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##Deploying a sharded production ready MongoDB cluster with Ansible
###A Primer into the MongoDB NoSQL database.
The above diagram shows how the MongoDB nosql differs from the traditional relational database model. In RDBMS the data of a user is stored in table and the records of users are stored in rows/columns, While in mongodb the 'table' is replaced by 'collection' and the individual 'records' are called 'documents'. One thing also to be noticed is that the data is stored as key/value pairs in BJSON format.
Another thing to be noticed is that nosql has a looser consistency model, as an example the second document in the users collection has an additonal field of 'last name'. Due to this flexibility the nosql database model can give us:
Better Horizontal scaling capability.
Also mongodb has inbuilt support for
Data Replication & HA
Which makes it good choice for users who have very large data to handle and less requirement for ACID.
MongoDB's Data replication .
Data backup is achieved in Mongodb via Replica sets. As the figure above show's a single replication set consists of a replication master (active) and several other replications slaves (passive). All the database operations like Add/Delete/Update happens on the replication master and the master replicates the data to the slave nodes. mongod is the process which is resposible for all the database activities as well as replication processes. The minimum recommended number of slave servers are 3.
MongoDB's Sharding (Horizontal Scaling) .
Sharding allows to achieve a very high performing database, by partioning the data into seperate chunks and allocating diffent ranges of chunks to diffrent shard servers. The figure above shows a collection which has 90 documents which has been sharded across the three shard server, The first shard getting ranges from 1- 29 etc... . When a client wants to access a certian document it contacts the query router (mongos process), which inturn would contact the 'configuration node' (lightweight mongod process) which keeps a record of which ranges of chunks are distributed across which shards.
Please do note that every shard server should be backed by a replica set, so that when data is written/queried copies of the data are available. So in a three shard deployment we would require 3 replica sets and primaries of each would act as the sharding server.
Here's a basic steps of how sharding works.
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A new database is created, and collections are added.
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New documents get updated as an when clients update, all the new documents goes into a single shard.
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when the size of collection in a shard exceeds the 'chunk_size' the collection is split and balanced across shards.
###Deploy MongoDB cluster via Ansible.
Deploy the Cluster.
The above diagram illustrates the deployment model for mongodb cluster via Ansible, This deployment models focuses on deploying a three shard servers, each having a replica set, the backup replica servers are other two shard primaries. The configuration server are co-located with the shard's. The mongos servers are best deployed on seperate servers. These are the minimum recomended configuration for a production grade mongodb deployment. Please note that the playbooks are capable of deploying N node cluster not necesarily three. Also all the processes are secured using keyfiles.
###Pre-Requisite's
Edit the group_vars/all file to reflect the below variables.
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iface: 'eth1' # the interface to be used for all communication.
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mongod_ports: # The hostname and tcp/ip port combination. mongo1: 2700 mongo2: 2701 mongo3: 2702
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The default directory for storing data is /data, please do change it if requried, also make sure it has sufficient space 10G recommended.
###Once the pre-requisite's have been done, we can procced with the site deployment. The following example deploys a three node MongoDB Cluster
The inventory file looks as follows:
#The site wide list of mongodb servers
[mongo_servers]
mongo1
mongo2
mongo3
#The list of servers where replication should happen, including the master server.
[replication_servers]
mongo1
mongo2
mongo3
#The list of mongodb configuration servers, make sure it is 1 or 3
[mongoc_servers]
mongo1
mongo2
mongo3
#The list of servers where mongos servers would run.
