Constructing and deploying MySQL Raft at Meta

• We’re rolling out MySQL Raft with the intention to finally exchange our present MySQL semisynchronous databases. 
• The largest win of MySQL Raft was simplification of the operation and making MySQL servers maintain 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 prospects 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 this planet. The deployment powers the social graph together with many different providers, like Messaging, Adverts, and Feed. Over the previous couple of years, we’ve got applied MySQL Raft, a Raft consensus engine that was built-in with MySQL to construct a replicated state machine. We have now migrated a big portion of our deployment to MySQL Raft and plan to completely exchange the present MySQL semisynchronous databases with it. The challenge 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 top availability, fault tolerance, and scaling reads, Meta’s MySQL datastore is a massively sharded, geo-replicated deployment with thousands and thousands of shards, holding petabytes of knowledge. The deployment consists of hundreds of machines operating over a number of areas and information facilities throughout a number of continents.

Beforehand, our replication answer used the MySQL semisynchronous (semisync) replication protocol. This was an information path–solely protocol. The MySQL main would use semisynchronous replication to 2 log-only replicas (logtailers) throughout the main area however outdoors 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 airplane operations (e.g., promotions, failover, and membership change) can be the duty of a set of Python daemons (henceforth known as automation). Automation would do the mandatory 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 can 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 duplicate to the first and add it to the service discovery retailer. A failover can be a extra complicated operation by which the tailing threads of the logtailers (semisynchronous ACKers) can be shut all the way down to fence the earlier useless main.

Why was MySQL Raft vital?

Previously, to assist assure security and keep away from information loss through the complicated 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 complicated and tougher to keep up over time as increasingly nook circumstances wanted to be patched.

We determined to take a very completely different strategy. We enhanced MySQL and made it a real distributed system. Realizing that management airplane operations like promotions and membership adjustments have been the set off of most points, we needed the management airplane and information airplane 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 reality of membership and management moved contained in the server (mysqld). This was the only 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 is predicated on Apache Kudu. We enhanced it considerably for the wants of MySQL and our deployment. We revealed this fork as an open supply challenge, kuduraft.

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

• FlexiRaft — assist for 2 completely different intersecting quorums: the info quorum and the chief election quorum
• Proxying — the power to make use of a proxy intermediate node to cut back community bandwidth
• Compression — the place we compress binary log (transaction) payloads as soon as earlier than distribution
• Log abstraction — to assist completely different bodily logfile implementations
• Main ban — the power to forestall 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 by way of the plugin APIs (related APIs had been used for semisync as nicely), whereas we created a separate API for MyRaft to interface again with MySQL server (callbacks).

MySQL Raft replication topologies

A Raft ring would encompass a number of MySQL cases (4 in the diagram) in several areas. The communication round-trip time (RTT) between these areas would vary from 10 to 100 milliseconds. A number of of those MySQLs (usually 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 very 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 information quorum for writes can 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 title 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 can be a reproduction in MySQL’s standpoint and can be making use of the transactions to its engine. It will not permit direct writes from consumer connections (read_only=1 is ready). A learner can be a non-voting member of the ring, e.g., the three MySQLs in non-primary-capable areas (above). It will be a reproduction in MySQL’s standpoint.

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 finished through the log abstraction enchancment to kuduraft. The MySQL transactions can 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 applicable headers and would usually 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 completely different from what occurs in customary MySQL. In a reproduction, 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 can 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 duplicate’s applier threads would decide up transactions from the binlog after which apply them to the engine. Throughout this course of, a brand new log file, the apply log, can be created. This apply log would play an essential position in crash restoration of replicas however is in any other case a nonreplicated log file.

So, in abstract:

In customary MySQL:

• Main 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:

• Main writes to binlog through 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 standpoint.

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 getting 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 go by way of group commit/ordered_commit circulate. GTIDs can 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 by way of the transaction to a binlog file. It will asynchronously begin transport the transaction to different followers to get ACKs and attain consensus.

The consumer thread, which is in “commit” of the transaction, can be blocked, ready for consensus from Raft. When Raft would get two out of three in-region votes, consensus commit can 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 can 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’ll 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) can be misplaced and the ready transaction in engine can be rolled again on course of restart. Since there was no further uncommitted transaction within the Raft log, no reconciliation with different members must be finished.
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 contributors. On crash restoration, the ready transaction in engine (e.g., InnoDB or MyRocks) can be rolled again (engine had not reached commit). Raft would undergo failover, and a brand new chief can 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: Much like no. 2 above, the ready transaction within the engine can 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 can 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 facet log from apply-log to binlog, and finally set the correct read_only settings. This mechanism is complicated and at the moment not open sourced.

