Question: What is MongoDB cluster sharding?

Answer

Mongodb cluster sharding is a database architecture pattern that distributes data across multiple machines. This method helps in supporting very large databases and high throughput operations by partitioning data into smaller, more manageable pieces called 'shards'. Each shard is hosted on separate shard servers, which can be either standalone mongod instances or replica sets, providing redundancy and high availability.

How Does Sharding Work?

Sharding in MongoDB involves three main components:

1. Shard: Each shard holds a portion of the data. In production environments, each shard is a replica set.
2. Mongos: The query router. Applications don't connect directly to the shards but instead connect through a mongos instance which routes queries to the appropriate shard(s).
3. Config Servers: These store the cluster's metadata and configuration settings. They track the location of the shards and manage the overall structure of the cluster.

Setting Up a Sharded Cluster

A basic example for setting up a sharded cluster involves the following steps:

1. Initialize the Config Server:

mongod --configsvr --replSet configReplSet --dbpath /data/configdb --port 27019

2. Start Mongos:

mongos --configdb configReplSet/<configReplSet_server_ip>:27019 --port 27017

3. Initialize Shard (as Replica Set if needed):

mongod --shardsvr --replSet shard1ReplSet --dbpath /data/shard1db --port 27018

4. Add Shard to Cluster:

Connect to the mongos instance:

mongo --host <mongos_ip> --port 27017

Then add the shard:

sh.addShard("<shard1ReplSet>/<shard1_server_ip>:27018");

Key Considerations

• Shard Keys: Choosing an effective shard key is critical as it determines how data is distributed across the shards. It should be a field that appears in most queries, ensuring that query loads can be evenly distributed.
• Balancing: MongoDB automatically balances data distribution across shards, moving chunks of data as necessary. Correct shard key selection helps minimize the need for rebalancing.
• Query Routing: The mongos component routes queries to the correct shard(s), based on the shard key.

Sharding enables MongoDB clusters to scale horizontally, supporting large datasets and high transaction rates by distributing operations across multiple servers. Properly implemented, sharding can provide significant performance improvements for read and write operations.