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DB Replication With Rails
 

DB Replication With Rails

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    DB Replication With Rails DB Replication With Rails Presentation Transcript

    • Using Replicated Databases with Rails Max Schöfmann 26. Feb. 2009
    • This talk is about... • Why replication? What does it look like? • Dealing with replicated DBs in a Rails app • not: Setting up replication (I‘m no DBA) (I will focus on MySQL here, but most is applicable to any DB)
    • So why...?
    • So why...?
    • So why...? • Load balancing • Redundancy
    • Load balancing Master Slave replicates writes reads App
    • Load balancing Slave 1 Master replicates Slave 2 repl icates s d rea reads writes App
    • Redundancy (read) Slave 1 Master Slave 2 repl icates reads writes App
    • Redundancy (r + w) Master Master replicates reads + writes reads + writes App
    • Redundancy (r + w) Master Master reads + writes App
    • Problems • Conflicting primary keys (only multi-master) • Replication lag (Reason: asynchronous replication)
    • Conflicting Primary Keys • auto_increment MODULO #Master • Only read from one master and use HA (migrating IP)
    • Failover using virtual IP „Shadow“-Master 10.0.0.1 Master 10.0.0.2 +10.0.0.10 replicates heartbeat reads + writes reads (optional) to/from virtual IP: 10.0.0.10 App
    • Failover using virtual IP New Master 10.0.0.1+10.0.0.10 Master 10.0.0.2 10.0.0.10 migrates from .2 to .1 reads + writes to/from virtual IP: 10.0.0.10 App
    • Failover using virtual IP • How? • hartbeat & Friends • Benefits: • App doesn‘t need to implement failover • no specially crafted primary keys needed • secondary master can be used for LB • add more slaves for more LB
    • Want it more Enterprisey? • MySQL Cluster • + synchronous replication • - RAM RAM RAM RAM RAM RAM RAM • - not ideal for every scenario
    • So we need a read/write split... • MySQL Proxy • DBSlayer • Various Rails plugins
    • MySQL Proxy Pro • Widely used (support!) • Powerful scripting • Delay adaptive balancing (helps against lag!) Con • Another moving part in your setup • Seems to have issues with UTF-8
    • DBSlayer • NYTimes • JSON via HTTP • http://code.nytimes.com/projects/dbslayer (haven‘t played with it yet)
    • Rails Plugins • Various plugins after the „Twitter-Incident“ • Magic Multi-Connections (Dr. Nic) • acts_as_readonlyable • MySQL replication adapter • Most need special care in your app (e.g. Foo.all :use_slave => true) (development seemed to have stalled, but check github!)
    • Rails Plugins • Still actively maintained: • masochism (Rick Olson) • data_fabric (Fiveruns) • multi_db (yours truly) • (dev of both started in spring ’08, based on the ideas of masochism) • mostly transparent in your app
    • How they work • „AR::B#connection“ returns a proxy • the proxy sends insert/delete/update... to the master and selects to the slave • and sends everything to the master during transactions • forcing the master is possible using: code } AR::B.connection.with_master {
    • multi_db is (a bit) different • Supports multiple slaves with blacklisting and immediate failover • Sends only „safe“ things to the slave and anything else to the master (safe when APIs change...) • dynamically creates delegation methods for perfomance • distributed as a GEM (use without Rails) http://github.com/schoefmax/multi_db
    • Fighting replication lag • Set certain controller actions master only • Declare certain models master only (all three plugins support this in different ways)
    • Comparison masochism data_fabric multi_db nesting with_master data sharding load balancing threadsafe! compatible* database.yml (* disable plugin without having to switch hosts in your database.yml - easy to forget...)
    • Thank you. http://github.com/schoefmax/multi_db http://github.com/fiveruns/data_fabric http://github.com/technoweenie/masochism http://jan.kneschke.de/projects/mysql/mysql-proxy http://code.nytimes.com/projects/dbslayer http://www.linux-ha.org/