiRaft

For the reason that Raft paper and Apache Kudu supported solely a single international quorum, it might not work effectively at Meta, the place rings have been giant however the knowledge path quorum wanted to be small.

To bypass this problem, we innovated on FlexiRaft, borrowing concepts from Flexible Paxos.

At a excessive degree, FlexiRaft permits Raft to have a unique knowledge commit quorum (small) however take a corresponding hit on the chief election quorum (giant). By following provable ensures of quorum intersection, FlexiRaft ensures that the longest log guidelines of Raft and the suitable quorum intersection will assure provable security.

FlexiRaft helps single area dynamic mode. On this mode, members are grouped collectively by their geo-region. The present quorum of Raft is determined by who the present chief is (therefore the identify “single area dynamic”). The info quorum is almost all of voters within the chief’s area. Throughout promotions, if phrases are steady, the Candidate will intersect with the final recognized chief’s area. FlexiRaft would additionally make sure that the quorum of the Candidate’s area can be attained, in any other case the following No-Op message may get caught. If within the uncommon case the phrases should not steady, Flexi Raft would attempt to determine a rising set of areas which must be intersected with for security or, within the worst case, would fall again to the N area intersection case of Versatile Paxos. Due to pre-elections and mock elections, the incidences of time period gaps are uncommon.

Management aircraft operations (promotions and membership adjustments)

With a view to serialize promotion and membership change occasions within the binlog, we hijacked the Rotate Occasion and Metadata occasion of the MySQL binary log format. These occasions would carry the equal of No-Op messages and add-member/remove-member operations of Raft. Apache Kudu didn’t have assist for joint consensus, therefore we solely permit one-at-a-time membership adjustments (you possibly can change the membership by just one entity in a single spherical to observe the principles of implicit quorum intersection).

Automation

With the implementation of MySQL Raft, we reached a really clear separation of considerations for the MySQL deployment. The MySQL server could be answerable for security by way of Raft’s replicated state machine. The no-data-loss assure could be provably enshrined within the server itself. Automation (Python scripts, daemons) would provoke management aircraft operations and monitor the well being of the fleet. It could additionally exchange members or do promotions by way of Raft throughout upkeep or when a bunch failure was detected. Now and again, automation may additionally change the regional placement of MySQL topology. Altering the automation to adapt to Raft was an enormous enterprise, spanning a number of years of growth and rollout effort.

Throughout extended upkeep occasions, automation would set management ban data on Raft. Raft would disallow these banned entities from changing into chief or evacuate them promptly on inadvertent election. The automation would additionally promote away from these areas into different areas.

Studying from rollouts and challenges encountered alongside the best way

Rolling out Raft to the fleet was an enormous studying for the staff. We initially developed Raft on MySQL 5.6 and needed to migrate to MySQL 8.0.

One of many key learnings was that whereas correctness was simpler to cause with Raft, the Raft protocol in itself doesn’t assist a lot within the concern of availability. Since our MySQL knowledge quorum was very small (two out of three in-region members), two dangerous entities within the area may just about shatter the quorum and convey down availability. The MySQL fleet undergoes quantity of churn each day (on account of upkeep, host failures, rebalancing operations), so initiating and doing membership adjustments promptly and accurately have been a key necessities for fixed availability. A big a part of the rollout effort was centered at doing logtailer and MySQL replacements promptly in order that the Raft quorums have been wholesome.

We needed to improve kuduraft to make it extra sturdy for availability. These enhancements weren’t a part of the core protocol however might be thought of as engineering add-ons to it. Kuduraft has the assist for pre-elections, however pre-elections are accomplished solely throughout a failover. Throughout a sleek switch of management, the designated Candidate strikes on to an actual election, bumping the time period. This results in caught leaders (kuduraft doesn’t do auto step-down). To handle this drawback, we added a mock elections characteristic, which was just like pre-elections however occurred solely upon a sleek switch of management. Since this was an async operation, it didn’t enhance promotion downtimes. A mock election would weed out instances the place an actual election would partially succeed and get caught.

Dealing with byzantine failures: Raft’s membership record is taken into account to be blessed by Raft itself. However through the provisioning of recent members, or due to races in automation, there might be weird instances of two totally different Raft rings intersecting. These zombie membership nodes needed to be weeded out and shouldn’t be capable of talk with one another. We carried out a characteristic to dam RPCs from such zombie members to the ring. This was, in some methods, a dealing with of a byzantine actor. We enhanced the Raft implementation after noticing these uncommon incidents that occurred in our deployment.  

Monitoring the MySQL Raft rollout

Whereas launching MySQL Raft, one of many targets was to scale back operational complexity for on-calls, in order that engineers may root-cause and mitigate points. We constructed a number of dashboards, CLI instruments, and scuba tables to watch Raft. We added copious logging to MySQL, particularly across the space of promotions and membership adjustments. We created CLIs for quorum and voting reviews on a hoop, which assist us rapidly establish when and why a hoop is unavailable (shattered quorum). The funding within the tooling and automation infrastructure went hand-in-hand and may need been a much bigger funding than the server adjustments. This funding paid off big-time and diminished operational and onboarding ache.

