Deploy a Replica Set

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Overview

Requirements
Considerations When Deploying a Replica Set
Procedure

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 mongod 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 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.

Considerations When Deploying a Replica Set

Architecture

In production, deploy each member of the replica set to its own machine. If possible, ensure that MongoDB listens on the default port of 27017.

For more information, see Replica Set Deployment Architectures.

Hostnames

Important

To avoid configuration updates due to IP address changes, use DNS hostnames instead of IP addresses. It is particularly important to use a DNS hostname instead of an IP address when configuring replica set members or sharded cluster members.

Use hostnames instead of IP addresses to configure clusters across a split network horizon. Starting in MongoDB 5.0, nodes that are only configured with an IP address will fail startup validation and will not start.

IP Binding

Use the --bind_ip option to ensure that MongoDB listens for connections from applications on configured addresses.

Changed in version 3.6:

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.

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.By default mongod and mongos that are bound to localhost only accept connections from clients that are running on the same computer. This binding behavior includes mongosh and other members of your replica set or sharded cluster. Remote clients cannot connect to binaries that are bound only to localhost.To override the default binding and bind to other IP addresses, use the net.bindIp configuration file setting or the --bind_ip command-line option to specify a list of hostnames or IP addresses.

Warning

Starting in MongDB 5.0, split horizon DNS nodes that are only configured with an IP address fail startup validation and report an error. See disableSplitHorizonIPCheck.

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:

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:

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

Connectivity

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

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. See the IP Binding considerations.

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.

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.

Configuration

Create the directory where MongoDB stores data files before deploying MongoDB.

Specify the mongod configuration in a configuration file stored in /etc/mongod.conf or a related location.

For more information about configuration options, see Configuration File Options.

Procedure

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 must have a distinct name.
Set net.bindIp option to the hostname/ip or a comma-delimited list of hostnames/ips.
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	`mongodb0.example.net`
Member 1	`mongodb1.example.net`
Member 2	`mongodb2.example.net`

The following example specifies the replica set name and the ip binding through the --replSet and --bind_ip command-line options:

Warning

mongod --replSet "rs0" --bind_ip localhost,<hostname(s)|ip address(es)>

For <hostname(s)|ip address(es)>, specify the hostname(s) and/or ip address(es) for your mongod instance that remote clients (including the other members of the replica set) can use to connect to the instance.

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

replication:
   replSetName: "rs0"
net:
   bindIp: localhost,<hostname(s)|ip address(es)>

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 `mongosh` to one of the `mongod` instances.

From the same machine where one of the mongod is running (in this tutorial, mongodb0.example.net), start mongosh. To connect to the mongod listening to localhost on the default port of 27017, simply issue:

mongosh

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

If your mongod is not running on the default port, specify the --port option for mongosh.

Initiate the replica set.

From mongosh, run rs.initiate() on replica set member 0.

Important

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

Important

rs.initiate( {
   _id : "rs0",
   members: [
      { _id: 0, host: "mongodb0.example.net:27017" },
      { _id: 1, host: "mongodb1.example.net:27017" },
      { _id: 2, host: "mongodb2.example.net:27017" }
   ]
})

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:

rs.conf()

The replica set configuration object resembles the following:

{
   "_id" : "rs0",
   "version" : 1,
   "protocolVersion" : NumberLong(1),
   "members" : [
      {
         "_id" : 0,
         "host" : "mongodb0.example.net:27017",
         "arbiterOnly" : false,
         "buildIndexes" : true,
         "hidden" : false,
         "priority" : 1,
         "tags" : {
         },
         "secondaryDelaySecs" : NumberLong(0),
         "votes" : 1
      },
      {
         "_id" : 1,
         "host" : "mongodb1.example.net:27017",
         "arbiterOnly" : false,
         "buildIndexes" : true,
         "hidden" : false,
         "priority" : 1,
         "tags" : {
         },
         "secondaryDelaySecs" : NumberLong(0),
         "votes" : 1
      },
      {
         "_id" : 2,
         "host" : "mongodb2.example.net:27017",
         "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("585ab9df685f726db2c6a840")
   }
}

Ensure that the replica set has a primary.

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

Deploy a Replica Set

Overview

Requirements

Considerations When Deploying a Replica Set

Architecture

Hostnames

Important

IP Binding

Warning

Warning

Connectivity

Configuration

Procedure

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

Warning

Connect `mongosh` to one of the `mongod` instances.

Initiate the replica set.

Important

Important

View the replica set configuration.

Ensure that the replica set has a primary.

Tip

See also:

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Overview

Requirements

Considerations When Deploying a Replica Set

Architecture

Hostnames

Important

IP Binding

Warning

Warning

Connectivity

Configuration

Procedure

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

Warning

Connect mongosh to one of the mongod instances.

Initiate the replica set.

Important

Important

View the replica set configuration.

Ensure that the replica set has a primary.

Tip

See also:

Deploy a Replica Set

Connect `mongosh` to one of the `mongod` instances.