Convert a Replica Set to a Replicated Sharded Cluster


Following this tutorial, you will convert a single 3-member replica set to a cluster that consists of 2 shards. Each shard will consist of an independent 3-member replica set.

The tutorial uses a test environment running on a local system UNIX-like system. You should feel encouraged to “follow along at home.” If you need to perform this process in a production environment, notes throughout the document indicate procedural differences.

The procedure, from a high level, is as follows:

  1. Create or select a 3-member replica set and insert some data into a collection.
  2. Start the config databases and create a cluster with a single shard.
  3. Create a second replica set with three new mongod instances.
  4. Add the second replica set as a shard in the cluster.
  5. Enable sharding on the desired collection or collections.


Install MongoDB according to the instructions in the MongoDB Installation Tutorial.

Deploy a Replica Set with Test Data

If have an existing MongoDB replica set deployment, you can omit the this step and continue from Deploy Sharding Infrastructure.

Use the following sequence of steps to configure and deploy a replica set and to insert test data.

  1. Create the following directories for the first replica set instance, named firstset:

    • /data/example/firstset1
    • /data/example/firstset2
    • /data/example/firstset3

    To create directories, issue the following command:

    mkdir -p /data/example/firstset1 /data/example/firstset2 /data/example/firstset3
  2. In a separate terminal window or GNU Screen window, start three mongod instances by running each of the following commands:

    mongod --dbpath /data/example/firstset1 --port 10001 --replSet firstset --oplogSize 700 --rest
    mongod --dbpath /data/example/firstset2 --port 10002 --replSet firstset --oplogSize 700 --rest
    mongod --dbpath /data/example/firstset3 --port 10003 --replSet firstset --oplogSize 700 --rest


    The --oplogSize 700 option restricts the size of the operation log (i.e. oplog) for each mongod instance to 700MB. Without the --oplogSize option, each mongod reserves approximately 5% of the free disk space on the volume. By limiting the size of the oplog, each instance starts more quickly. Omit this setting in production environments.

  3. In a mongo shell session in a new terminal, connect to the mongodb instance on port 10001 by running the following command. If you are in a production environment, first read the note below.

    mongo localhost:10001/admin


    Above and hereafter, if you are running in a production environment or are testing this process with mongod instances on multiple systems, replace “localhost” with a resolvable domain, hostname, or the IP address of your system.

  4. In the mongo shell, initialize the first replica set by issuing the following command:

    db.runCommand({"replSetInitiate" :
                        {"_id" : "firstset", "members" : [{"_id" : 1, "host" : "localhost:10001"},
                                                          {"_id" : 2, "host" : "localhost:10002"},
                                                          {"_id" : 3, "host" : "localhost:10003"}
            "info" : "Config now saved locally.  Should come online in about a minute.",
            "ok" : 1
  5. In the mongo shell, create and populate a new collection by issuing the following sequence of JavaScript operations:

    use test
    switched to db test
    people = ["Marc", "Bill", "George", "Eliot", "Matt", "Trey", "Tracy", "Greg", "Steve", "Kristina", "Katie", "Jeff"];
    for(var i=0; i<1000000; i++){
                                 name = people[Math.floor(Math.random()*people.length)];
                                 user_id = i;
                                 boolean = [true, false][Math.floor(Math.random()*2)];
                                 added_at = new Date();
                                 number = Math.floor(Math.random()*10001);
                                 db.test_collection.save({"name":name, "user_id":user_id, "boolean": boolean, "added_at":added_at, "number":number });

    The above operations add one million documents to the collection test_collection. This can take several minutes, depending on your system.

    The script adds the documents in the following form:

{ "_id" : ObjectId("4ed5420b8fc1dd1df5886f70"), "name" : "Greg", "user_id" : 4, "boolean" : true, "added_at" : ISODate("2011-11-29T20:35:23.121Z"), "number" : 74 }

Deploy Sharding Infrastructure

This procedure creates the three config databases that store the cluster’s metadata.


For development and testing environments, a single config database is sufficient. In production environments, use three config databases. Because config instances store only the metadata for the sharded cluster, they have minimal resource requirements.

  1. Create the following data directories for three config database instances:

    • /data/example/config1
    • /data/example/config2
    • /data/example/config3

    Issue the following command at the system prompt:

    mkdir -p /data/example/config1 /data/example/config2 /data/example/config3
  2. In a separate terminal window or GNU Screen window, start the config databases by running the following commands:

    mongod --configsvr --dbpath /data/example/config1 --port 20001
    mongod --configsvr --dbpath /data/example/config2 --port 20002
    mongod --configsvr --dbpath /data/example/config3 --port 20003
  3. In a separate terminal window or GNU Screen window, start mongos instance by running the following command:

    mongos --configdb localhost:20001,localhost:20002,localhost:20003 --port 27017 --chunkSize 1


    If you are using the collection created earlier or are just experimenting with sharding, you can use a small --chunkSize (1MB works well.) The default chunkSize of 64MB means that your cluster must have 64MB of data before the MongoDB’s automatic sharding begins working.

    In production environments, do not use a small shard size.

    The configDB options specify the configuration databases (e.g. localhost:20001, localhost:20002, and localhost:2003). The mongos instance runs on the default “MongoDB” port (i.e. 27017), while the databases themselves are running on ports in the 30001 series. In the this example, you may omit the --port 27017 option, as 27017 is the default port.

  4. Add the first shard in mongos. In a new terminal window or GNU Screen session, add the first shard, according to the following procedure:

    1. Connect to the mongos with the following command:

      mongo localhost:27017/admin
    2. Add the first shard to the cluster by issuing the addShard command:

      db.runCommand( { addShard : "firstset/localhost:10001,localhost:10002,localhost:10003" } )
    3. Observe the following message, which denotes success:

      { "shardAdded" : "firstset", "ok" : 1 }

Deploy a Second Replica Set

This procedure deploys a second replica set. This closely mirrors the process used to establish the first replica set above, omitting the test data.

