MongoDB Documentation, Release 3.0.4 Add Members to a Replica Set Overview Requirements Procedures 1. An active replica set. 2. A new MongoDB system capable of supporting your data set, accessible by the active replica set through the network. • More recent than the oldest operation in the primary’s oplog . The new member must be able to become current by applying operations from the primary’s oplog. • Make sure the new member’s data directory does not contain data. The new member will copy the data from an existing member. • A valid copy of the data files from a member of the same replica set. See Backup and Restore with Filesystem Snapshots (page 243) document for more information. This tutorial explains how to add an additional member to an existing replica set . For background on replication deployment patterns, see the Replica Set Deployment Architectures (page 572) document. Maximum Voting Members A replica set can have a maximum of seven voting members (page 580). To add a member to a replica set that already has seven votes, you must either add the member as a non-voting member (page 583) or remove a vote from an existing member . Control Scripts In production deployments you can configure a control script to manage member processes. Existing Members You can use these procedures to add new members to an existing set. You can also use the same procedure to “re-add” a removed member. If the removed member’s data is still relatively recent, it can recover and catch up easily. Data Files If you have a backup or snapshot of an existing member, you can move the data files (e.g. the dbPath directory) to a new system and use them to quickly initiate a new member. The files must be: Important: Always use filesystem snapshots to create a copy of a member of the existing replica set. Do not use mongodump and mongorestore to seed a new replica set member. Otherwise, use the MongoDB installation tutorial (page 5) and the Deploy a Replica Set (page 603) tutorials. Prepare the Data Directory Before adding a new member to an existing replica set , prepare the new member’s data directory using one of the following strategies: If the new member is in a recovering state, it must exit and become a secondary before MongoDB can copy all data as part of the replication process. This process takes time but does not require administrator intervention. Chapter 9. Replication 616