Introduction

In this tutorial, we will be exploring how to balance and rebalance shards in MongoDB. Sharding is a method for storing data across multiple machines. MongoDB uses sharding to support deployments with very large data sets and high throughput operations.

Goals:
- Understand the concept and need for balancing shards in MongoDB.
- Learn how to balance and rebalance shards manually.

What you will learn:
- Basics of shards and sharding in MongoDB.
- MongoDB's auto-balancing of data.
- Triggering manual rebalancing of shards.

Prerequisites:
- Familiarity with MongoDB and its basic operations.
- Basic understanding of sharding.

Step-by-Step Guide

Understanding Shards and Sharding in MongoDB:

Shards are used to store data records in MongoDB. When the size of the data exceeds the capacity of a single machine, sharding is used to distribute the data among multiple machines.

MongoDB automatically balances the data stored in your shards. This is done by moving chunks of data between shards. However, sometimes you might need to manually trigger a rebalance.

Manual Shard Balancing:

MongoDB provides the sh.startBalancer() and sh.stopBalancer() methods to control the balancer state.

1. To check the balancer state, use the sh.getBalancerState() command.

2. To start the balancer, use the sh.startBalancer() command.

3. To stop the balancer, use the sh.stopBalancer() command.

Code Examples

Checking the balancer state:

// check the current balancer state
sh.getBalancerState();

The output will be either true (balancer is enabled) or false (balancer is disabled).

Starting the balancer:

// start the balancer
sh.startBalancer();

After running this command, MongoDB will start balancing the chunks between the shards.

Stopping the balancer:

// stop the balancer
sh.stopBalancer();

After running this command, MongoDB will stop balancing the chunks between the shards.

Summary

In this tutorial, we've learned about shards and their balancing in MongoDB. We've understood how MongoDB automatically balances data and how we can manually trigger rebalancing.

Next steps would be to understand more complex aspects of sharding, like configuring shard zones and tag-aware sharding.

Practice Exercises

1. Start a MongoDB instance, create a sharded collection, and check its balancer status.
2. Manually start and stop the balancer, and observe the effect on data distribution.
3. Try to identify a scenario where automatic balancing would not be sufficient and manual intervention would be needed.

Note: As you work through these exercises, remember that balancing and rebalancing should be done carefully, considering the load on the database and the size of the data.