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Deploy a Replica Set

This tutorial describes how to create a three-member replica set from three existing mongod instances running with access control disabled.

To deploy a replica set with enabled access control, see Deploy Replica Set With Keyfile Access Control. If you wish to deploy a replica set from a single MongoDB instance, see Convert a Standalone to a Replica Set. For more information on replica set deployments, see the Replication and Replica Set Deployment Architectures documentation.


Three member replica sets provide enough redundancy to survive most network partitions and other system failures. These sets also have sufficient capacity for many distributed read operations. Replica sets should always have an odd number of members. This ensures that elections will proceed smoothly. For more about designing replica sets, see the Replication overview.

The basic procedure is to start the mongod instances that will become members of the replica set, configure the replica set itself, and then add the mongod instances to it.


For production deployments, you should maintain as much separation between members as possible by hosting the mongod instances on separate machines. When using virtual machines for production deployments, you should place each mongod instance on a separate host server serviced by redundant power circuits and redundant network paths.

Before you can deploy a replica set, you must install MongoDB on each system that will be part of your replica set. If you have not already installed MongoDB, see the installation tutorials.

Before creating your replica set, you should verify that your network configuration allows communication among all members; i.e. each member must be able to connect to every other member. For instructions on how to check your connection, see Test Connections Between all Members.

Considerations When Deploying a Replica Set


In a production, deploy each member of the replica set to its own machine and if possible bind to the standard MongoDB port of 27017. Use the bind_ip option to ensure that MongoDB listens for connections from applications on configured addresses.

See Replica Set Deployment Architectures for more information.


Ensure that network traffic can pass between all members of the set and all clients in the network securely and efficiently. Consider the following:

  • Establish a virtual private network. Ensure that your network topology routes all traffic between members within a single site over the local area network.
  • Configure access control to prevent connections from unknown clients to the replica set.
  • Configure networking and firewall rules so that incoming and outgoing packets are permitted only on the default MongoDB port and only from within your deployment.

Finally ensure that each member of a replica set is accessible by way of resolvable DNS or hostnames. You should either configure your DNS names appropriately or set up your systems’ /etc/hosts file to reflect this configuration.


Specify the run time configuration on each system in a configuration file stored in /etc/mongod.conf or a related location. Create the directory where MongoDB stores data files before deploying MongoDB.

For more information about the run time options used above and other configuration options, see Configuration File Options.


The following procedure outlines the steps to deploy a replica set when access control is disabled.


Start each member of the replica set with the appropriate options.

For each member, start a mongod instance with the following settings:

  • Set replication.replSetName option to the replica set name,

    If your application connects to more than one replica set, each set should have a distinct name. Some drivers group replica set connections by replica set name.

  • Set any other settings as appropriate for your deployment.

In this tutorial, the three mongod instances are associated with the following hosts:

Replica Set Member Hostname
Member 0
Member 1
Member 2

The following example specifies the replica set name through the --replSet command-line option:

mongod --replSet "rs0"

Alternatively, you can also specify the replica set name in a configuration file:

   replSetName: "rs0"

To start mongod with a configuration file, specify the configuration file’s path with the --config option:

mongod --config <path-to-config>

In production deployments, you can configure a init script to manage this process. Init scripts are beyond the scope of this document.


Connect a mongo shell to one of the mongod instances.

From the same machine where one of the mongod is running (in this tutorial,, start the mongo shell. To connect to the mongod listening to localhost on the default port of 27017, simply issue:


Depending on your path, you may need to specify the path to the mongo binary.


Initiate the replica set.

From the mongo shell, run rs.initiate() on replica set member 0.


Run rs.initiate() on just one and only one mongod instance for the replica set.

rs.initiate( {
   _id : "rs0",
   members: [
      { _id: 0, host: "" },
      { _id: 1, host: "" },
      { _id: 2, host: "" }

MongoDB initiates a replica set, using the default replica set configuration.


View the replica set configuration.

Use rs.conf() to display the replica set configuration object:


The replica set configuration object resembles the following:

   "_id" : "rs0",
   "version" : 1,
   "protocolVersion" : NumberLong(1),
   "members" : [
         "_id" : 0,
         "host" : "",
         "arbiterOnly" : false,
         "buildIndexes" : true,
         "hidden" : false,
         "priority" : 1,
         "tags" : {

         "slaveDelay" : NumberLong(0),
         "votes" : 1
         "_id" : 1,
         "host" : "",
         "arbiterOnly" : false,
         "buildIndexes" : true,
         "hidden" : false,
         "priority" : 1,
         "tags" : {

         "slaveDelay" : NumberLong(0),
         "votes" : 1
         "_id" : 2,
         "host" : "",
         "arbiterOnly" : false,
         "buildIndexes" : true,
         "hidden" : false,
         "priority" : 1,
         "tags" : {

         "slaveDelay" : NumberLong(0),
         "votes" : 1

   "settings" : {
      "chainingAllowed" : true,
      "heartbeatIntervalMillis" : 2000,
      "heartbeatTimeoutSecs" : 10,
      "electionTimeoutMillis" : 10000,
      "catchUpTimeoutMillis" : -1,
      "getLastErrorModes" : {

      "getLastErrorDefaults" : {
         "w" : 1,
         "wtimeout" : 0
      "replicaSetId" : ObjectId("585ab9df685f726db2c6a840")

Ensure that the replica set has a primary.

Use rs.status() to identify the primary in the replica set.