  1. Create the following data directories for the members of the second replica set, named secondset:

    • /data/example/secondset1
    • /data/example/secondset2
    • /data/example/secondset3
  2. In three new terminal windows, start three instances of mongod with the following commands:

    mongod --dbpath /data/example/secondset1 --port 10004 --replSet secondset --oplogSize 700 --rest
    mongod --dbpath /data/example/secondset2 --port 10005 --replSet secondset --oplogSize 700 --rest
    mongod --dbpath /data/example/secondset3 --port 10006 --replSet secondset --oplogSize 700 --rest


    As above, the second replica set uses the smaller oplogSizeMB configuration. Omit this setting in production environments.

  3. In the mongo shell, connect to one mongodb instance by issuing the following command:

    mongo localhost:10004/admin
  4. In the mongo shell, initialize the second replica set by issuing the following command:

    db.runCommand({"replSetInitiate" :
                        {"_id" : "secondset",
                         "members" : [{"_id" : 1, "host" : "localhost:10004"},
                                      {"_id" : 2, "host" : "localhost:10005"},
                                      {"_id" : 3, "host" : "localhost:10006"}
         "info" : "Config now saved locally.  Should come online in about a minute.",
         "ok" : 1
  5. Add the second replica set to the cluster. Connect to the mongos instance created in the previous procedure and issue the following sequence of commands:

    use admin
    db.runCommand( { addShard : "secondset/localhost:10004,localhost:10005,localhost:10006" } )

    This command returns the following success message:

    { "shardAdded" : "secondset", "ok" : 1 }
  6. Verify that both shards are properly configured by running the listShards command. View this and example output below:

           "shards" : [
                         "_id" : "firstset",
                         "host" : "firstset/localhost:10001,localhost:10003,localhost:10002"
                         "_id" : "secondset",
                         "host" : "secondset/localhost:10004,localhost:10006,localhost:10005"
         "ok" : 1

Enable Sharding

MongoDB must have sharding enabled on both the database and collection levels.

Enabling Sharding on the Database Level

Issue the enableSharding command. The following example enables sharding on the “test” database:

db.runCommand( { enableSharding : "test" } )
{ "ok" : 1 }

Create an Index on the Shard Key

MongoDB uses the shard key to distribute documents between shards. Once selected, you cannot change the shard key. Good shard keys:

  • have values that are evenly distributed among all documents,
  • group documents that are often accessed at the same time into contiguous chunks, and
  • allow for effective distribution of activity among shards.

Typically shard keys are compound, comprising of some sort of hash and some sort of other primary key. Selecting a shard key depends on your data set, application architecture, and usage pattern, and is beyond the scope of this document. For the purposes of this example, we will shard the “number” key. This typically would not be a good shard key for production deployments.

Create the index with the following procedure:

use test

See also

The Shard Key Overview and Shard Key sections.

Shard the Collection

Issue the following command:

use admin
db.runCommand( { shardCollection : "test.test_collection", key : {"number":1} })
{ "collectionsharded" : "test.test_collection", "ok" : 1 }

The collection test_collection is now sharded!

Over the next few minutes the Balancer begins to redistribute chunks of documents. You can confirm this activity by switching to the test database and running db.stats() or db.printShardingStatus().

As clients insert additional documents into this collection, mongos distributes the documents evenly between the shards.

In the mongo shell, issue the following commands to return statics against each cluster:

use test

Example output of the db.stats() command:

     "raw" : {
             "firstset/localhost:10001,localhost:10003,localhost:10002" : {
                     "db" : "test",
                     "collections" : 3,
                     "objects" : 973887,
                     "avgObjSize" : 100.33173458522396,
                     "dataSize" : 97711772,
                     "storageSize" : 141258752,
                     "numExtents" : 15,
                     "indexes" : 2,
                     "indexSize" : 56978544,
                     "fileSize" : 1006632960,
                     "nsSizeMB" : 16,
                     "ok" : 1
             "secondset/localhost:10004,localhost:10006,localhost:10005" : {
                     "db" : "test",
                     "collections" : 3,
                     "objects" : 26125,
                     "avgObjSize" : 100.33286124401914,
                     "dataSize" : 2621196,
                     "storageSize" : 11194368,
                     "numExtents" : 8,
                     "indexes" : 2,
                     "indexSize" : 2093056,
                     "fileSize" : 201326592,
                     "nsSizeMB" : 16,
                     "ok" : 1
     "objects" : 1000012,
     "avgObjSize" : 100.33176401883178,
     "dataSize" : 100332968,
     "storageSize" : 152453120,
     "numExtents" : 23,
     "indexes" : 4,
     "indexSize" : 59071600,
     "fileSize" : 1207959552,
     "ok" : 1

Example output of the db.printShardingStatus() command:

--- Sharding Status ---
sharding version: { "_id" : 1, "version" : 3 }
       {  "_id" : "firstset",  "host" : "firstset/localhost:10001,localhost:10003,localhost:10002" }
       {  "_id" : "secondset",  "host" : "secondset/localhost:10004,localhost:10006,localhost:10005" }
       {  "_id" : "admin",  "partitioned" : false,  "primary" : "config" }
       {  "_id" : "test",  "partitioned" : true,  "primary" : "firstset" }
                  test.test_collection chunks:
                                               secondset     5
                                               firstset      186


In a few moments you can run these commands for a second time to demonstrate that chunks are migrating from firstset to secondset.

When this procedure is complete, you will have converted a replica set into a cluster where each shard is itself a replica set.