Data replication strategies in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding replication consistency models

Which data replication consistency model ensures that all replicas return the most recent write for a given data item?

A Eventual consistency

B Causal consistency

C Strong consistency

D Read-your-writes consistency

💡 Hint

Think about the model that guarantees immediate visibility of writes across all replicas.

Strong consistency means every read reflects the latest write, ensuring all replicas are synchronized immediately.

❓ Architecture

intermediate

Choosing replication topology for low latency

You want to design a data replication system for a global application that requires low read latency for users worldwide. Which replication topology is best suited?

A Single primary with multiple asynchronous replicas distributed globally

B Single primary with multiple synchronous replicas

C Multi-primary (master-master) replication

D No replication, single centralized database

💡 Hint

Consider a topology that allows fast reads worldwide but may tolerate some delay in writes.

Asynchronous replicas distributed globally allow local reads with low latency, while writes propagate asynchronously to replicas.

❓ scaling

advanced

Scaling write throughput with replication

Which replication strategy best helps scale write throughput in a distributed database system?

A Multi-primary (master-master) replication

B No replication, single node writes

C Single primary with asynchronous replicas

D Single primary with synchronous replicas

💡 Hint

Think about allowing multiple nodes to accept writes simultaneously.

Multi-primary replication allows multiple nodes to accept writes, increasing write throughput by distributing load.

❓ tradeoff

advanced

Tradeoffs in synchronous vs asynchronous replication

What is the main tradeoff when choosing synchronous replication over asynchronous replication?

A Asynchronous replication guarantees no data loss but increases write latency

B Synchronous replication improves write latency but risks data loss

C Asynchronous replication improves write latency and guarantees strong consistency

D Synchronous replication guarantees no data loss but increases write latency

💡 Hint

Consider how waiting for replicas affects write speed and data safety.

Synchronous replication waits for replicas to confirm writes, increasing latency but preventing data loss.

❓ estimation

expert

Estimating replication lag impact on read freshness

A system uses asynchronous replication with an average replication lag of 200ms. If the system receives 1000 writes per second, what is the expected number of writes not visible on a replica at any moment?

A Approximately 50 writes

B Approximately 200 writes

C Approximately 100 writes

D Approximately 500 writes

💡 Hint

Calculate lag in seconds multiplied by writes per second.

200ms = 0.2 seconds; 0.2 * 1000 writes/sec = 200 writes lagged on average.

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