1. Introduction

Goal of the Tutorial

This tutorial aims to provide insight into MongoDB's capacity to handle multiple simultaneous transactions. We will take a deep dive into concurrency control and isolation levels in MongoDB.

Learning Objectives

By the end of this tutorial, you would have a good understanding of:
- What is Concurrency Control and Isolation
- How MongoDB handles concurrency control and isolation
- Practical examples of how to implement these concepts

Prerequisites

Before starting, you should have basic knowledge of MongoDB, Node.js, and JavaScript. Familiarity with the concept of database transactions would be beneficial but not necessary.

2. Step-by-Step Guide

Concurrency Control

Concurrency control in MongoDB is managed by what's known as a 'lock'. When a transaction is being processed, MongoDB locks the data being accessed to prevent other transactions from modifying it until it's done.

There are two types of locks in MongoDB:
- Shared locks: These locks allow multiple transactions to read (not write) the same data simultaneously.
- Exclusive locks: These locks are used when data is being modified. Only one transaction can hold an exclusive lock on data at a time.

Isolation

Isolation in MongoDB ensures that the execution of transactions concurrently will result in a system state that would be obtained if transactions were executed serially. MongoDB provides different levels of isolation and you can choose the one that best fits your needs.

3. Code Examples

Example 1: Shared Lock

// We start a session
const session = db.getMongo().startSession()

// We start a transaction
session.startTransaction({readConcern: {level: 'snapshot'}})

// We perform a find operation
session.getDatabase('test').getCollection('myCollection').find({_id: 1})

// We commit the transaction
session.commitTransaction()

// We end the session
session.endSession()

Example 2: Exclusive Lock

// We start a session
const session = db.getMongo().startSession()

// We start a transaction
session.startTransaction({readConcern: {level: 'snapshot'}})

// We perform an update operation
session.getDatabase('test').getCollection('myCollection').updateOne({_id: 1}, {$set: {field1: "newValue"}})

// We commit the transaction
session.commitTransaction()

// We end the session
session.endSession()

4. Summary

In this tutorial, we covered the basics of concurrency control and isolation in MongoDB. We learned about shared and exclusive locks and how they help MongoDB manage concurrent transactions. We also saw practical examples of how to use these concepts in Node.js.

For further learning, you can study about different isolation levels provided by MongoDB and when to use each one.

5. Practice Exercises

Exercise 1:

Write a script to perform a read operation inside a transaction.

Exercise 2:

Write a script to perform a write operation inside a transaction.

Exercise 3:

Write a script to perform both read and write operations inside a single transaction.

Remember, practice is key to mastering any concept. Keep practicing and keep learning!