Quorum Fixer

Though it’s undesirable, quorums do get shattered every so often, resulting in availability loss. The standard case is when automation doesn’t detect unhealthy situations/logtailers within the ring and doesn’t exchange them rapidly. This may occur due to poor detection, employee queue overload, or a scarcity of spare host capability. Correlated failures, when a number of entities within the quorum go down on the identical time, are much less typical. These don’t occur typically, as a result of the deployments attempt to isolate failure domains throughout essential entities of the quorum via correct placement choices. Lengthy story quick: At scale, sudden issues occur, regardless of present safeguards. Instruments must be obtainable to mitigate such conditions in manufacturing. We constructed Quorum Fixer in anticipation of this.

Quorum Fixer is a handbook remediation software authored in Python that squelches the writes on the ring. It does out-of-band checks to determine the longest log entity. It forcibly adjustments the quorum expectations for a pacesetter election inside Raft, in order that the chosen entity turns into a pacesetter. After profitable promotion, we reset the quorum expectation again, and the ring sometimes turns into wholesome.

It was a aware resolution to not run this software robotically, as a result of we wish to root trigger and establish all instances of quorum loss and repair bugs alongside the best way (not have them silently be fastened by automation).

Rolling out MySQL Raft

Transitioning from semisynchronous to MySQL Raft over an enormous deployment is tough. For this we created a software (in Python) known as enable-raft. Allow-raft orchestrates the transition from semisynchronous to Raft by loading the plugin and setting the suitable configs (mysql sys-vars) on every of the entities. This course of entails a small downtime for the ring. The software was made sturdy over time and might roll out Raft at scale in a short time. We’ve used it to securely roll out Raft.

Testing and shadow workflow

For sure, doing a change within the core replication pipeline of MySQL is a really tough mission. Since knowledge security is at stake, testing was key for confidence. We leveraged shadow testing and failure injection considerably through the mission. We’d inject hundreds of failovers and elections on take a look at rings earlier than each RPM bundle supervisor rollout. We’d set off replacements and membership adjustments on the take a look at property to set off the essential code paths.

Lengthy-running testing with knowledge correctness checks have been additionally key. We’ve automation that runs nightly on the shards, guaranteeing consistency of primaries and replicas. We’re alerted to any such mismatch, and we debug it.

Efficiency

The efficiency of the write path latency for Raft was equal to semisync. The semisync equipment is barely less complicated and therefore anticipated to be leaner, nonetheless we optimized Raft to get the identical latencies as semi-sync. We optimized kuduraft to not add any extra CPU to the fleet despite pulling in lots of extra duties that beforehand had been exterior the server binary.

Raft made order-of-magnitude enhancements to promotions and failover instances. Sleek promotions, that are the majority of management adjustments within the fleet, improved considerably, and we will sometimes end a promotion in 300 milliseconds. Within the semisync setups, for the reason that service discovery retailer could be the supply of fact, the shoppers noticing the end of promotion could be for much longer, resulting in extra elevated finish consumer downtimes on a shard.

Raft sometimes does a failover inside 2 seconds. It’s because we heartbeat for Raft well being each 500 milliseconds and begin an election when three successive heartbeats fail. Within the semisync world, this step was orchestration heavy and would take 20 to 40 seconds. Raft thereby gave a 10x enchancment in downtimes for failover instances. 

Subsequent steps

Raft has helped clear up issues with the operational administration of MySQL at Meta by offering provable security and ease. Our targets of getting a palms off-management of MySQL consistency, and having instruments for the uncommon instances of availability loss, are principally met. Raft now opens up important alternatives sooner or later, as we will deal with enhancing the providing to the providers that use MySQL. One of many asks’ from our service homeowners is to have configurable consistency. Configurable consistency will permit the homeowners on the time of onboarding, to pick out whether or not the service wants X-region quorums or quorums that ask for copies in some particular geographies (e.g., Europe and america). FlexiRaft has seamless assist for such configurable quorums, and we plan to start out rolling out this assist sooner or later. Such quorums will correspondingly result in increased commit latencies, however use instances have to have the ability to trade-off between consistency and latency (e.g., PACELC theorem).

Due to the proxying characteristic (capability to ship messages utilizing a multihop distribution topology), Raft also can save community bandwidth throughout the Atlantic. We plan to make use of Raft to duplicate from america to Europe solely as soon as, after which use Raft’s proxying characteristic to distribute inside Europe. This can enhance latency, however it will likely be nominal on condition that the majority of the latency is within the cross-Atlantic switch and the additional hop is way shorter.

A number of the extra speculative concepts in Meta’s database deployments and distributed consensus house are about exploring leaderless protocols, like Epaxos. Our present deployments and providers have labored with the assumptions that include sturdy chief protocols, however we’re beginning to see a trickle of necessities the place providers would profit from extra uniform write latency within the WAN. One other concept that we’re contemplating is to disentangle the log from the state machine (the database) right into a disaggregated log setup. This can permit the staff to handle the considerations of the log and replication individually from the considerations of the database storage and SQL execution engine.