Five New Replica Set Features in 1.7.x | MongoDB Blog` These five new features added to MongoDB's 1.7.4 release include initial sync from a secondary, slave delay, hidden servers, freeze a member, and fast sync. Initial sync allows you to control where a new node will sync from, while slave delay makes a slave postpone replaying operations from the master. Hidden servers won't appear in isMaster() results and can be useful for maintenance or failover. Freeze a member forces it to remain a secondary for a given number of seconds, while fast sync allows you to bring up a new member quickly with a reasonably up-to-date backup.