Quiz: Transactions and Migrations

Test your knowledge on transactions, Flyway, and Liquibase.

30 questions - Click on each question to reveal the answer.

Questions 1-10: Transactions

Question 1: What is a transaction?

Answer: A transaction is an atomic unit of work that guarantees ACID:

Atomicity: all or nothing
Consistency: valid state before/after
Isolation: independent transactions
Durability: persisted after commit

Question 2: How to declare a transactional method?

Answer:

@Service
public class TransferService {
    
    @Transactional
    public void transfer(Long from, Long to, BigDecimal amount) {
        accountRepository.debit(from, amount);
        accountRepository.credit(to, amount);
    }
}

Question 3: What happens if an exception is thrown in a transaction?

Answer:

RuntimeException (unchecked): automatic rollback
Exception (checked): no rollback by default

@Transactional(rollbackFor = Exception.class)  // Rollback also for checked
public void doWork() throws IOException { }

Question 4: How to force a rollback for a checked exception?

Answer:

@Transactional(rollbackFor = {IOException.class, CustomException.class})
public void process() throws IOException { }

// Or rollback for all
@Transactional(rollbackFor = Exception.class)

Question 5: How to prevent rollback for a RuntimeException?

Answer:

@Transactional(noRollbackFor = BusinessException.class)
public void process() {
    // BusinessException will not cause a rollback
}

Question 6: What are the transaction isolation levels?

Answer:

Level	Description
READ_UNCOMMITTED	Reads uncommitted data
READ_COMMITTED	Reads only committed (default)
REPEATABLE_READ	Identical repeatable reads
SERIALIZABLE	Full isolation (slower)

@Transactional(isolation = Isolation.SERIALIZABLE)

Question 7: What is transaction propagation?

Answer: How a transaction behaves when called from another:

@Transactional(propagation = Propagation.REQUIRED)  // Default
// Uses existing transaction or creates a new one

@Transactional(propagation = Propagation.REQUIRES_NEW)
// Always creates a new transaction

@Transactional(propagation = Propagation.MANDATORY)
// Must be called within an existing transaction

Question 8: When to use REQUIRES_NEW?

Answer: When an operation must be committed independently:

@Transactional
public void processOrder(Order order) {
    orderService.save(order);
    auditService.logAction(order);  // Must persist even if order fails
}

@Service
public class AuditService {
    @Transactional(propagation = Propagation.REQUIRES_NEW)
    public void logAction(Order order) {
        // New independent transaction
    }
}

Question 9: Why doesn't @Transactional work on a private method?

Answer: Spring uses AOP proxies that only intercept external calls. Private methods and internal calls do not go through the proxy.

public void methodA() {
    methodB();  // Internal call - no proxy!
}

@Transactional
private void methodB() { }  // Doesn't work

Question 10: How to solve the problem of internal calls?

Answer:

Extract into another bean
Use self-injection
Use TransactionTemplate

// Option 1: Another bean
@Service
public class HelperService {
    @Transactional
    public void doWork() { }
}

// Option 2: TransactionTemplate
transactionTemplate.execute(status -> {
    // Transactional code
    return result;
});

Questions 11-20: Flyway

Question 11: What is Flyway?

Answer: Flyway is a database migration tool:

Versions the DB schema
Applies migrations in order
Keeps migration history
Supports SQL and Java

Question 12: How to name Flyway migration files?

Answer: Convention: V\{version\}__\{description\}.sql

V1__create_users_table.sql
V2__add_email_column.sql
V3__create_orders_table.sql
V1.1__add_index.sql  (sub-version)

The double underscore __ is mandatory.

Question 13: Where to place migration files?

Answer: Default: src/main/resources/db/migration/

src/main/resources/
└── db/
    └── migration/
        ├── V1__create_users.sql
        └── V2__add_orders.sql

Question 14: Which table does Flyway use for tracking?

Answer: flyway_schema_history contains:

Version
Description
Checksum
Execution date
Success/Failure

Question 15: How to configure Flyway in Spring Boot?

