Deploy a Self-Managed Replica Set
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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.
This page covers how to deploy a replica set for self-managed deployments.
To learn more about deploying a replica set for deployments hosted in MongoDB Atlas, see Create a Cluster.
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
.
Note
Outside of a rolling upgrade, all mongod
members of
a replica set should use the same major version of
MongoDB.
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 fail startup validation and do not start.
IP Binding
Use the --bind_ip
option to ensure that
MongoDB listens for connections from applications on configured
addresses.
Warning
Before you bind your instance to a publicly-accessible IP address, you must secure your cluster from unauthorized access. For a complete list of security recommendations, see Security Checklist for Self-Managed Deployments. 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 Self-Managed Configuration File Options.
Deploy a Replica Set in the Terminal
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 Self-Managed Replica Set With Keyfile Authentication. If you wish to deploy a replica set from a single MongoDB instance, see Convert a Standalone Self-Managed mongod to a Replica Set. For more information on replica set deployments, see the Replication and Replica Set Deployment Architectures documentation.
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
Before you bind your instance to a publicly-accessible IP address, you must secure your cluster from unauthorized access. For a complete list of security recommendations, see Security Checklist for Self-Managed Deployments. At minimum, consider enabling authentication and hardening network infrastructure.
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 only one mongod
instance
for the replica set.
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 fail startup validation and do not start.
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.