Database replication (master-slave) in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of master-slave database replication?

To copy data from the master database to one or more slave databases to improve read performance and provide data redundancy.

beginner

In master-slave replication, which database handles write operations?

The master database handles all write operations, while slaves handle read operations.

intermediate

What is a common challenge in master-slave replication related to data consistency?

Slaves may lag behind the master, causing eventual consistency rather than immediate consistency.

intermediate

How does asynchronous replication differ from synchronous replication in master-slave setups?

Asynchronous replication allows the master to continue without waiting for slaves to confirm data receipt, while synchronous waits for confirmation, ensuring stronger consistency but higher latency.

intermediate

Name one advantage and one disadvantage of master-slave replication.

Advantage: Improves read scalability by distributing reads to slaves. Disadvantage: Single point of failure at the master and potential data lag on slaves.

In master-slave replication, which database is responsible for processing write requests?

A Master

B Slave

C Both master and slave

D Neither

The master database handles all write operations, while slaves handle read operations.

What is a common issue caused by asynchronous replication in master-slave setups?

A Master cannot accept reads

B Slaves block writes

C Slaves may have outdated data temporarily

D Slaves overwrite master data

Asynchronous replication can cause slaves to lag behind the master, leading to temporary outdated data.

Which of the following is NOT a benefit of master-slave replication?

A Automatic failover without configuration

B Improved read scalability

C Data redundancy

D Load distribution for reads

Master-slave replication does not provide automatic failover by default; additional setup is needed.

What happens if the master database fails in a typical master-slave setup without failover?

A Reads and writes continue normally

B Slaves automatically become masters

C Slaves reject all queries

D Writes stop until master is restored

Without failover, the master failure stops all write operations until it is fixed.

Which replication type waits for slaves to confirm data receipt before completing a write?

A Multi-master replication

B Synchronous replication

C Master-slave replication

D Asynchronous replication

Synchronous replication waits for slaves to confirm data receipt, ensuring stronger consistency.

Explain how master-slave replication works and its main benefits and drawbacks.

Think about who writes and who reads, and what happens if the master fails.

Describe the difference between synchronous and asynchronous replication in a master-slave database system.

Consider how quickly the master waits for slaves before confirming writes.

Practice

1. What is the primary role of the master in a master-slave database replication setup?

easy

A. Handle all write operations and propagate changes to slaves

B. Serve read requests only

C. Backup data from slaves

D. Monitor network traffic between nodes

Solution

1. Step 1: Understand master responsibilities

   The master database is responsible for handling all write operations to ensure data consistency.

2. Step 2: Understand slave responsibilities

   Slaves replicate data from the master and typically handle read requests to reduce load on the master.

3. Final Answer:

   Handle all write operations and propagate changes to slaves -> Option A

4. Quick Check:

   Master = writes + replication [OK]

Hint: Master handles writes; slaves handle reads [OK]

Common Mistakes:

• Thinking slaves handle writes
• Confusing backup role with master
• Assuming master only monitors

2. Which of the following is the correct sequence for setting up master-slave replication?

easy

A. Configure slaves only, master is optional

B. Configure slaves, then configure master, then start replication

C. Start replication, then configure master and slaves

D. Configure master, then configure slaves, then start replication

Solution

1. Step 1: Configure master first

   The master must be configured first to accept writes and log changes for replication.

2. Step 2: Configure slaves after master

   Slaves are then configured to connect to the master and replicate its data.

3. Final Answer:

   Configure master, then configure slaves, then start replication -> Option D

4. Quick Check:

   Master setup before slaves [OK]

Hint: Master setup precedes slaves setup [OK]

Common Mistakes:

• Configuring slaves before master
• Starting replication before configuration
• Ignoring master configuration

3. Consider a master-slave setup where the master receives 100 writes per second and each slave replicates with a 2-second lag. If a read request is sent to a slave immediately after a write, what is the likely result?

medium

A. The slave returns the latest data including the write

B. The slave crashes due to replication lag

C. The slave returns stale data missing the latest write

D. The slave rejects the read request

Solution

1. Step 1: Understand replication lag effect

   Replication lag means slaves are behind the master by 2 seconds, so recent writes are not yet applied.

2. Step 2: Analyze read request timing

   A read immediately after a write will hit the slave before replication applies the change, so stale data is returned.

3. Final Answer:

   The slave returns stale data missing the latest write -> Option C

4. Quick Check:

   Replication lag causes stale reads [OK]

Hint: Replication lag means slaves have delayed data [OK]

Common Mistakes:

• Assuming slaves always have latest data
• Thinking slaves reject reads during lag
• Believing slaves crash due to lag

4. A system uses master-slave replication but experiences inconsistent reads from slaves. Which change can fix this issue?

medium

A. Allow writes on slaves to sync data faster

B. Increase monitoring and reduce replication lag

C. Disable replication and use only master for reads

D. Add more slaves without changing replication

Solution

1. Step 1: Identify cause of inconsistent reads

   Inconsistent reads usually happen due to replication lag causing slaves to have outdated data.

2. Step 2: Fix by reducing replication lag

   Improving monitoring and optimizing replication reduces lag, making slave reads more consistent.

3. Final Answer:

   Increase monitoring and reduce replication lag -> Option B

4. Quick Check:

   Reduce lag to fix inconsistent reads [OK]

Hint: Reduce lag to fix stale slave reads [OK]

Common Mistakes:

• Allowing writes on slaves breaks consistency
• Disabling replication loses read scaling
• Adding slaves without lag fix doesn't help

5. You are designing a high-availability system using master-slave replication. Which approach best ensures minimal downtime and data loss during master failure?

hard

A. Automatically promote a slave to master and redirect writes

B. Stop all writes until the master is manually restored

C. Allow writes on slaves during master downtime

D. Use only one master without slaves to avoid complexity

Solution

1. Step 1: Understand master failure impact

   If the master fails, writes cannot proceed unless a new master is available.

2. Step 2: Use automatic failover

   Promoting a slave to master automatically and redirecting writes ensures availability and minimal data loss.

3. Step 3: Evaluate other options

   Stopping writes causes downtime; writing on slaves breaks consistency; single master lacks redundancy.

4. Final Answer:

   Automatically promote a slave to master and redirect writes -> Option A

5. Quick Check:

   Automatic failover ensures availability [OK]

Hint: Auto-promote slave for failover [OK]

Common Mistakes:

• Stopping writes causes downtime
• Writing on slaves breaks data integrity
• Ignoring failover leads to outages