Commit and abort behavior in MongoDB

Introduction

Commit and abort control if changes to the database are saved or canceled. This helps keep data correct and safe.

When you want to save a group of changes only if all succeed.

When you want to cancel changes if something goes wrong.

When updating multiple documents and want all updates to apply together.

When you want to avoid partial updates that can cause errors.

When working with transactions to keep data consistent.

Syntax

MongoDB

const session = client.startSession();
await session.startTransaction();
try {
  // database operations
  await session.commitTransaction();
} catch (error) {
  await session.abortTransaction();
} finally {
  session.endSession();
}

Use startTransaction() to begin a transaction.

Use commitTransaction() to save changes.

Use abortTransaction() to cancel changes.

Examples

Insert a document and commit if successful, else abort.

MongoDB

const session = client.startSession();
await session.startTransaction();
try {
  await collection.insertOne({name: 'Alice'}, {session});
  await session.commitTransaction();
} catch (e) {
  await session.abortTransaction();
} finally {
  session.endSession();
}

Update and delete documents in one transaction, commit if all succeed.

MongoDB

const session = client.startSession();
await session.startTransaction();
try {
  await collection.updateOne({name: 'Bob'}, {$set: {age: 30}}, {session});
  await collection.deleteOne({name: 'Charlie'}, {session});
  await session.commitTransaction();
} catch (e) {
  await session.abortTransaction();
} finally {
  session.endSession();
}

Sample Program

This program starts a transaction, inserts a user, updates the user, and commits the changes. If any step fails, it aborts the transaction.

MongoDB

const { MongoClient } = require('mongodb');

async function run() {
  const client = new MongoClient('mongodb://localhost:27017');
  await client.connect();
  const db = client.db('testdb');
  const collection = db.collection('users');

  const session = client.startSession();
  await session.startTransaction();

  try {
    await collection.insertOne({name: 'Diana'}, {session});
    await collection.updateOne({name: 'Diana'}, {$set: {age: 25}}, {session});
    await session.commitTransaction();
    console.log('Transaction committed');
  } catch (error) {
    await session.abortTransaction();
    console.log('Transaction aborted');
  } finally {
    session.endSession();
    await client.close();
  }
}

run();

Important Notes

Transactions work only on replica sets or sharded clusters in MongoDB.

Always end the session with endSession() to free resources.

Use transactions to keep multiple operations safe and consistent.

Summary

Commit saves all changes made in a transaction.

Abort cancels all changes made in a transaction.

Use transactions to group multiple operations safely.

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