FlexiRaft

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

To avoid this situation, we innovated on FlexiRaft, borrowing concepts from Flexible Paxos.

At a excessive degree, FlexiRaft permits Raft to have a unique information 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 will depend on who the present chief is (therefore the title “single area dynamic”). The information quorum is almost all of voters within the chief’s area. Throughout promotions, if phrases are steady, the Candidate will intersect with the final identified chief’s area. FlexiRaft would additionally be sure that the quorum of the Candidate’s area can also be attained, in any other case the next No-Op message may get caught. If within the uncommon case the phrases usually are not steady, Flexi Raft would strive to determine a rising set of areas which have to be intersected with for security or, within the worst case, would fall again to the N area intersection case of Versatile Paxos. Because of pre-elections and mock elections, the incidences of time period gaps are uncommon.

Management airplane operations (promotions and membership adjustments)

In an effort 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 may 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 issues for the MySQL deployment. The MySQL server can be accountable for security through Raft’s replicated state machine. The no-data-loss assure can be provably enshrined within the server itself. Automation (Python scripts, daemons) would provoke management airplane operations and monitor the well being of the fleet. It will additionally exchange members or do promotions through Raft throughout upkeep or when a number failure was detected. On occasion, automation may additionally change the regional placement of MySQL topology. Altering the automation to adapt to Raft was a large endeavor, 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 way in which

Rolling out Raft to the fleet was an enormous studying for the crew. 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 information quorum was very small (two out of three in-region members), two unhealthy entities within the area may just about shatter the quorum and produce down availability. The MySQL fleet undergoes a superb quantity of churn on daily basis (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 targeted 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 could be thought of as engineering add-ons to it. Kuduraft has the assist for pre-elections, however pre-elections are finished solely throughout a failover. Throughout a swish 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 function, which was much like pre-elections however occurred solely upon a swish switch of management. Since this was an async operation, it didn’t enhance promotion downtimes. A mock election would weed out circumstances the place an actual election would partially succeed and get caught.

Dealing with byzantine failures: Raft’s membership checklist is taken into account to be blessed by Raft itself. However through the provisioning of latest members, or due to races in automation, there could possibly be weird circumstances of two completely different Raft rings intersecting. These zombie membership nodes needed to be weeded out and shouldn’t be capable of talk with one another. We applied a function 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 objectives was to cut 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 experiences on a hoop, which assist us shortly establish when and why a hoop is unavailable (shattered quorum). The funding within the tooling and automation infrastructure went hand-in-hand and might need been an even 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 from time to time, resulting in availability loss. The standard case is when automation doesn’t detect unhealthy cases/logtailers within the ring and doesn’t exchange them shortly. This could occur due to poor detection, employee queue overload, or an absence 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 usually, as a result of the deployments attempt to isolate failure domains throughout important entities of the quorum by way of correct placement choices. Lengthy story quick: At scale, surprising issues occur, regardless of current safeguards. Instruments have to be out there to mitigate such conditions in manufacturing. We constructed Quorum Fixer in anticipation of this.

Quorum Fixer is a guide 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 usually turns into wholesome.

It was a acutely aware resolution to not run this software routinely, as a result of we wish to root trigger and establish all circumstances of quorum loss and repair bugs alongside the way in which (not have them silently be mounted by automation).

Rolling out MySQL Raft

Transitioning from semisynchronous to MySQL Raft over a large 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 includes 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 have now used it to soundly roll out Raft.

Testing and shadow workflow

Evidently, doing a change within the core replication pipeline of MySQL is a really tough challenge. Since information security is at stake, testing was key for confidence. We leveraged shadow testing and failure injection considerably through the challenge. 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 important code paths.

Lengthy-running testing with information correctness checks have been additionally key. We have now 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 regardless of pulling in lots of extra duties that beforehand had been outdoors the server binary.

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

Raft usually does a failover inside 2 seconds. It is 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 circumstances. 

Subsequent steps

Raft has helped clear up issues with the operational administration of MySQL at Meta by offering provable security and ease. Our objectives of getting a arms off-management of MySQL consistency, and having instruments for the uncommon circumstances of availability loss, are largely met. Raft now opens up important alternatives sooner or later, as we are able to give attention to enhancing the providing to the providers that use MySQL. One of many asks’ from our service house owners is to have configurable consistency. Configurable consistency will permit the house owners on the time of onboarding, to pick whether or not the service wants X-region quorums or quorums that ask for copies in some particular geographies (e.g., Europe and the USA). 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 circumstances have to have the ability to trade-off between consistency and latency (e.g., PACELC theorem).

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

A few 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 robust 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 crew to handle the issues of the log and replication individually from the issues of the database storage and SQL execution engine.