Commit and abort behavior in MongoDB - Mini Project: Build & Apply

Commit and Abort Behavior in MongoDB Transactions

📖 Scenario: You are managing a small online bookstore database. You want to ensure that when a customer places an order, the stock quantity is updated only if the order is successfully recorded. If anything goes wrong, no changes should be saved.

🎯 Goal: Build a MongoDB transaction that inserts a new order and updates the stock quantity of a book. If any step fails, the transaction should abort, leaving the database unchanged.

📋 What You'll Learn

Create a MongoDB collection named books with a document for a book titled 'Learn MongoDB' with stock quantity 10.

Create a MongoDB collection named orders to store customer orders.

Start a session and a transaction.

Insert a new order document with customer name 'Alice' and book title 'Learn MongoDB'.

Update the stock quantity of 'Learn MongoDB' by reducing it by 1.

Commit the transaction if all operations succeed.

Abort the transaction if any operation fails.

💡 Why This Matters

🌍 Real World

Transactions ensure that multiple related database operations either all succeed or all fail, which is critical for applications like online stores to keep data consistent.

💼 Career

Understanding commit and abort behavior in MongoDB transactions is essential for backend developers and database administrators working with modern NoSQL databases.

1

Create the initial books collection with one book document

Create a MongoDB collection called books and insert one document with { title: 'Learn MongoDB', stock: 10 }.

MongoDB

// Your code here to create the books collection and insert the document

Hint

Use insertOne on the books collection with the specified document.

2

Create the orders collection and start a session

Create a MongoDB collection called orders and start a client session with const session = db.getMongo().startSession().

MongoDB

db.books.insertOne({ title: 'Learn MongoDB', stock: 10 })
// Your code here to create orders collection and start session

Hint

Use db.createCollection('orders') to create the orders collection and db.getMongo().startSession() to start a session.

3

Start a transaction and insert an order document

Use session.startTransaction() to begin a transaction. Then insert a document { customer: 'Alice', book: 'Learn MongoDB' } into the orders collection using the session.

MongoDB

db.books.insertOne({ title: 'Learn MongoDB', stock: 10 })
db.createCollection('orders')
const session = db.getMongo().startSession()
// Your code here to start transaction and insert order

Hint

Call startTransaction() on the session, then insert the order document passing { session } as an option.

4

Update stock and commit or abort the transaction

Update the books collection to reduce the stock of 'Learn MongoDB' by 1 using the session. Then commit the transaction with session.commitTransaction(). If an error occurs, abort the transaction with session.abortTransaction().

MongoDB

db.books.insertOne({ title: 'Learn MongoDB', stock: 10 })
db.createCollection('orders')
const session = db.getMongo().startSession()
session.startTransaction()
db.orders.insertOne({ customer: 'Alice', book: 'Learn MongoDB' }, { session })
// Your code here to update stock and commit or abort transaction

Hint

Use updateOne with $inc to reduce stock, then commit or abort the transaction inside a try-catch block.

Practice

1. What happens when you commit a transaction in MongoDB?

easy

A. All changes made during the transaction are saved permanently.

B. All changes made during the transaction are discarded.

C. The transaction is paused and can be resumed later.

D. Only some changes are saved, others are lost.

Solution

1. Step 1: Understand commit behavior

   Committing a transaction means saving all changes made during that transaction permanently to the database.

2. Step 2: Compare with abort

   Unlike abort, which cancels changes, commit finalizes them.

3. Final Answer:

   All changes made during the transaction are saved permanently. -> Option A

4. Quick Check:

   Commit = Save changes [OK]

Hint: Commit means save all changes made in the transaction [OK]

Common Mistakes:

• Confusing commit with abort
• Thinking commit only saves some changes
• Believing commit pauses the transaction

2. Which of the following is the correct syntax to abort a transaction in MongoDB using the shell?

easy

A. transaction.abort()

B. session.abortTransaction()

C. db.abort()

D. session.commit()

Solution

1. Step 1: Recall MongoDB transaction abort syntax

   The correct method to abort a transaction is session.abortTransaction().

2. Step 2: Eliminate incorrect options

   session.commit() is invalid, db.abort() and transaction.abort() are not valid MongoDB shell commands.

3. Final Answer:

   session.abortTransaction() -> Option B

4. Quick Check:

   Abort transaction = session.abortTransaction() [OK]

Hint: Abort transaction uses session.abortTransaction() method [OK]

Common Mistakes:

• Using commit() instead of abortTransaction()
• Trying to abort with db.abort() which doesn't exist
• Confusing transaction object with session

3. Consider this MongoDB transaction code snippet:

const session = client.startSession();
session.startTransaction();
await collection.insertOne({name: 'Alice'}, {session});
await session.abortTransaction();
const count = await collection.countDocuments({name: 'Alice'});
console.log(count);

What will be printed?

medium

A. 0

B. 1

C. undefined

D. Error

Solution

1. Step 1: Analyze transaction operations

   The document insertion happens inside a transaction that is aborted immediately after.

2. Step 2: Understand abort effect on data

   Aborting the transaction cancels the insert, so the document is not saved.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Abort cancels insert, count = 0 [OK]

Hint: Aborted inserts do not persist, count remains zero [OK]

Common Mistakes:

• Assuming insert stays after abort
• Expecting count to be 1
• Thinking abort throws error

4. You wrote this MongoDB transaction code but it throws an error:

const session = client.startSession();
session.startTransaction();
await collection.insertOne({item: 'book'}, {session});
session.commitTransaction();
session.endSession();

What is the likely cause of the error?

medium

A. insertOne() is missing session option

B. startTransaction() is called twice

C. Missing await before commitTransaction()

D. endSession() must be called before commitTransaction()

Solution

1. Step 1: Check async calls in transaction

   commitTransaction() returns a promise and must be awaited to complete properly.

2. Step 2: Identify missing await causes error

   Without await, the code may proceed before commit finishes, causing errors.

3. Final Answer:

   Missing await before commitTransaction() -> Option C

4. Quick Check:

   Async commit needs await [OK]

Hint: Always await commitTransaction() to avoid errors [OK]

Common Mistakes:

• Not awaiting commitTransaction()
• Calling startTransaction() multiple times
• Calling endSession() too early

5. You want to update two documents atomically in MongoDB. You start a transaction, update the first document, then update the second. If the second update fails, what should you do to ensure no partial changes remain?

hard

A. Manually delete the first update after failure.

B. Commit the transaction to save the first update only.

C. Ignore the failure and continue.

D. Abort the transaction to cancel all changes.

Solution

1. Step 1: Understand atomicity in transactions

   Transactions ensure all-or-nothing changes. If one update fails, the whole transaction should be canceled.

2. Step 2: Apply abort to cancel partial changes

   Aborting the transaction cancels all changes, preventing partial updates.

3. Final Answer:

   Abort the transaction to cancel all changes. -> Option D

4. Quick Check:

   Fail in transaction = abort all changes [OK]

Hint: Abort transaction to avoid partial updates on failure [OK]

Common Mistakes:

• Committing partial updates
• Trying to manually fix partial changes
• Ignoring failure and continuing