[mongosservers]
mongos1
mongos2
Build the site with the following command:
ansible-playbook -i hosts site.yml
###Verifying the deployed MongoDB Cluster
Once completed we can check replication set availibitly by connecting to individual primary replication set nodes, 'mongo --host 192.168.1.1 --port 2700 and issue the command to query the status of replication set, we should get a similar output.
web2:PRIMARY> rs.status()
{
"set" : "web2",
"date" : ISODate("2013-03-19T10:26:35Z"),
"myState" : 1,
"members" : [
{
"_id" : 0,
"name" : "web2:2013",
"health" : 1,
"state" : 1,
"stateStr" : "PRIMARY",
"uptime" : 102,
"optime" : Timestamp(1363688755000, 1),
"optimeDate" : ISODate("2013-03-19T10:25:55Z"),
"self" : true
},
{
"_id" : 1,
"name" : "web3:2013",
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 40,
"optime" : Timestamp(1363688755000, 1),
"optimeDate" : ISODate("2013-03-19T10:25:55Z"),
"lastHeartbeat" : ISODate("2013-03-19T10:26:33Z"),
"pingMs" : 1
}
],
"ok" : 1
}
we can check the status of the Shards as follows: connect to the mongos service 'mongos --host 192.168.1.1 --port 8888' and issue the following command to get the status of the Shards.
mongos> sh.status()
--- Sharding Status ---
sharding version: { "_id" : 1, "version" : 3 }
shards:
{ "_id" : "web2", "host" : "web2/web2:2013,web3:2013" }
{ "_id" : "web3", "host" : "web3/web2:2014,web3:2014" }
databases:
{ "_id" : "admin", "partitioned" : false, "primary" : "config" }
###We can also make sure the Sharding works by creating a database,collection and populate it with documents and check if the chunks of the collection are balanced equally across nodes. The below diagram illustrates the verification step.
The above mentioned steps can be tested with an automated playbook.
Issue the following command to run the test. In variable passed make sure the servername is one of any mongos server.
ansible-playbook -i hosts playbooks/testsharding.yml -e servername=mongos
Once the playbook completes, we check if the shadring has succeded by logging on to any mongos server and issuing the following command. The output display the number of chunks spread across the shards.
mongos> sh.status()
--- Sharding Status ---
sharding version: { "_id" : 1, "version" : 3 }
shards:
{ "_id" : "bensible", "host" : "bensible/bensible:20103,web2:20103,web3:20103" }
{ "_id" : "web2", "host" : "web2/bensible:20105,web2:20105,web3:20105" }
{ "_id" : "web3", "host" : "web3/bensible:20102,web2:20102,web3:20102" }
databases:
{ "_id" : "admin", "partitioned" : false, "primary" : "config" }
{ "_id" : "test", "partitioned" : true, "primary" : "web3" }
test.test_collection chunks:
bensible 7
web2 6
web3 7
Scaling the Cluster
To add a new node to the configured MongoDb Cluster, setup the inventory file as follows:
#The site wide list of mongodb servers
[mongoservers]
mongo1
mongo2
mongo3
mongo4
#The list of servers where replication should happen, make sure the new node is listed here.
[replicationservers]
mongo4
mongo3
mongo1
mongo2
#The list of mongodb configuration servers, make sure it is 1 or 3
[mongocservers]
mongo1
mongo2
mongo3
#The list of servers where mongos servers would run.
[mongosservers]
mongos1
mongos2
Make sure you have the new node added in the replicationservers section and execute the following command:
ansible-playbook -i hosts site.yml
###Verification.
The verification of the newly added node can be as easy checking the sharding status and see the chunks being rebalanced to the newly added node.
$/usr/bin/mongo localhost:8888/admin -u admin -p 123456
mongos> sh.status()
--- Sharding Status ---
sharding version: { "_id" : 1, "version" : 3 }
shards:
{ "_id" : "bensible", "host" : "bensible/bensible:20103,web2:20103,web3:20103" }
{ "_id" : "web2", "host" : "web2/bensible:20105,web2:20105,web3:20105" }
{ "_id" : "web3", "host" : "web3/bensible:20102,web2:20102,web3:20102" }
{ "_id" : "web4", "host" : "web4/bensible:20101,web3:20101,web4:20101" }
databases:
{ "_id" : "admin", "partitioned" : false, "primary" : "config" }
{ "_id" : "test", "partitioned" : true, "primary" : "bensible" }
test.test_collection chunks:
web4 3
web3 6
web2 6
bensible 5