Replication basics in MySQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Identify the Master Server Status

Given the output of SHOW MASTER STATUS; on a MySQL server, what does the File column represent?

MySQL

SHOW MASTER STATUS;

A The name of the binary log file currently being written by the master.

B The name of the relay log file on the slave server.

C The name of the error log file for the master server.

D The name of the configuration file used by the master.

💡 Hint

Think about what the master writes to keep track of changes for replication.

The File column in SHOW MASTER STATUS; shows the current binary log file that the master is writing to. This log contains all changes that slaves will replicate.

🧠 Conceptual

intermediate

Role of Relay Logs in Replication

In MySQL replication, what is the primary purpose of the relay log on the slave server?

A To log errors encountered during replication.

B To store the changes read from the master's binary log before applying them locally.

C To keep a backup of the master's binary logs.

D To store the binary logs generated by the slave server.

💡 Hint

Relay logs act as a temporary storage on the slave.

Relay logs on the slave store the events read from the master's binary log. The slave then applies these events to its own data to stay synchronized.

📝 Syntax

advanced

Correct Syntax to Start Slave Replication

Which of the following commands correctly starts the replication process on a MySQL slave server?

A START SLAVE;

B BEGIN SLAVE;

C RUN SLAVE;

D INITIATE SLAVE;

💡 Hint

The command uses the word 'START'.

The correct command to start replication on a slave is START SLAVE;. Other options are invalid commands.

🔧 Debug

advanced

Troubleshooting Replication Error

A MySQL slave shows the error Last_SQL_Error: Error 'Duplicate entry' for key 'PRIMARY'. What is the most likely cause?

A The relay log on the slave is missing.

B The master server's binary log is corrupted.

C The slave tried to apply a change that conflicts with existing data, causing a duplicate primary key.

D The slave server has lost network connection to the master.

💡 Hint

Think about what a duplicate primary key error means in data replication.

This error means the slave tried to insert a row with a primary key that already exists, indicating data inconsistency or previous partial replication.

❓ optimization

expert

Improving Replication Performance

Which of the following changes is most effective to reduce replication lag on a MySQL slave?

A Increase the size of the master's binary log files.

B Set the slave's <code>read_only</code> variable to OFF.

C Disable the relay log on the slave server.

D Enable multi-threaded replication by setting <code>slave_parallel_workers</code> to a value greater than 0.

💡 Hint

Think about how the slave can apply changes faster.

Enabling multi-threaded replication allows the slave to apply multiple transactions in parallel, reducing lag. Other options either do not affect lag or can cause issues.

Practice

1. What is the main purpose of MySQL replication?

easy

A. To copy data automatically from one database server to another

B. To create backups manually on the same server

C. To speed up SQL query execution on a single server

D. To encrypt data during transmission

Solution

1. Step 1: Understand replication concept

   Replication means copying data from one database (called master) to another (called slave) automatically.

2. Step 2: Identify the main purpose

   The main goal is to keep data synchronized between servers without manual copying.

3. Final Answer:

   To copy data automatically from one database server to another -> Option A

4. Quick Check:

   Replication = automatic data copy [OK]

Hint: Replication means automatic data copying between servers [OK]

Common Mistakes:

• Confusing replication with backup
• Thinking replication speeds up queries on one server
• Assuming replication encrypts data

2. Which command is used to start replication on a MySQL slave server?

easy

A. START SLAVE

B. START REPLICATION

C. BEGIN SLAVE

D. RUN SLAVE

Solution

1. Step 1: Recall replication start command

   In MySQL, the command to start the slave replication process is START SLAVE.

2. Step 2: Check other options

   Other options like START REPLICATION, BEGIN SLAVE, or RUN SLAVE are not valid MySQL commands.

3. Final Answer:

   START SLAVE -> Option A

4. Quick Check:

   Start replication = START SLAVE [OK]

Hint: Use START SLAVE to begin replication on slave server [OK]

Common Mistakes:

• Using incorrect commands like START REPLICATION
• Confusing with transaction commands
• Forgetting to start the slave after setup

3. Given the command CHANGE MASTER TO MASTER_HOST='192.168.1.10', MASTER_USER='replica', MASTER_PASSWORD='pass'; executed on a slave, what does it do?

medium

A. Deletes the current master configuration

B. Starts replication immediately with given credentials

C. Creates a new master server at 192.168.1.10

D. Sets the slave to connect to the master at IP 192.168.1.10 using user replica

Solution

1. Step 1: Understand CHANGE MASTER TO command

   This command configures the slave server to connect to a master server with specified host and user credentials.

2. Step 2: Analyze given parameters

   It sets the master host IP to 192.168.1.10 and uses the user 'replica' with password 'pass' for connection.

3. Final Answer:

   Sets the slave to connect to the master at IP 192.168.1.10 using user replica -> Option D

4. Quick Check:

   CHANGE MASTER TO = configure slave connection [OK]

Hint: CHANGE MASTER TO sets slave's master connection info [OK]

Common Mistakes:

• Thinking it starts replication immediately
• Assuming it creates a new master server
• Believing it deletes master config

4. You run START SLAVE; but replication does not start. Which of these is a likely cause?

medium

A. The master server is down

B. The slave user password is empty

C. The slave has not been configured with CHANGE MASTER TO

D. The slave server is running MySQL version 8

Solution

1. Step 1: Check slave configuration

   If the slave is not configured with CHANGE MASTER TO, it does not know which master to connect to.

2. Step 2: Understand replication start requirements

   Without proper master info, START SLAVE cannot initiate replication, even if master is up or password is set.

3. Final Answer:

   The slave has not been configured with CHANGE MASTER TO -> Option C

4. Quick Check:

   Missing CHANGE MASTER TO blocks replication start [OK]

Hint: Configure slave with CHANGE MASTER TO before START SLAVE [OK]

Common Mistakes:

• Assuming START SLAVE works without configuration
• Blaming MySQL version without checking config
• Ignoring master server status

5. You want to set up a slave that replicates only the database named sales. Which command helps you filter replication to this database?

hard

A. CHANGE MASTER TO MASTER_DB='sales';

B. CHANGE REPLICATION FILTER REPLICATE_DO_DB = ('sales');

C. SET REPLICATION FILTER TO 'sales';

D. CHANGE REPLICATION FILTER REPLICATE_DO_DB = 'sales';

Solution

1. Step 1: Understand replication filtering

   MySQL uses replication filters like REPLICATE_DO_DB to replicate only specific databases.

2. Step 2: Identify correct syntax

   The correct syntax uses CHANGE REPLICATION FILTER REPLICATE_DO_DB = ('sales'); with parentheses around the database name.

3. Final Answer:

   CHANGE REPLICATION FILTER REPLICATE_DO_DB = ('sales'); -> Option B

4. Quick Check:

   Use REPLICATE_DO_DB with parentheses for filtering [OK]

Hint: Use CHANGE REPLICATION FILTER with REPLICATE_DO_DB and parentheses [OK]

Common Mistakes:

• Using non-existent commands like SET REPLICATION FILTER
• Omitting parentheses around database name
• Trying to filter with CHANGE MASTER TO