Deploy a Replica Set for Testing and Development

Overview

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.

Requirements

For test and development systems, you can run your mongod instances on a local system, or within a virtual instance.

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.

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

mongod --bind_ip localhost,My-Example-Associated-Hostname

mongosh --host My-Example-Associated-Hostname
mongosh --host 198.51.100.1

Procedure

1. Create the necessary data directories for each member by issuing a command similar to the following:

   mkdir -p /srv/mongodb/rs0-0  /srv/mongodb/rs0-1 /srv/mongodb/rs0-2

   This will create directories called "rs0-0", "rs0-1", and "rs0-2", which will contain the instances' database files.

2. Start your mongod instances in their own shell windows by issuing the following commands:

   Warning

   Before binding to a non-localhost (e.g. publicly accessible) IP address, ensure you have secured your cluster from unauthorized access. For a complete list of security recommendations, see Security Checklist. At minimum, consider enabling authentication and hardening network infrastructure.

   First member:

   mongod --replSet rs0 --port 27017 --bind_ip localhost,<hostname(s)|ip address(es)> --dbpath /srv/mongodb/rs0-0  --oplogSize 128

   Second member:

   mongod --replSet rs0 --port 27018 --bind_ip localhost,<hostname(s)|ip address(es)> --dbpath /srv/mongodb/rs0-1  --oplogSize 128

   Third member:

   mongod --replSet rs0 --port 27019 --bind_ip localhost,<hostname(s)|ip address(es)> --dbpath /srv/mongodb/rs0-2 --oplogSize 128

   This starts each instance as a member of a replica set named rs0, each running on a distinct port, and specifies the path to your data directory with the --dbpath setting. If you are already using the suggested ports, select different ports.

   The instances bind to both the localhost and the ip address of the host.

   The --oplogSize setting reduces the disk space that each mongod instance uses. [1] This is ideal for testing and development deployments as it prevents overloading your machine. For more information on this and other configuration options, see Configuration File Options.

3. Connect to one of your mongod instances through mongosh. You will need to indicate which instance by specifying its port number. For the sake of simplicity and clarity, you may want to choose the first one, as in the following command;

   mongosh --port 27017

4. In mongosh, use rs.initiate() to initiate the replica set. You can create a replica set configuration object in mongosh environment, as in the following example:

   rsconf = {
     _id: "rs0",
     members: [
       {
        _id: 0,
        host: "<hostname>:27017"
       },
       {
        _id: 1,
        host: "<hostname>:27018"
       },
       {
        _id: 2,
        host: "<hostname>:27019"
       }
      ]
   }

   replacing <hostname> with your system's hostname, and then pass the rsconf file to rs.initiate() as follows:

   rs.initiate( rsconf )

5. Display the current replica configuration by issuing the following command:

   rs.conf()

   The replica set configuration object resembles the following:

   {
      "_id" : "rs0",
      "version" : 1,
      "protocolVersion" : NumberLong(1),
      "members" : [
         {
            "_id" : 0,
            "host" : "<hostname>:27017",
            "arbiterOnly" : false,
            "buildIndexes" : true,
            "hidden" : false,
            "priority" : 1,
            "tags" : {
            },
            "secondaryDelaySecs" : NumberLong(0),
            "votes" : 1
         },
         {
            "_id" : 1,
            "host" : "<hostname>:27018",
            "arbiterOnly" : false,
            "buildIndexes" : true,
            "hidden" : false,
            "priority" : 1,
            "tags" : {
            },
            "secondaryDelaySecs" : NumberLong(0),
            "votes" : 1
         },
         {
            "_id" : 2,
            "host" : "<hostname>:27019",
            "arbiterOnly" : false,
            "buildIndexes" : true,
            "hidden" : false,
            "priority" : 1,
            "tags" : {
            },
            "secondaryDelaySecs" : NumberLong(0),
            "votes" : 1
         }
      ],
      "settings" : {
         "chainingAllowed" : true,
         "heartbeatIntervalMillis" : 2000,
         "heartbeatTimeoutSecs" : 10,
         "electionTimeoutMillis" : 10000,
         "catchUpTimeoutMillis" : -1,
         "getLastErrorModes" : {
         },
         "getLastErrorDefaults" : {
            "w" : 1,
            "wtimeout" : 0
         },
         "replicaSetId" : ObjectId("598f630adc9053c6ee6d5f38")
      }
   }

Check the status of your replica set at any time with the rs.status() operation.