MongoDB Sharded Cluster ¶

This tutorial shows deploying a sharded MongoDB cluster on india.futuresystems.org.

Tip

Approximate time: 30 minutes

Table of Contents

MongoDB Sharded Cluster

MongoDB Sharding Overview ¶

lesson/database/images/mongodb-overview.png

Figure 1. Sharding Mongodb Image reference: http://docs.mongodb.org/master/MongoDB-sharding-guide.pdf

Shards store the data. To provide high availability and data consistency, in a production sharded cluster, each shard is a replica set 1
Query Routers, or mongos instances, interface with client applications and direct operations to the appropriate shard or shards. The query router processes and targets operations to shards and then returns results to the clients. A sharded cluster can contain more than one query router to divide the client request load. A client sends requests to one query router. Most sharded clusters have many query routers.
Config servers store the cluster’s metadata. This data contains a mapping of the cluster’s data set to the shards. The query router uses this metadata to target operations to specific shards. Production sharded clusters have exactly 3 config servers

Start Virtual Cluster for Database ¶

In this test environment, three instances are required to connect mongodb databases on the cloud. Try cm cluster command to create virtual machines in the OpenStack India. See the page:

Install Mongodb Server ¶

Install MongoDB server on each of the three virtual instances. If you have other operating systems, please use its package management program e.g. yum or brew.

(Ubuntu)
sudo apt-get install mongodb-server

(CentOS)
sudo yum install mongodb-server

(Mac OS)
sudo brew install mongodb-server

Configure Instances ¶

Create data directories for each of the three config server instances. By default, a config server stores its data files in the /data/configdb directory. You can choose a different location. To create a data directory, issue a command similar to the following:
```
sudo mkdir -p /data/configdb
```
Start the three config server instances. Start each by issuing a command using the following syntax:
```
mongod --configsvr --dbpath <path> --port <port>
```
The default port for config servers is 27019. You can specify a different port. The following example starts a config server using the default port and default data directory:
```
sudo mongod --configsvr --dbpath /data/configdb --port 27019 &
```

For additional command options, see mongod or Configuration File Options.

Note

All config servers must be running and available when you first initiate a sharded cluster.

The mongos instances are lightweight and do not require data directories. You can run a mongos instance on a system that runs other cluster components, such as on an application server or a server running a mongod process. By default, a mongos instance runs on port 27017.

When you start the mongos instance, specify the hostnames of the three config servers, either in the configuration file or as command line parameters.

To start a mongos instance, issue a command using the following syntax:

mongos --configdb <config server hostnames>

For example, to start a mongos that connects to config server instance running on the following hosts and on the default ports:

albert_1-i
albert_2-i
albert_3-i

You would issue the following command:

mongos --configdb albert_1-i:27019,albert_2-i:27019,albert_3-i:27019

albert_1-i, albert_2-i, and albert_3-i are hostnames for internal IPs

Each mongos in a sharded cluster must use the same configDB string, with identical host names listed in identical order.

If you start a mongos instance with a string that does not exactly match the string used by the other mongos instances in the cluster, the mongos return a Config Database String Error error and refuse to start.

Add Shards to the Cluster ¶

A shard can be a standalone mongod or a replica set. In a production environment, each shard should be a replica set. Use the procedure in Deploy a Replica Set to deploy replica sets for each shard.

From a mongo shell, connect to the mongos instance. Issue a command using the following syntax:
```
mongo --host <hostname of machine running mongos> --port <port mongos listens on>
```
For example, if a mongos is accessible at mongos0.example.net on port 27017, issue the following command:
```
mongo --host albert_1-i --port 27017
```
Add each shard to the cluster using the sh.addShard() method, as shown in the examples below. Issue sh.addShard() separately for each shard. If the shard is a replica set, specify the name of the replica set and specify a member of the set. In production deployments, all shards should be replica sets.

Note

You can instead use the addShard database command, which lets you specify a name and maximum size for the shard. If you do not specify these, MongoDB automatically assigns a name and maximum size. To use the database command, see addShard.

The following are examples of adding a shard with sh.addShard():

 * To add a shard for a replica set named rs1 with a member running on port
   27017 on mongodb0.example.net, issue the following command::

         sh.addShard( "rs1/albert_1-i:27017" )

   For MongoDB versions prior to 2.0.3, you must specify all members of the
   replica set. For example::


         sh.addShard(
         "rs1/albert_1-i:27017,albert_2-i:27017,albert_3-i:27017"
         )

* To add a shard for a standalone mongod on port 27017 of
  mongodb0.example.net, issue the following command::

       sh.addShard( "albert_1-i:27017" )

Note

It might take some time for chunks to migrate to the new shard.

Enable Sharding for a Database ¶

Before you can shard a collection, you must enable sharding for the collection’s database. Enabling sharding for a database does not redistribute data but make it possible to shard the collections in that database.

Once you enable sharding for a database, MongoDB assigns a primary shard for that database where MongoDB stores all data before sharding begins.

From a mongo shell, connect to the mongos instance. Issue a command using the following syntax:
```
mongo --host <hostname of machine running mongos> --port <port mongos listens on>
```
Issue the sh.enableSharding() method, specifying the name of the database for which to enable sharding. Use the following syntax:
```
sh.enableSharding("<database>")
```

Optionally, you can enable sharding for a database using the enableSharding command, which uses the following syntax:

db.runCommand( { enableSharding: <database> } )

Enable Sharding for a Collection ¶

You enable sharding on a per-collection basis.

Determine what you will use for the shard key. Your selection of the shard key affects the efficiency of sharding. See the selection considerations listed in the Considerations for Selecting Shard Key.
If the collection already contains data you must create an index on the shard key using ensureIndex(). If the collection is empty then MongoDB will create the index as part of the sh.shardCollection() step.
Enable sharding for a collection by issuing the sh.shardCollection() method in the mongo shell. The method uses the following syntax:
```
sh.shardCollection("<database>.<collection>", shard-key-pattern)
```
Replace the <database>.<collection> string with the full namespace of your database, which consists of the name of your database, a dot (e.g. .), and the full name of the collection. The shard-key-pattern represents your shard key, which you specify in the same form as you would an index key pattern.
```
EXAMPLE
The following sequence of commands shards four collections:

sh.shardCollection("records.people", { "zipcode": 1, "name": 1 } )
sh.shardCollection("people.addresses", { "state": 1, "_id": 1 } )
sh.shardCollection("assets.chairs", { "type": 1, "_id": 1 } )
sh.shardCollection("events.alerts", { "_id": "hashed" } )
```

In order, these operations shard:

The people collection in the records database using the shard key {“zipcode”: 1, “name”: 1}. This shard key distributes documents by the value of the zipcode field. If a number of documents have the same value for this field, then that chunk will be splittable by the values of the name field.
The addresses collection in the people database using the shard key {“state”: 1, “_id”: 1}. This shard key distributes documents by the value of the state field. If a number of documents have the same value for this field, then that chunk will be splittable by the values of the _id field.
The chairs collection in the assets database using the shard key {“type”: 1, “_id”: 1}. This shard key distributes documents by the value of the type field. If a number of documents have the same value for this field, then that chunk will be splittable by the values of the _id field.
The alerts collection in the events database using the shard key {“_id”: “hashed”}.

New in version 2.4.

This shard key distributes documents by a hash of the value of the _id field. MongoDB computes the hash of the _id field for the hashed index, which should provide an even distribution of documents across a cluster.

Next Step ¶

We have an additional tutorial for using cloud resources on FutureSystems. In the next page, we explore several examples such as IPython, Chef, etc.