Sharding in MongoDB
Sharding, can also be called horizontal scaling. The scaling approach divides the data set and distributes the data over multiple servers. Each of this server can be called a shard. Each shard is an independent database, and collectively, the shards make up a single logical database. Learn MongoDB online training from industrial experts for more skills and techniques.
What other components are required for MongoDB sharding?
Like mentioned above, the Shards stores the data. For ensuring that shards are highly available, you need to have replica sets . We will go into more details later around replica sets, but for now the easiest way to explain this is — you could have a primary shard which you can write to and a second shard which is paired with the primary shard hosting the same data which you can read from. When you do sharding, you would need to know which shard to get the data from. This is done by the Query Router. The Query Router direct requests to the appropriate shard or shards. It processes and targets operations to shards and then returns results to the clients.
How does the query router know which shard to get the data from ? This information is stored in Config servers. They store the cluster’s metadata. This data contains a mapping of the cluster’s data set to the shards. The query router uses this data to route requests to the right shards.
Turning a Replica Set to a Sharded Cluster
In a Sharded Cluster, another 2 new roles will be added: mongos and mongod config. mongos is a routing service for MongoDB Sharded Clusters, it determines the location of the data in the cluster, and forwards operations to the right shard. mongos requires mongod config, which stores the metadata of the cluster. All mongos instances must specify the mongod config hosts to the — configdb setting in the same order, and the mongos will read from the first config server (if it cannot connect to the config server, it will move on to the next on the list).
Previously, our mongod replication instance listened on TCP port 27017. We are going to change this to listen to another port (27018) since mongos will take over port 27017 to serve queries from clients.mongod config will use port 27019 to serve the cluster metadata.
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