This document discusses MongoDB replication using replica sets. It describes how to configure and administer replica sets, which allow for asynchronous master-slave replication and automatic failover between members. Replica sets maintain multiple copies of data across multiple servers, provide redundancy and high availability, and can elect a new primary if one fails. The document outlines different replication topologies and member types in a replica set, and how replica sets integrate with sharded clusters in MongoDB.

1. Replication and MongoDBDwight Merriman (@dmerr)10gen

2. BasicsA bit like MySQL replicationAsynchronous master/slaveLet’s try it…

3. Command Line--master [--oplogSize <MB>]--slave –source <host> [--only <db>]

4. The local dbDoesn’t replicateOn master:local.oplog.$mainlocal.slavesOn slave:local.sources > use local > db.sources.find()

5. Administration> // master> use local> db.printReplicationInfo()> db.slaves.find()> db.oplog.$main.findOne()// slave> use local> db.printSlaveReplicationInfo()

6. TopologiesM->SM->S ->S ->SM->SM-/M<->M *very restrictive

7. Replica Pairs--pairwith

8. Replica PairsReplica Sets A cluster of N servers

9. Any (one) node can be primary

10. Consensus election of primary

11. Automatic failover

12. Automatic recovery

13. All writes to primary

14. Reads can be to primary or a second

15. Rack and data center aware

16. ETA: v1.6 July 2010 (“stable”)Replica Sets – Design ConceptsA write is only truly committed once it has replicated to a majority of servers in the set. (We can wait for confirmation for this though, with getLastError.)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).

17. A SetMember 1Member 3Member 2

18. A SetMember 1Member 3Member 2PRIMARY

19. A SetMember 1Member 3PRIMARYMember 2DOWN

20. A SetMember 1Member 3PRIMARYMember 2RECOVER-ING