MongoDB: The Definitive Guide, 2nd Edition

Chapter 9. Setting Up a Replica Set

This chapter introduces MongoDB’s replication system: replica sets. It covers:

• What replica sets are

• How to set up a replica set

• What configuration options are available for replica set members

Introduction to Replication

Since the first chapter, we’ve been using a standalone server, a single mongod server. It’s an easy way to get started but a dangerous way to run in production: what if your server crashes or becomes unavailable? Your database will at least be unavailable for a little while. If there are problems with the hardware, you may have to move your data to another machine. In the worst case, disk or network issues could leave you with corrupt or inaccessible data.

Replication is a way of keeping identical copies of your data on multiple servers and is recommended for all production deployments. Replication keeps your application running and your data safe, even if something happens to one or more of your servers.

With MongoDB, you set up replication by creating a replica set. A replica set is a group of servers with one primary, the server taking client requests, and multiple secondaries, servers that keep copies of the primary’s data. If the primary crashes, the secondaries can elect a new primary from amongst themselves.

If you are using replication and a server goes down, you can still access your data from the other servers in the set. If the data on a server is damaged or inaccessible, you can make a new copy of the data from one the other ...