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Deploy a Geographically Redundant Replica Set

Overview

This tutorial outlines the process for deploying a replica set with members in multiple locations. The tutorial addresses three-member replica sets and five-member replica sets. If you have an even number of replica set members, add an arbiter to deploy an odd number replica set.

For more information on distributed replica sets, see Replica Sets Distributed Across Two or More Data Centers. See also Replica Set Deployment Architectures and see Replication.

Considerations

Architecture

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.

Connectivity

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.

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.

Distribution of the Members

If possible, use an odd number of data centers, and choose a distribution of members that maximizes the likelihood that even with a loss of a data center, the remaining replica set members can form a majority or at minimum, provide a copy of your data.

Voting Members

Never deploy more than seven voting members.

Prerequisites

For all configurations in this tutorial, deploy each replica set member on a separate system. Although you may deploy more than one replica set member on a single system, doing so reduces the redundancy and capacity of the replica set. Such deployments are typically for testing purposes.

This tutorial assumes you have installed MongoDB on each system that will be part of your replica set. If you have not already installed MongoDB, see the installation tutorials.

Procedures

Deploy a Geographically Redundant Three-Member Replica Set

For a geographically redundant three-member replica set deployment, you must decide how to distribute your system. Some possible distributions for the three members are:

  • Across Three Data Centers: One members to each site.
  • Across Two Data Centers: Two members to Site A and one member to Site B. If one of the members of the replica set is an arbiter, distribute the arbiter to Site A with a data-bearing member.

Note

Distributing across two data centers provides benefit over a single data center. However, in a two data center distribution, if the data center with two members goes down, the replica set becomes read-only. As such, for config server replica sets (CSRS), distribute across three data centers if possible.

1

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 mongodb0.example.net
Member 1 mongodb1.example.net
Member 2 mongodb2.example.net

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:

replication:
   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.

2

Connect a mongo shell to one of the mongod instances.

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

mongo

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

3

Initiate the replica set.

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

Important

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

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.

4

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" : {

         },
         "slaveDelay" : NumberLong(0),
         "votes" : 1
      },
      {
         "_id" : 1,
         "host" : "mongodb1.example.net:27017",
         "arbiterOnly" : false,
         "buildIndexes" : true,
         "hidden" : false,
         "priority" : 1,
         "tags" : {

         },
         "slaveDelay" : NumberLong(0),
         "votes" : 1
      },
      {
         "_id" : 2,
         "host" : "mongodb2.example.net:27017",
         "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")
   }
}
5

Optional. Configure the member eligibility for becoming primary.

In some cases, you may prefer that the members in one data center be elected primary before the members in the other data centers. You can modify the priority of the members such that the members in the one data center has higher priority than the members in the other data centers.

Some members of the replica set, such as members that have networking restraint or limited resources, should not be able to become primary in a failover. Configure members that should not become primary to have priority 0.

For example, to lower the relative eligibility of the member located in one of the sites (in this example, mongodb2.example.net), set the member’s priority to 0.5.

  1. View the replica set configuration to determine the members array position for the member. Keep in mind the array position is not the same as the _id:

    rs.conf()
    
  2. Copy the replica set configuration object to a variable (to cfg in the example below). Then, in the variable, set the correct priority for the member. Then pass the variable to rs.reconfig() to update the replica set configuration.

    For example, to set priority for the third member in the array (i.e., the member at position 2), issue the following sequence of commands:

    cfg = rs.conf()
    cfg.members[2].priority = 0.5
    rs.reconfig(cfg)
    

    Note

    The rs.reconfig() shell method can force the current primary to step down, causing an election. When the primary steps down, all clients will disconnect. This is the intended behavior. While most elections complete within a minute, always make sure any replica configuration changes occur during scheduled maintenance periods.

After these commands return, you have a geographically redundant three-member replica set.

6

Ensure that the replica set has a primary.

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

Deploy a Geographically Redundant Five-Member Replica Set

For a geographically redundant five-member replica set deployment, you must decide how to distribute your system. Some possible distributions for the five members are:

  • Across Three Data Centers: Two members in Site A, two members in Site B, one member in Site C.
  • Across Four Data Centers: Two members in one site, and one member in the other three sites.
  • Across Five Data Centers: One members in each site.
  • Across Two Data Centers: Three members in Site A and two members in Site B. If possible, avoid distributing config server replica set across only two data centers.

Note

Distributing replica set members across two data centers provides benefit over a single data center. However, in a two data center distribution, if the data center with the majority of the members goes down, the replica set becomes read-only.

If possible, distribute members across at least three data centers. For config server replica sets (CSRS), the best practice is to distribute across three (or more depending on the number of members) centers.

The following five-member replica set includes an arbiter.

1

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 five 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
Member 3 mongodb3.example.net
Member 4 mongodb4.example.net

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:

replication:
   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.

2

Connect a mongo shell to one of the mongod instances.

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

mongo

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

3

Initiate the replica set.

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

Important

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

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" },
      { _id: 3, host: "mongodb3.example.net:27017" },
      { _id: 4, host: "mongodb4.example.net:27017", arbiterOnly: true }
   ]
})

The last member is added as an arbiter.

For the following MongoDB versions, pv1 increases the likelihood of w:1 rollbacks compared to pv0 for replica sets with arbiters:

  • MongoDB 3.4.1
  • MongoDB 3.4.0
  • MongoDB 3.2.11 or earlier

See Replica Set Protocol Versions.

4

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" : {

         },
         "slaveDelay" : NumberLong(0),
         "votes" : 1
      },
      {
         "_id" : 1,
         "host" : "mongodb1.example.net:27017",
         "arbiterOnly" : false,
         "buildIndexes" : true,
         "hidden" : false,
         "priority" : 1,
         "tags" : {

         },
         "slaveDelay" : NumberLong(0),
         "votes" : 1
      },
      {
         "_id" : 2,
         "host" : "mongodb2.example.net:27017",
         "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")
   }
}
5

Optional. Configure the member eligibility for becoming primary.

In some cases, you may prefer that the members in one data center be elected primary before the members in the other data centers. You can modify the priority of the members such that the members in the one data center has higher priority than the members in the other data centers.

Some members of the replica set, such as members that have networking restraint or limited resources, should not be able to become primary in a failover. Configure members that should not become primary to have priority 0.

For example, to lower the relative eligibility of the member located in one of the sites (in this example, mongodb2.example.net), set the member’s priority to 0.5.

  1. View the replica set configuration to determine the members array position for the member. Keep in mind the array position is not the same as the _id:

    rs.conf()
    
  2. Copy the replica set configuration object to a variable (to cfg in the example below). Then, in the variable, set the correct priority for the member. Then pass the variable to rs.reconfig() to update the replica set configuration.

    For example, to set priority for the third member in the array (i.e., the member at position 2), issue the following sequence of commands:

    cfg = rs.conf()
    cfg.members[2].priority = 0.5
    rs.reconfig(cfg)
    

    Note

    The rs.reconfig() shell method can force the current primary to step down, causing an election. When the primary steps down, all clients will disconnect. This is the intended behavior. While most elections complete within a minute, always make sure any replica configuration changes occur during scheduled maintenance periods.

After these commands return, you have a geographically redundant five-member replica set.

6

Ensure that the replica set has a primary.

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