Transaction handling in GraphQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a transaction in database terms?

A transaction is a group of operations performed as a single unit. All operations must complete successfully, or none are applied, to keep data correct.

intermediate

What does ACID stand for in transaction handling?

ACID means Atomicity, Consistency, Isolation, and Durability. These are rules to make sure transactions work safely and correctly.

intermediate

Why is isolation important in transactions?

Isolation keeps transactions separate so they don't interfere with each other. This avoids problems like reading wrong or partial data.

beginner

What happens if a transaction fails during execution?

If a transaction fails, all changes made during it are undone (rolled back) so the database stays correct and unchanged.

advanced

How do you start and end a transaction in GraphQL with a database?

You start a transaction by telling the database to begin, then run your operations, and finally commit to save or rollback to undo if errors happen.

What does the 'Atomicity' property ensure in a transaction?

A All parts of the transaction succeed or none do

B Data is always consistent after transaction

C Transactions run at the same time without interference

D Changes are saved permanently

Atomicity means the transaction is all or nothing: either all steps succeed or none are applied.

Which of the following is NOT part of ACID properties?

A Atomicity

B Consistency

C Isolation

D Indexing

Indexing is a database optimization, not an ACID property.

What does 'rollback' do in transaction handling?

A Undoes all changes made in the transaction

B Starts a new transaction

C Saves all changes permanently

D Locks the database

Rollback reverses all changes made during the transaction to keep data safe.

Why is isolation important when multiple transactions run at the same time?

A To speed up the database

B To allow partial data reads

C To prevent transactions from interfering with each other

D To save memory

Isolation keeps transactions separate so they don't affect each other's data.

In GraphQL, how do you ensure multiple database operations are treated as one transaction?

A By using a single query

B By starting a transaction, running operations, then committing or rolling back

C By running operations in parallel

D By using multiple mutations without control

You explicitly start a transaction, perform operations, then commit or rollback to keep data consistent.

Explain the ACID properties and why they matter in transaction handling.

Think about how each property helps keep data correct and reliable.

Describe the steps to handle a transaction in a GraphQL API connected to a database.

Consider how you control the database actions inside your GraphQL resolver.

Practice

1. What is the main purpose of using transactions in a database when working with GraphQL?

easy

A. To group multiple actions so they all succeed or fail together

B. To speed up query execution by caching results

C. To automatically generate GraphQL schemas

D. To encrypt data sent over the network

Solution

1. Step 1: Understand what transactions do

   Transactions ensure that a set of database operations are treated as a single unit.

2. Step 2: Identify the purpose in GraphQL context

   This means either all changes succeed or none do, keeping data consistent.

3. Final Answer:

   To group multiple actions so they all succeed or fail together -> Option A

4. Quick Check:

   Transactions = atomic group actions [OK]

Hint: Transactions keep multiple changes safe together [OK]

Common Mistakes:

• Thinking transactions speed up queries
• Confusing transactions with schema generation
• Assuming transactions encrypt data

2. Which of the following is the correct way to start a transaction in a GraphQL resolver using a typical backend?

easy

A. const transaction = await db.rollback();

B. const transaction = db.commit();

C. const transaction = await db.startTransaction();

D. const transaction = db.query('BEGIN');

Solution

1. Step 1: Identify how to start a transaction

   Starting a transaction usually involves calling a method like startTransaction() asynchronously.

2. Step 2: Check options for correctness

   const transaction = await db.startTransaction(); correctly uses await and startTransaction(), while others are commit, rollback, or raw query.

3. Final Answer:

   const transaction = await db.startTransaction(); -> Option C

4. Quick Check:

   Start transaction = startTransaction() [OK]

Hint: Start transactions with async startTransaction() method [OK]

Common Mistakes:

• Using commit or rollback to start a transaction
• Using raw query strings instead of methods
• Missing await for async calls

3. Given this GraphQL resolver snippet, what will be the output if an error occurs during updateUser()?

async function updateProfile(parent, args, context) {
  const trx = await context.db.startTransaction();
  try {
    await trx.updateUser(args.userId, args.data);
    await trx.commit();
    return 'Success';
  } catch (e) {
    await trx.rollback();
    return 'Failed';
  }
}

medium

A. 'Success'

B. 'Failed'

C. Throws an unhandled exception

D. Returns null

Solution

1. Step 1: Analyze the try block

   The code tries to update the user and commit the transaction.

2. Step 2: Understand error handling

   If updateUser throws an error, the catch block rolls back and returns 'Failed'.

3. Final Answer:

   'Failed' -> Option B

4. Quick Check:

   Error during update = rollback + 'Failed' [OK]

Hint: Errors trigger rollback and return 'Failed' [OK]

Common Mistakes:

• Assuming commit runs even on error
• Expecting unhandled exceptions instead of catch
• Thinking it returns 'Success' always

4. Identify the error in this GraphQL resolver transaction handling code:

async function saveOrder(parent, args, context) {
  const trx = context.db.startTransaction();
  try {
    await trx.insertOrder(args.order);
    await trx.commit();
    return 'Order saved';
  } catch (error) {
    await trx.rollback();
    return 'Error saving order';
  }
}

medium

A. Missing await before startTransaction()

B. No rollback in catch block

C. commit() should be called before insertOrder()

D. Returning string instead of object

Solution

1. Step 1: Check transaction start syntax

   startTransaction() is asynchronous and must be awaited.

2. Step 2: Identify missing await impact

   Without await, trx is a Promise, causing errors on trx.insertOrder().

3. Final Answer:

   Missing await before startTransaction() -> Option A

4. Quick Check:

   Async startTransaction() needs await [OK]

Hint: Always await async transaction start calls [OK]

Common Mistakes:

• Forgetting await on async calls
• Calling commit before operations
• Ignoring rollback on errors

5. You want to update a user's profile and their related settings in one transaction using GraphQL. Which approach ensures both updates succeed or fail together?

async function updateUserAndSettings(parent, args, context) {
  const trx = await context.db.startTransaction();
  try {
    await trx.updateUser(args.userId, args.userData);
    await trx.updateSettings(args.userId, args.settingsData);
    await trx.commit();
    return 'Update successful';
  } catch (error) {
    await trx.rollback();
    return 'Update failed';
  }
}

hard

A. Use separate transactions for user and settings updates

B. Do not use transactions for related updates

C. Commit after each update separately

D. Wrap both updates in a single transaction as shown

Solution

1. Step 1: Understand transaction grouping

   Both updates must be inside one transaction to succeed or fail together.

2. Step 2: Analyze the code approach

   The code wraps both updates in one transaction, committing only if both succeed.

3. Final Answer:

   Wrap both updates in a single transaction as shown -> Option D

4. Quick Check:

   Single transaction = atomic multi-update [OK]

Hint: Group related updates in one transaction [OK]

Common Mistakes:

• Using separate transactions causing partial updates
• Committing too early after first update
• Skipping transactions for multi-step changes