MySQL and the CAP theorem: relevance & misconceptions
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The CAP theorem is often used to describe the tradeoffs of available versus consistent systems. But the CAP theorem is often misunderstood and misrepresented. In this session we investigate the CAP properties of various MySQL replication setups, and show that it is impossible to claim a setup is AP or that it is CP. We will see how the properties can be non binary, and change across time, geography and workload. We will also observe where CAP's terms can be made irrelevant. This session takes the practical engineer's perspective and shows how we can gain the upper hand with reasonable tradeoffs.
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MySQL and the CAP theorem: relevance & misconceptions
- 1. MySQL and the CAP theorem: relevance & misconceptions Dissecting,affirmingandrefutingCAPassumptionsin realproductionsystems. ShlomiNoach GitHub FOSDEM2019
- 4. Incentive
- 6. CAP Conjecture SuggestedbyEricBrewer,1998-1999 Terms: • [atomic]Consistency • [high]Availability • Partitiontolerance
- 10. CAP Theorem: Availability Everynon-crashednodemustrespondtorequestsinafinite amountoftime. Thereisnoconstraintontheactualamountoftime. Thoughitisnotspecified,itisimpliedthatresponsemustbe valid,non-error. ContrastwithBrewer’sdefinitonofHighavailability: consumer alwayshasaccesstodataviaatleastonereplica.
- 11. CAP Theorem: Partition Tolerance Thesystemisabletooperateonnetworkpartitioning. Partitiontoleranceisconsideredasagivencondition,since networkpartitioningcananddoestakeplaceregardlessofa system’sdesign.
- 12. CAP Theorem Adistributeddatastore[webservice]cannotprovidemorethan twooutofthethreeproperties.Betterillustratedas: • Ifthenetworkisgood,youmayachievebothAvailibilityand Consistency. • Ifthenetworkispartitioned,youmustchoosebetween AvailabilityandConsistency.
- 13. Proof of CAP theorem Simplified,notbymuch. Giventwonodesreplicatingfromeachother ! ! n1 n2
- 14. Proof of CAP theorem Wepartitionthenetworkbetweenthetwonodestoaninfinite amountoftime. ! ! n1 n2
- 15. Proof of CAP theorem Wewritedatatoonenode.IfthesystemisAvailable,thewrite compeltesinafiniteamountoftime. ! ! n1 n2
- 16. Proof of CAP theorem Wereaddatafromtheothernode.IfthesystemisAvailable thatreadcompletesinafinitemountoftime. ! ! n1 n2
- 17. Proof of CAP theorem Duringthatfinitetimethenetworkwaspartitioned,henceour readcouldnotreflectchangesmadebythewrite. QED ! ! n1 n2
- 20. CAP does not discuss DisasterRecovery Failover Latency(seePACELC) Durability,noderecovery SLA,uptime,nines Scalability,capacity
- 21. CAP & MySQL in production
- 23. Standard MySQL replication: CAP properties NotConsistent:asynchronousreplication,replicationlag. NotAvailable: • Replicanodesareunabletoservewrites(returnwitherror). • Ifmastercrashes,noservertobeabletotakewrites. • CAPdoesnotdiscussfailovers/SLA. ! ! ! ! ! !
- 24. Standard MySQL replication: consistency in practice Readfrommaster? Scalability/capacity ! ! ! ! ! !
- 25. Standard MySQL replication: consistency in practice ProxySQLGTIDcasualreads: • Appliestosameconnectiononly. • Ifmasterwithinhostgroup,canfailbacktomaster • NotCAPConsistent.Aproxyisn’taknowncomponentinCAP Theorem. https://proxysql.com/blog/proxysql-gtid-causal-reads ! ! ! ! ! ! "
- 26. Standard MySQL replication: consistency in practice ! ! ! ! ! ! frenotomonitorreplicationlag: • Suggestswhenreplicasareuptodate • HAProxyroutesqueriestoreplicas,offloadingtrafficfrom master • NotCAPConsistent.NotpartofCAP. https://githubengineering.com/mitigating-replication-lag-and-reducing-read-load-with-freno/ "
- 27. Semi-sync replication Singlemaster,multiplereplicas,semi-synctoall/subsetof replicas. ! ! ! ! ! !
