Commit and abort behavior in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does commit mean in MongoDB transactions?

Commit means to save all the changes made during a transaction permanently to the database.

beginner

What happens when you abort a transaction in MongoDB?

Aborting a transaction cancels all changes made during the transaction, so the database stays as it was before the transaction started.

intermediate

Why use transactions with commit and abort in MongoDB?

Transactions ensure that multiple related changes happen all together or not at all, keeping data correct and safe.

beginner

How do you start a transaction in MongoDB?

You start a transaction by calling session.startTransaction() on a client session.

beginner

What is the role of session.commitTransaction() and session.abortTransaction()?

session.commitTransaction() saves changes permanently, while session.abortTransaction() cancels all changes made in the transaction.

What does commitTransaction() do in MongoDB?

A Starts a new transaction

B Cancels all changes made in the transaction

C Saves all changes made in the transaction permanently

D Deletes the database

commitTransaction() saves all changes made during the transaction permanently to the database.

If you want to cancel all changes in a MongoDB transaction, which method do you use?

A commitTransaction()

B abortTransaction()

C startTransaction()

D endTransaction()

abortTransaction() cancels all changes made during the transaction.

When does a MongoDB transaction become permanent?

A When you start the transaction

B When you abort the transaction

C When you close the database

D When you commit the transaction

Changes become permanent only after commitTransaction() is called.

Which of these is true about MongoDB transactions?

A They allow multiple operations to be treated as one unit

B They automatically save changes without commit

C They cannot be aborted once started

D They delete all data on abort

Transactions group multiple operations so they succeed or fail together.

What is the first step to use transactions in MongoDB?

A Start a client session and call startTransaction()

B Call commitTransaction()

C Call abortTransaction()

D Close the database connection

You must start a client session and call startTransaction() to begin a transaction.

Explain in your own words what happens when you commit and abort a transaction in MongoDB.

Think about saving or canceling a group of changes together.

Describe the steps to perform a transaction in MongoDB including commit and abort.

Imagine you are making a group of changes and deciding to save or cancel them.

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