MongoDB - Replication and Replica Sets

Presented at MongoDB LA. Jan 2012

Transcript

1. Sridhar Nanjundeswaran, 10gen sridhar@10gen.com @snanjund © Copyright 2010 10gen Inc.
2. Agenda • Introduction to replica sets • Durability and Consistency • Options and configuration • Common deployment scenarios • Behind the Scenes
3. INTRO TO REPLICA SETS
4. Replica Set - Creation • 2 or more nodes form the set Node 1 Node 2 Node 3
5. Replica Set - Initialize • Initialize -> Election • Primary + data replication from primary to secondary Node 1 Secondary Node 2 Secondary Node 3 Primary Replication Replication Heartbeat
6. Replica Set - Failure • Primary down/network failure • Automatic election of new primary if majority exists Node 1 Secondary Node 2 Secondary Node 3 Primary Heartbeat Primary Election
7. Replica Set - Failover • New primary elected • Replication established from new primary Node 1 Secondary Node 2 Primary Node 3 Primary Heartbeat
8. Replica Set - Recovery • Down node comes up • Rejoins sets • Recovery and then secondary Node 1 Secondary Node 2 Primary Node 3 Secondary Replication Replication Heartbeat
9. DURABILITY AND CONSISTENCY
10. Strong Consistency Primary Secondary Secondary Read Write Driver Client
11. Eventual Consistency Primary Read Write Driver Read Secondary Secondary Client Application
12. Durability • Fire and forget • Wait for error • Wait for journal sync • Wait for fsync • Wait for replication
13. Fire and Forget Driver Primary write apply in memory
14. Wait for error Driver Primary getLastError apply in memory
15. Wait for journal sync Driver Primary getLastError apply in memory write j:true Write to journal
16. Wait for fsync Driver Primary getLastError apply in memory write fsync:true fsync
17. Wait for replication Driver Primary getLastError apply in memory write w:2 Secondary replicate
18. OPTIONS AND CONFIGURATION
19. Priorities • Floating point number between 0..1000 • Highest member that is up to date wins • Up to date == within 10 seconds of primary • If a higher priority member catches up, it will force election and win
20. Slave Delay • Lags behind master by configurable time delay • Automatically hidden from clients • Protects against operator errors • Fat fingering • Application corrupts data
21. Tagging • New in 2.0.0 • Control over where data is written to • Each member can have one or more tags e.g. • tags: {dc: "ny"} • tags: {dc: "ny",
 ip: "192.168",
 rack: "row3rk7"} • Replica set defines rules for where data resides • Rules can change without change application code
22. Tagging - example { _id : "mySet", members : [ {_id : 0, host : "A", tags : {"dc": "ny"}}, {_id : 1, host : "B", tags : {"dc": "ny"}}, {_id : 2, host : "C", tags : {"dc": "sf"}}, {_id : 3, host : "D", tags : {"dc": "sf"}}, {_id : 4, host : "E", tags : {"dc": "cloud"}}] settings : { getLastErrorModes : { allDCs : {"dc" : 3}, someDCs : {"dc" : 2}} } } > db.blogs.insert({...}) > db.runCommand({getLastError : 1, w : "allDCs"})
23. Others • Arbiters • Vote in elections • Don’t store a copy of data • Use as tie breaker • Hidden • Not reported in isMaster • Hidden from slaveOk reads
24. COMMON DEPLOYMENT SCENARIOS
25. Replica Set – 1 Data Center • Single datacenter • Single switch & power • Points of failure: • Power • Network • Datacenter • Two node failure • Automatic recovery of single node crash Member 1 Member 2 Member 3
26. Replica Set – 2 data centers • Multi datacenter • DR node for safety • Can’t do multi data center durable write safely since only 1 node in distant DC Member 1 Member 2 Member 3 DC1 DC2
27. Replica Set – 3 Data Centers • Three data centers • Can survive full data center loss • Can do w= { dc : 2 } to guarantee write in 2 data centers Member 1 Member 2 Member 3 Member 4 DC2 Member 5 - DR DC1 DC3
28. BEHIND THE SCENES
29. Local database • config • Oplog • Capped collection • Idempotent version of operation stored db.replsettest.update({},{$inc:{set:1}}) { "ts" : …, "h" : …, "op" : "u", "ns" : "mongo_la.replsettest", "o2" : { "_id" : ObjectId("4f177bc768dad821278224d8") }, "o" : { "$set" : { "set" : 2 } } }
30. @mongodb © Copyright 2010 10gen Inc. conferences, appearances, and meetups http://www.10gen.com/events http://bit.ly/mongofb Facebook | Twitter | LinkedIn http://linkd.in/joinmongo download at mongodb.org We’re Hiring ! sridhar@10gen.com @snanjund