- 28. Semi-sync replication: CAP properties StillnotConsistent. • Thoughwegetdurability. StillnotAvailable. • semi-syncwriteAvailabilitydependsonconfiguration;on identitiesofsemi-syncreplicas. ! ! ! ! ! !
- 30. Vitess SQLlayes:vttablet,proxytobackend MySQLstandardreplicationbackendshards Suggestedconfiguration: • semi-sync • onlysendtraffictomaster " " " ! ! ! ! ! !
- 31. Vitess: CAP properties Givensuggestedconfiguration: • NotConsistent • NotAvailable " " " ! ! ! ! ! !
- 33. CAP != E=MC²
- 34. InnoDB Cluster, single writer Singlewriternode,paxosconsensus ! ! ! < 8.0.14
- 35. InnoDB Cluster, single writer: CAP properties NotAvailable:ifnetworkpartitionedseverely,quorumcanbe lostandnoleadertoacceptwrites. NotConsistent;changesare: • propagatedtomajorityofnodes,notall. • madedurable,notapplied. ! ! ! < 8.0.14
- 36. InnoDB Cluster, single writer: availability, consistency in practice MySQLRouterasProxy,routetraffictoleader. ProxySQLforper-connectionconsistentreads.! ! ! " < 8.0.14
- 37. InnoDB Cluster, multi writer Multiwriternodes,paxosconsensus ! ! ! < 8.0.14
- 38. InnoDB Cluster, multi writer: CAP properties NotAvailable,asbefore. • Debatable:arewriteconflictscompatiblewithAvailability? Notconsistent,asbefore. ! ! ! < 8.0.14
- 39. Galera: CAP properties wsrep_sync_wait=0 • NotAvailable,notConsistent wsrep_sync_wait != 0 (e.g.=3) • NotAvailable • Consistentuptotimeoutonisolatednodes:i.e.not Consistent. ! ! !
- 40. InnoDB Cluster, >= 8.0.14 group_replication_consistency= BEFORE_AND_AFTER Supportsconsistentreads,uptotimeoutonisolatednode. NotAvailable,notConsistent. STONITHforisolatednodes. ! ! !
- 41. Is there any AP setup?
- 42. AP setups Disjointmasters ! !! ! !! ! ! ! ! ! Writablemaster-master,asyncreplication Writablecircular[n>2]mastertopology,asyncreplication
- 45. Harvest, Yield, and ScalableTolerant Systems,ArmandoFox,EricA.Brewer https://pdfs.semanticscholar.org/5015/8bc1a8a67295ab7bce0550886a9859000dc2.pdf Brewer’s Conjecture and the Feasibility of Consistent, Available, Partition-TolerantWeb Services,SethGilbert,NancyLynch https://www.glassbeam.com/sites/all/themes/glassbeam/images/blog/10.1.1.67.6951.pdf A Critique of the CAPTheorem,MartinKleppmann https://arxiv.org/pdf/1509.05393.pdf CAPTwelve Years Later: How the "Rules" Have Changed,EricBrewer https://www.infoq.com/articles/cap-twelve-years-later-how-the-rules-have-changed Please stop calling databases CP or AP,MartinKleppmann https://martin.kleppmann.com/2015/05/11/please-stop-calling-databases-cp-or-ap.html Further reading
- 46. NewSQL database systems are failing to guarantee consistency, and I blame Spanner,Daniel Abadi http://dbmsmusings.blogspot.com/2018/09/newsql-database-systems-are-failing-to.html Problems with CAP, andYahoo’s little known NoSQL system,DanielAbadi http://dbmsmusings.blogspot.com/2010/04/problems-with-cap-and-yahoos-little.html PACELC theorem,Wikipedia https://en.wikipedia.org/wiki/PACELC_theorem Further reading
- 47. CAP theorem is mostly misunderstood,SuguSougoumarane http://ssougou.blogspot.com/2017/09/cap-theorem-is-mostly-misunderstood.html "A Critique of the CAPTheorem”,JuliaEvans https://jvns.ca/blog/2016/11/19/a-critique-of-the-cap-theorem/ CAPTheorem,FoundationDB https://apple.github.io/foundationdb/cap-theorem.html Further reading