Deploy a Replica Set for Testing and Development¶

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Overview
Requirements
Considerations
Procedure

This procedure describes deploying a replica set in a development or test environment. For a production deployment, refer to the Deploy a Replica Set tutorial.

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 Authentication. 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.

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¶

Tip

When possible, use a logical DNS hostname instead of an ip address, particularly when configuring replica set members or sharded cluster members. The use of logical DNS hostnames avoids configuration changes due to ip address changes.

IP Binding¶

Starting in MongoDB 3.6, MongoDB binaries, mongod and mongos, bind to localhost by default. If the net.ipv6 configuration file setting or the --ipv6 command line option is set for the binary, the binary additionally binds to the localhost IPv6 address.

Previously, starting from MongoDB 2.6, only the binaries from the official MongoDB RPM (Red Hat, CentOS, Fedora Linux, and derivatives) and DEB (Debian, Ubuntu, and derivatives) packages bind to localhost by default.

When bound only to the localhost, these MongoDB 3.6 binaries can only accept connections from clients (including the mongo shell, other members in your deployment for replica sets and sharded clusters) that are running on the same machine. Remote clients cannot connect to the binaries bound only to localhost.

To override and bind to other ip addresses, you can use the net.bindIp configuration file setting or the --bind_ip command-line option to specify a list of hostnames or ip addresses.

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.

For example, the following mongod instance binds to both the localhost and the hostname My-Example-Associated-Hostname, which is associated with the ip address 198.51.100.1:

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

In order to connect to this instance, remote clients must specify the hostname or its associated ip address 198.51.100.1:

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mongo --host My-Example-Associated-Hostname

mongo --host 198.51.100.1

In this test deployment, the three members run on the same machine.

Replica Set Naming¶

Important

These instructions should only be used for test or development deployments.

The examples in this procedure create a new replica set named rs0.

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.

Procedure¶

Tip

When possible, use a logical DNS hostname instead of an ip address, particularly when configuring replica set members or sharded cluster members. The use of logical DNS hostnames avoids configuration changes due to ip address changes.

Create the necessary data directories for each member by issuing a command similar to the following:
copy
```
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.
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:
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```
mongod --replSet rs0 --port 27017 --bind_ip localhost,<hostname(s)|ip address(es)> --dbpath /srv/mongodb/rs0-0  --oplogSize 128
```
Second member:
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```
mongod --replSet rs0 --port 27018 --bind_ip localhost,<hostname(s)|ip address(es)> --dbpath /srv/mongodb/rs0-1  --oplogSize 128
```
Third member:
copy
```
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.
Connect to one of your mongod instances through the mongo shell. 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;
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```
mongo --port 27017
```

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

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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:

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rs.initiate( rsconf )

Display the current replica configuration by issuing the following command:

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rs.conf()

The replica set configuration object resembles the following:

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{
   "_id" : "rs0",
   "version" : 1,
   "protocolVersion" : NumberLong(1),
   "members" : [
      {
         "_id" : 0,
         "host" : "<hostname>:27017",
         "arbiterOnly" : false,
         "buildIndexes" : true,
         "hidden" : false,
         "priority" : 1,
         "tags" : {

         },
         "slaveDelay" : NumberLong(0),
         "votes" : 1
      },
      {
         "_id" : 1,
         "host" : "<hostname>:27018",
         "arbiterOnly" : false,
         "buildIndexes" : true,
         "hidden" : false,
         "priority" : 1,
         "tags" : {

         },
         "slaveDelay" : NumberLong(0),
         "votes" : 1
      },
      {
         "_id" : 2,
         "host" : "<hostname>:27019",
         "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("598f630adc9053c6ee6d5f38")
   }
}

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