- Replica sets in MongoDB allow for replication across multiple servers, with one server acting as the primary and able to accept writes, and other secondary servers replicating the primary. - If the primary fails, the replica set will automatically elect a new primary from the secondary servers and continue operating without interruption. - The replica set configuration specifies the members, their roles, and settings like heartbeat frequency to monitor member health and elect a primary if needed.

1. Replica Sets (MongoDB v1.6) Dwight Merriman (@dmerr) 10gen dmerr.tumblr.com

2. Basics MongoDB replication is a bit like MySQL replication Asynchronous master/slave at its core Variations: Master / slave Replica Pairs (deprecated – use replica sets) Replica Sets (today’s topic)

3. Replica Sets A cluster of N servers Any (one) node can be primary Consensus election of primary Automatic failover Automatic recovery All writes to primary Reads can be to primary (default) or a secondary v1.6

4. With Sharding

5. Replica Sets – Design Concepts A write is only truly committed once it has replicated to a majority of servers in the set. Thus, for important writes, the developer waits for confirmation of the cluster-wide commit with the getLastError command. Writes which are committed at the master of the set may be visible before the true cluster-wide commit has occurred. This property, which is more relaxed than some traditional products, makes theoretically achievable performance and availability higher. On a failover, if there is data which has not replicated form the primary, the data is dropped (see #1).

6. A Set Member 1 Member 2 Member 3

7. A Set Member 1 Member 2 PRIMARY Member 3

8. A Set Member 1 Member 2 DOWN Member 3 PRIMARY negotiate new master

9. A Set Member 1 Member 2 DOWN Member 3 PRIMARY

10. A Set Member 1 Member 2 RECOVER-ING Member 3 PRIMARY

11. A Set Member 1 Member 2 Member 3 PRIMARY

12. Configuration { _id : <setname>, members: [ { _id : <ordinal>, host : <hostname[:port]>, [, priority: <priority>] [, arbiterOnly : true] [, votes : n] [, slaveDelay : s] [, hidden : true ] } , ... ], settings: { [getLastErrorDefaults: <lasterrdefaults>] [, heartbeatSleep : <seconds>] [, heartbeatTimeout : <seconds>] [, heartbeatConnRetries : <n>] } }

13. Initiation > cfg = { ... _id : &quot;acme_a&quot;, ... members : [ ... { _id : 0, host : &quot;sf1.acme.com&quot; }, ... { _id : 1, host : &quot;sf2.acme.com&quot; }, ... { _id : 2, host : &quot;sf3.acme.com&quot; } ] } > use admin > db.runCommand({replSetInitiate:cfg})

14. Set Member Types Normal (priority == 1) Passive (priority == 0) DR (priority < 1.0) (coming soon) Arbiter (no data, but can vote)

15. Commands { isMaster : 1 } { replSetGetStatus : 1 } { replSetInitiate : <config> } { replSetStepDown : true } { replSetReconfig : <config> } > rs.help()

16. Demo

17. Questions? Contact 10gen for on-site advisory sessions on MongoDB architecture, schema design, performance tuning, training… [email_address] irc://irc.freenode.net/#mongodb http://groups.google.com/group/mongodb-user