Data replication strategies in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is data replication in system design?

Data replication is the process of copying and maintaining database or data storage copies across multiple servers or locations to improve availability, fault tolerance, and performance.

intermediate

Explain the difference between synchronous and asynchronous replication.

Synchronous replication waits for data to be copied to all replicas before confirming success, ensuring consistency but adding latency. Asynchronous replication confirms success immediately and copies data later, improving speed but risking temporary inconsistency.

beginner

What is master-slave replication?

Master-slave replication means one server (master) handles writes and updates, while one or more servers (slaves) copy data from the master and serve read requests, improving read scalability.

advanced

Describe multi-master replication and its main challenge.

Multi-master replication allows multiple servers to accept writes and replicate changes to each other. The main challenge is conflict resolution when the same data is changed on different masters simultaneously.

intermediate

What is eventual consistency in replication?

Eventual consistency means that replicas may temporarily differ but will become consistent over time once all updates propagate, trading immediate consistency for better availability and performance.

Which replication strategy waits for all replicas to confirm before completing a write?

A Eventual consistency

B Asynchronous replication

C Master-slave replication

D Synchronous replication

Synchronous replication waits for all replicas to confirm the write to ensure consistency.

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

A Master

B Slave

C Both master and slave

D None

The master server handles all write operations; slaves replicate data and serve reads.

What is a key challenge of multi-master replication?

A Scaling reads

B Data backup

C Conflict resolution

D Latency reduction

Multi-master replication must handle conflicts when multiple masters update the same data.

Which replication type offers lower latency but may cause temporary inconsistency?

A Asynchronous replication

B Multi-master replication

C Master-slave replication

D Synchronous replication

Asynchronous replication confirms writes immediately, risking temporary inconsistency.

What does eventual consistency guarantee?

A Immediate data consistency

B Data will become consistent over time

C No data replication

D Synchronous writes

Eventual consistency means replicas will match after some time, not instantly.

Explain the main differences between synchronous and asynchronous data replication.

Think about when the system confirms a write is successful.

Describe master-slave and multi-master replication strategies and their use cases.

Consider read/write patterns and conflict handling.

Practice

1. Which data replication strategy ensures that all copies of data are updated at the same time before confirming a write operation?

easy

A. Synchronous replication

B. Asynchronous replication

C. Master-slave replication

D. Multi-master replication

Solution

1. Step 1: Understand synchronous replication

   Synchronous replication waits for all copies to confirm the write before completing the operation.

2. Step 2: Compare with other strategies

   Asynchronous replication does not wait, master-slave and multi-master describe topology, not timing.

3. Final Answer:

   Synchronous replication -> Option A

4. Quick Check:

   Strong consistency = Synchronous replication [OK]

Hint: Strong consistency means waiting for all copies to update [OK]

Common Mistakes:

• Confusing asynchronous with synchronous replication
• Mixing replication timing with replication topology
• Assuming master-slave means synchronous

2. Which of the following is the correct description of asynchronous replication?

easy

A. Replicas confirm the data before the primary confirms the write

B. Data is copied to replicas without waiting for confirmation from them

C. Data is copied simultaneously to all replicas with confirmation

D. Data is copied to replicas before the primary confirms the write

Solution

1. Step 1: Define asynchronous replication

   It copies data to replicas but does not wait for their confirmation before confirming the write.

2. Step 2: Eliminate other options

   Replicas confirm the data before the primary confirms the write requires replica confirmation before primary ack (synchronous); B copies before primary ack (incorrect); C describes synchronous behavior.

3. Final Answer:

   Data is copied to replicas without waiting for confirmation from them -> Option B

4. Quick Check:

   Asynchronous = no wait for replica confirmation [OK]

Hint: Asynchronous means no waiting for replicas to confirm [OK]

Common Mistakes:

• Thinking asynchronous waits for replicas
• Confusing timing of data copy
• Mixing synchronous and asynchronous definitions

3. Consider a system using master-slave replication. If the master receives a write request, what happens next?

medium

A. The write is applied simultaneously to master and slaves synchronously

B. The write is applied to all slaves first, then to the master

C. The write is only applied to slaves, not the master

D. The write is applied to the master and then asynchronously copied to slaves

Solution

1. Step 1: Understand master-slave replication flow

   Writes go to the master first, then are copied to slaves, usually asynchronously.

2. Step 2: Check other options

   Slaves do not receive writes first or alone; synchronous writes to all are not typical in master-slave.

3. Final Answer:

   The write is applied to the master and then asynchronously copied to slaves -> Option D

4. Quick Check:

   Master handles writes first in master-slave [OK]

Hint: Master handles writes first, slaves get updates later [OK]

Common Mistakes:

• Assuming slaves can accept writes first
• Confusing synchronous with asynchronous replication
• Thinking slaves are independent masters

4. A system using multi-master replication faces conflicts when two masters update the same data simultaneously. What is a common way to fix this issue?

medium

A. Switch to asynchronous replication only

B. Disable writes on all but one master

C. Use conflict resolution strategies like timestamps or version vectors

D. Remove all replicas and use a single database

Solution

1. Step 1: Identify conflict in multi-master replication

   Simultaneous writes cause conflicts that need resolution strategies.

2. Step 2: Evaluate conflict resolution methods

   Timestamps or version vectors help decide which write wins or how to merge.

3. Final Answer:

   Use conflict resolution strategies like timestamps or version vectors -> Option C

4. Quick Check:

   Conflict resolution = timestamps/version vectors [OK]

Hint: Resolve conflicts with timestamps or version vectors [OK]

Common Mistakes:

• Disabling multi-master writes defeats purpose
• Thinking asynchronous replication solves conflicts
• Removing replicas is impractical

5. You are designing a global application with users worldwide needing fast reads and writes. Which replication strategy best fits this use case?

hard

A. Multi-master replication with conflict resolution

B. Master-slave replication with synchronous writes

C. Single master with asynchronous replication

D. No replication, single database instance

Solution

1. Step 1: Analyze global user needs

   Users worldwide require fast reads and writes from multiple locations.

2. Step 2: Match replication strategy

   Multi-master allows writes at multiple locations with conflict resolution, improving speed and availability.

3. Step 3: Eliminate other options

   Master-slave with synchronous writes is slow globally; single master limits write locations; no replication is slow and risky.

4. Final Answer:

   Multi-master replication with conflict resolution -> Option A

5. Quick Check:

   Global fast writes = multi-master + conflict resolution [OK]

Hint: Global writes need multi-master with conflict handling [OK]

Common Mistakes:

• Choosing synchronous master-slave for global writes
• Ignoring conflict resolution in multi-master
• Using single master limits write speed globally