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Deploy a Sharded Cluster

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  • Overview
  • Considerations
  • Procedure

This tutorial involves creating a new sharded cluster that consists of a mongos, the config server replica set, and two shard replica sets.

Each member of a sharded cluster must be able to connect to all other members in the cluster. This includes all shards and config servers. Ensure that network and security systems, including all interface and firewalls, allow these connections.

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.

If you use either localhost or its IP address as the hostname portion of any host identifier, you must use that identifier as the host setting for any other MongoDB component in the cluster.

For example, the sh.addShard() method takes a host parameter for the hostname of the target shard. If you set host to localhost, you must then use localhost as the host for all other shards in the cluster.

This tutorial does not include the required steps for configuring Internal/Membership Authentication or Role-Based Access Control.

In production environments, sharded clusters should employ at minimum x.509 security for internal authentication and client access.

The following steps deploys a config server replica set.

For a production deployment, deploy a config server replica set with at least three members. For testing purposes, you can create a single-member replica set.

Note

The config server replica set must not use the same name as any of the shard replica sets.

For this tutorial, the config server replica set members are associated with the following hosts:

Config Server Replica Set Member
Hostname
Member 0
cfg1.example.net
Member 1
cfg2.example.net
Member 2
cfg3.example.net
1

When starting each mongod, specify the mongod settings either via a configuration file or the command line.

2

Connect mongosh to one of the config server members.

mongosh --host <hostname> --port <port>
3

From mongosh, run the rs.initiate() method.

rs.initiate() can take an optional replica set configuration document. In the replica set configuration document, include:

  • The _id set to the replica set name specified in either the replication.replSetName or the --replSet option.

  • The configsvr field set to true for the config server replica set.

  • The members array with a document per each member of the replica set.

Important

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

rs.initiate(
{
_id: "myReplSet",
configsvr: true,
members: [
{ _id : 0, host : "cfg1.example.net:27019" },
{ _id : 1, host : "cfg2.example.net:27019" },
{ _id : 2, host : "cfg3.example.net:27019" }
]
}
)

See Replica Set Configuration for more information on replica set configuration documents.

Once the config server replica set (CSRS) is initiated and up, proceed to creating the shard replica sets.

For a production deployment, use a replica set with at least three members. For testing purposes, you can create a single-member replica set.

Note

Shard replica sets must not use the same name as the config server replica set.

For each shard, use the following steps to create the shard replica set:

1

When starting each mongod, specify the mongod settings either via a configuration file or the command line.

2

Connect mongosh to one of the replica set members.

mongosh --host <hostname> --port <port>
3

From mongosh, run the rs.initiate() method.

rs.initiate() can take an optional replica set configuration document. In the replica set configuration document, include:

  • The _id field set to the replica set name specified in either the replication.replSetName or the --replSet option.

  • The members array with a document per each member of the replica set.

The following example initiates a three member replica set.

Important

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

rs.initiate(
{
_id : "myReplSet",
members: [
{ _id : 0, host : "s1-mongo1.example.net:27018" },
{ _id : 1, host : "s1-mongo2.example.net:27018" },
{ _id : 2, host : "s1-mongo3.example.net:27018" }
]
}
)

Start a mongos using either a configuration file or a command line parameter to specify the config servers.

At this point, your sharded cluster consists of the mongos and the config servers. You can now connect to the sharded cluster using mongosh.

Connect mongosh to the mongos. Specify the host and port on which the mongos is running:

mongosh --host <hostname> --port <port>

Once you have connected mongosh to the mongos, continue to the next procedure to add shards to the cluster.

In a mongosh session that is connected to the mongos, use the sh.addShard() method to add each shard to the cluster.

The following operation adds a single shard replica set to the cluster:

sh.addShard( "<replSetName>/s1-mongo1.example.net:27018,s1-mongo2.example.net:27018,s1-mongo3.example.net:27018")

Repeat these steps until the cluster includes all desired shards.

To shard a collection, connect mongosh to the mongos and use the sh.shardCollection() method.

Note

Sharding and Indexes

If the collection already contains data, you must create an index that supports the shard key before sharding the collection. If the collection is empty, MongoDB creates the index as part of sh.shardCollection().

MongoDB provides two strategies to shard collections:

  • Hashed sharding uses a hashed index of a single field as the shard key to partition data across your sharded cluster.

    sh.shardCollection("<database>.<collection>", { <shard key field> : "hashed" } )
  • Range-based sharding can use multiple fields as the shard key and divides data into contiguous ranges determined by the shard key values.

    sh.shardCollection("<database>.<collection>", { <shard key field> : 1, ... } )

Your selection of shard key affects the efficiency of sharding, as well as your ability to take advantage of certain sharding features such as zones. To learn how to choose an effective shard key, see Choose a Shard Key.

Starting in version 4.0, mongosh provides the method convertShardKeyToHashed(). This method uses the same hashing function as the hashed index and can be used to see what the hashed value would be for a key.

Tip

See also:

←  Ranged ShardingZones →
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