Read replicas in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the primary purpose of read replicas

What is the main reason to use read replicas in a database system?

A To reduce network latency by storing data closer to users

B To provide backup storage for disaster recovery only

C To increase the write throughput by distributing write operations

D To improve read scalability by distributing read operations

💡 Hint

Think about how read replicas help when many users want to access data simultaneously.

Read replicas are primarily used to distribute read requests, improving read scalability and reducing load on the primary database.

❓ Architecture

intermediate

Choosing the correct architecture for read replicas

Which architecture best describes a system using read replicas to handle high read traffic?

A One primary database handling writes and multiple read replicas handling read queries

B Multiple primary databases each handling a subset of writes and reads independently

C Multiple read replicas handling writes and a primary database handling reads

D A single primary database handling all reads and writes with no replicas

💡 Hint

Consider which node handles writes and which nodes handle reads in a typical read replica setup.

In a read replica architecture, the primary database handles all writes, while read replicas serve read queries to scale read capacity.

❓ scaling

advanced

Scaling read replicas for global users

You have a global user base with users in multiple continents. How should you deploy read replicas to minimize read latency?

A Deploy read replicas only in the primary database region to keep data consistent

B Deploy read replicas in multiple geographic regions close to user clusters

C Deploy read replicas randomly without considering user location

D Use a single read replica in a central location with the fastest network

💡 Hint

Think about how placing data closer to users affects latency.

Deploying read replicas geographically close to users reduces network latency and improves read performance globally.

❓ tradeoff

advanced

Tradeoffs of using read replicas for strong consistency

What is a common tradeoff when using read replicas in systems that require strong consistency?

A Read replicas may serve stale data due to replication lag

B Read replicas always provide up-to-date data with no delay

C Read replicas improve write performance by delaying replication

D Read replicas eliminate the need for a primary database

💡 Hint

Consider what happens when data changes on the primary but replicas have not yet updated.

Replication lag means read replicas might not have the latest data immediately, causing potential stale reads.

❓ estimation

expert

Estimating read replica capacity for a high-traffic system

Your system receives 10,000 read requests per second and 1,000 write requests per second. Each read replica can handle 2,000 reads per second. How many read replicas do you need to handle the read load without overloading the primary?

A 4 read replicas

B 10 read replicas

C 5 read replicas

D 6 read replicas

💡 Hint

Divide total read requests by capacity per replica and round up.

To handle 10,000 reads per second with each replica handling 2,000, you need 10,000 / 2,000 = 5 replicas.

Practice

1. What is the primary purpose of using read replicas in a system design?

easy

A. To backup data by creating copies on different servers

B. To increase write throughput by adding more write nodes

C. To improve read scalability by offloading read queries from the primary database

D. To encrypt data for security purposes

Solution

1. Step 1: Understand the role of read replicas

   Read replicas are copies of the primary database used mainly to handle read requests.

2. Step 2: Identify the benefit in system design

   By directing read queries to replicas, the primary database is less loaded, improving read scalability.

3. Final Answer:

   To improve read scalability by offloading read queries from the primary database -> Option C

4. Quick Check:

   Read replicas = improve read scalability [OK]

Hint: Read replicas handle reads, not writes, to reduce primary load [OK]

Common Mistakes:

• Thinking read replicas improve write performance
• Confusing read replicas with backup systems
• Assuming read replicas encrypt data

2. Which of the following is the correct way to describe replication in read replicas?

easy

A. Replication is synchronous, so replicas always have the latest data instantly.

B. Replication duplicates only write queries, not read queries.

C. Replication happens only when the primary database restarts.

D. Replication is asynchronous, so replicas may lag behind the primary database.

Solution

1. Step 1: Understand replication types

   Read replicas usually use asynchronous replication, meaning data updates are sent with some delay.

2. Step 2: Identify the correct replication behavior

   Because replication is asynchronous, replicas may not always have the latest data instantly.

3. Final Answer:

   Replication is asynchronous, so replicas may lag behind the primary database. -> Option D

4. Quick Check:

   Replication = asynchronous lag possible [OK]

Hint: Replication delay means replicas can be slightly outdated [OK]

Common Mistakes:

• Assuming replication is always synchronous
• Thinking replication triggers only on restart
• Confusing replication of queries with replication of data

3. Consider a system with one primary database and two read replicas. If the primary receives 100 write requests and 300 read requests per second, how should the read requests be handled to optimize performance?

medium

A. Send all 300 read requests to the read replicas and 100 writes to the primary

B. Send all 400 requests to the primary database

C. Send half of the read requests to the primary and half to the replicas

D. Send all requests randomly to any database

Solution

1. Step 1: Separate reads and writes

   Writes must go to the primary database to maintain data consistency.

2. Step 2: Offload reads to replicas

   All read requests should be directed to read replicas to reduce load on the primary and improve read response.

3. Final Answer:

   Send all 300 read requests to the read replicas and 100 writes to the primary -> Option A

4. Quick Check:

   Reads to replicas, writes to primary = best practice [OK]

Hint: Direct reads to replicas, writes to primary for best performance [OK]

Common Mistakes:

• Sending writes to replicas causing data inconsistency
• Overloading primary with all requests
• Randomly distributing requests without separation

4. A system uses read replicas but users report seeing stale data sometimes. What is the most likely cause and how can it be fixed?

medium

A. Primary database is overloaded; fix by adding more replicas

B. Replication is asynchronous causing delay; fix by using synchronous replication

C. Read replicas are down; fix by restarting replicas

D. Network latency is high; fix by reducing read requests

Solution

1. Step 1: Identify cause of stale data

   Stale data usually happens because asynchronous replication delays updates to replicas.

2. Step 2: Fix by changing replication method

   Using synchronous replication ensures replicas have the latest data but may impact performance.

3. Final Answer:

   Replication is asynchronous causing delay; fix by using synchronous replication -> Option B

4. Quick Check:

   Stale data = async replication delay [OK]

Hint: Stale reads? Check replication delay type [OK]

Common Mistakes:

• Restarting replicas without checking replication
• Adding replicas without fixing replication lag
• Reducing reads instead of addressing replication

5. You are designing a high-traffic e-commerce platform. To handle millions of read requests per second, you plan to use read replicas. Which combination of strategies best ensures scalability and data freshness?

hard

A. Use multiple asynchronous read replicas with eventual consistency and implement cache invalidation on writes

B. Use a single synchronous read replica to guarantee data freshness but risk read bottlenecks

C. Send all reads and writes to the primary database to avoid replication delays

D. Use asynchronous replicas without any cache or consistency strategy

Solution

1. Step 1: Understand scalability needs

   Millions of reads require multiple replicas to distribute load effectively.

2. Step 2: Balance data freshness and performance

   Asynchronous replicas scale well but have lag; cache invalidation helps keep data fresh for users.

3. Step 3: Evaluate other options

   Single synchronous replica limits scalability; sending all to primary causes bottlenecks; no cache hurts freshness.

4. Final Answer:

   Use multiple asynchronous read replicas with eventual consistency and implement cache invalidation on writes -> Option A

5. Quick Check:

   Multiple async replicas + cache = scalable & fresh [OK]

Hint: Combine async replicas with cache for scale and freshness [OK]

Common Mistakes:

• Choosing single synchronous replica causing bottlenecks
• Ignoring cache leading to stale reads
• Sending all traffic to primary causing overload