Answer:

spring:
  flyway:
    enabled: true
    locations: classpath:db/migration
    baseline-on-migrate: true
    validate-on-migrate: true

Question 16: What is a repeatable migration?

Answer: Migration re-executed if its content changes:

R__create_views.sql  (R instead of V)

Used for views, stored procedures, reference data.

Question 17: How to undo a Flyway migration?

Answer: Flyway Community does not support rollback. Solutions:

Create a new migration that undoes
Use Flyway Teams (paid)
Restore a backup

-- V3__undo_v2.sql
ALTER TABLE users DROP COLUMN email;

Question 18: How to ignore a migration already applied manually?

Answer:

spring:
  flyway:
    baseline-on-migrate: true
    baseline-version: 1

Or CLI command: flyway baseline

Question 19: How to execute Java code in a migration?

Answer:

public class V3__ComplexMigration implements JavaMigration {
    
    @Override
    public void migrate(Context context) throws Exception {
        try (Statement stmt = context.getConnection().createStatement()) {
            // Complex migration in Java
        }
    }
}

Question 20: How to validate migrations without executing them?

Answer:

# CLI
flyway validate

# Or in config
spring:
  flyway:
    validate-on-migrate: true

Checks that the checksums match.

Questions 21-30: Liquibase

Question 21: What is the difference between Flyway and Liquibase?

Answer:

Flyway	Liquibase
SQL or Java	XML, YAML, JSON, SQL
Simpler	More flexible
No rollback (Community)	Integrated rollback
Linear versioning	Changesets with ID

Question 22: How to structure a Liquibase changelog file?

Answer:

# db/changelog/db.changelog-master.yaml
databaseChangeLog:
  - changeSet:
      id: 1
      author: dev
      changes:
        - createTable:
            tableName: users
            columns:
              - column:
                  name: id
                  type: bigint
                  autoIncrement: true
                  constraints:
                    primaryKey: true

Question 23: How does Liquibase track applied changes?

Answer: Via tables:

DATABASECHANGELOG: changeset history
DATABASECHANGELOGLOCK: lock to prevent parallel executions

Question 24: How to define a rollback in Liquibase?

Answer:

- changeSet:
    id: 1
    author: dev
    changes:
      - addColumn:
          tableName: users
          columns:
            - column:
                name: phone
                type: varchar(20)
    rollback:
      - dropColumn:
          tableName: users
          columnName: phone

Question 25: How to execute a Liquibase rollback?

Answer:

# Rollback the last changeset
liquibase rollbackCount 1

# Rollback to a tag
liquibase rollback myTag

# Rollback to a date
liquibase rollbackToDate 2024-01-01

Question 26: How to configure Liquibase in Spring Boot?

Answer:

spring:
  liquibase:
    enabled: true
    change-log: classpath:db/changelog/db.changelog-master.yaml
    contexts: dev

Question 27: How to use Liquibase contexts?

Answer: Execute certain changesets based on the environment:

- changeSet:
    id: 1
    author: dev
    context: dev, test  # Only in dev and test
    changes:
      - insert:
          tableName: users
          columns:
            - column: {name: name, value: 'Test User'}

spring:
  liquibase:
    contexts: dev

Question 28: How to generate a changelog from an existing DB?

Answer:

liquibase generateChangeLog \
  --changeLogFile=changelog.yaml \
  --url=jdbc:postgresql://localhost/mydb \
  --username=user \
  --password=pass

Question 29: How to validate a changelog without executing it?

Answer:

liquibase validate

Checks syntax and checksums.

Question 30: How to tag a version for rollback?

Answer:

- changeSet:
    id: release-1.0
    author: dev
    changes:
      - tagDatabase:
          tag: v1.0.0

# Rollback to this tag
liquibase rollback v1.0.0

Score

Score	Level
27-30	Excellent - Mastered transactions and migrations
21-26	Good - Some revisions needed
15-20	Average - Review the chapters
Less than 15	Needs review

Back to Module 7

Questions 1-10: Transactions​

Questions 11-20: Flyway​

Questions 21-30: Liquibase​

Score​

Questions 1-10: Transactions

Questions 11-20: Flyway

Questions 21-30: Liquibase

Score