Read-only transactions in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a read-only transaction in Spring Boot?

A read-only transaction is a transaction that only reads data and does not modify it. It tells the database and Spring that no data changes will happen, which can improve performance and safety.

beginner

How do you declare a read-only transaction in Spring Boot?

You use the @Transactional annotation with the attribute readOnly=true, like this:
@Transactional(readOnly = true) on a method or class.

intermediate

Why use read-only transactions?

Read-only transactions can make your app faster by avoiding unnecessary locks and checks in the database. They also help prevent accidental data changes during read operations.

intermediate

What happens if you try to write data inside a read-only transaction?

Depending on the database and configuration, it may throw an error or ignore the write. Spring tries to optimize for reads, so writes inside a read-only transaction are discouraged and can cause unexpected behavior.

intermediate

Can read-only transactions improve database concurrency?

Yes, because they reduce locking and resource use, allowing more read operations to happen at the same time without blocking each other.

How do you mark a method as read-only in Spring Boot?

A @Transactional(readOnly = true)

B @ReadOnlyTransaction

C @Transactional(write = false)

D @ReadOnly(true)

The correct way is to use @Transactional with readOnly = true.

What is a benefit of using read-only transactions?

A They prevent any database connection

B They allow writing data faster

C They disable transactions completely

D They improve performance by reducing locks

Read-only transactions reduce locking and improve performance for read operations.

What might happen if you write data inside a read-only transaction?

A An error or exception may occur

B Spring Boot will ignore the transaction

C The write will always succeed

D The transaction will become read-write automatically

Writing inside a read-only transaction can cause errors or unexpected behavior.

Which annotation attribute controls read-only behavior in Spring transactions?

A readonly

B readOnly

C read_only

D readOnlyFlag

The attribute is named readOnly with camel case.

Read-only transactions are best used for:

A Updating user profiles

B Deleting records

C Fetching data without changes

D Inserting new data

They are designed for operations that only read data.

Explain what a read-only transaction is and why it is useful in Spring Boot.

Think about how telling the system you won't change data can help.

Describe how to declare a read-only transaction in Spring Boot and what happens if you try to write data inside it.

Focus on the annotation and consequences of writing inside.

Practice

1. What is the main purpose of using @Transactional(readOnly = true) in Spring Boot?

easy

A. To allow data modifications within the transaction

B. To optimize performance by indicating the method only reads data

C. To disable transaction management entirely

D. To automatically commit changes after method execution

Solution

1. Step 1: Understand the role of read-only transactions

   Read-only transactions tell Spring the method will only read data, not modify it.

2. Step 2: Recognize performance benefits

   This allows Spring and the database to optimize the transaction for reading, improving performance.

3. Final Answer:

   To optimize performance by indicating the method only reads data -> Option B

4. Quick Check:

   Read-only = optimize read performance [OK]

Hint: Read-only means no data changes allowed, just reading [OK]

Common Mistakes:

• Thinking readOnly=true allows data changes
• Confusing readOnly with disabling transactions
• Assuming it commits changes automatically

2. Which of the following is the correct way to declare a read-only transaction on a method in Spring Boot?

easy

A. @Transactional(readOnly = true)

B. @Transactional(readOnly)

C. @Transactional(enabled = true)

D. @Transactional(readOnly = false)

Solution

1. Step 1: Recall the correct syntax for read-only transactions

   The correct attribute is readOnly = true inside the @Transactional annotation.

2. Step 2: Check each option

   @Transactional(readOnly = true) uses the exact correct syntax. Others are either wrong attribute names or values.

3. Final Answer:

   @Transactional(readOnly = true) -> Option A

4. Quick Check:

   Correct syntax uses readOnly = true [OK]

Hint: Use readOnly = true exactly inside @Transactional [OK]

Common Mistakes:

• Using readOnly without = true
• Using readOnly = false by mistake
• Using non-existent attributes like enabled

3. Consider this Spring Boot method:

@Transactional(readOnly = true)
public List<User> getUsers() {
    userRepository.save(new User("John"));
    return userRepository.findAll();
}

What will happen when this method runs?

medium

A. An exception will be thrown because save is called in a read-only transaction

B. The save call will be ignored, but findAll will return existing users

C. The new user "John" will be saved and returned in the list

D. The method will run normally without any restrictions

Solution

1. Step 1: Understand read-only transaction restrictions

   Read-only transactions prevent data modifications like save or update operations.

2. Step 2: Analyze the method behavior

   Calling save inside a read-only transaction causes Spring or the database to throw an exception.

3. Final Answer:

   An exception will be thrown because save is called in a read-only transaction -> Option A

4. Quick Check:

   Save in read-only transaction = exception [OK]

Hint: Save inside read-only transaction causes error [OK]

Common Mistakes:

• Assuming save silently fails
• Thinking save works normally in read-only
• Ignoring transaction settings

4. You have this method:

@Transactional(readOnly = true)
public void updateUserName(Long id, String name) {
    User user = userRepository.findById(id).orElseThrow();
    user.setName(name);
}

Why might this method fail to update the user's name?

medium

A. Because the method is missing @Transactional annotation

B. Because findById does not return a user

C. Because setName is not a valid method

D. Because readOnly = true prevents any data changes inside the transaction

Solution

1. Step 1: Understand effect of readOnly = true on data changes

   Read-only transactions prevent changes from being saved to the database.

2. Step 2: Analyze the method's update attempt

   Even though the user object is modified, the transaction will not commit changes due to readOnly=true.

3. Final Answer:

   Because readOnly = true prevents any data changes inside the transaction -> Option D

4. Quick Check:

   readOnly = true blocks data updates [OK]

Hint: readOnly = true blocks saving changes [OK]

Common Mistakes:

• Assuming object changes auto-save without commit
• Thinking findById always fails
• Ignoring transaction annotation effects

5. You want to create a service method that fetches user data without risking accidental updates and improves performance. Which approach is best?

hard

A. Do not use any transaction annotation and perform all operations directly

B. Use @Transactional without readOnly and manually avoid updates

C. Annotate the method with @Transactional(readOnly = true) and avoid any save/update calls

D. Use @Transactional(readOnly = false) to allow updates if needed

Solution

1. Step 1: Identify the goal of safe read-only data fetching

   The goal is to read data safely without accidental changes and improve performance.

2. Step 2: Choose the best annotation and practice

   Using @Transactional(readOnly = true) explicitly marks the method as read-only, enabling optimizations and preventing writes.

3. Final Answer:

   Annotate the method with @Transactional(readOnly = true) and avoid any save/update calls -> Option C

4. Quick Check:

   Use readOnly = true for safe, optimized reads [OK]

Hint: Use readOnly = true to prevent accidental writes [OK]

Common Mistakes:

• Skipping readOnly and risking accidental writes
• Not using transactions at all
• Using readOnly = false when no updates needed