Replication has been one of the most useful features of MongoDB since the very early days. In general, replication refers to the process of synchronizing data across different servers.

The upsides for replication:

• Protect from data loss
• High availability of data
• Disaster recovery
• Avoiding downtime for maintenance
• Scaling reads since we can read from multiple servers
• Scaling writes (only if we can write to multiple servers)

In this chapter, we will also discuss cluster operations on a replica set. Administration tasks such as maintenance, resyncing a member, changing the oplog size, reconfiguring a member, and chained replication will be covered later in this chapter.