CQRS (Command Query Responsibility Segregation) in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the main purpose of the Command in CQRS.

HLD

In CQRS, the Command side is responsible for [1] data.

Drag options to blanks, or click blank then click option'

Acaching

Bquerying

Cmodifying

Dindexing

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing Command with Query responsibilities.

The Command side handles all data changes or modifications.

2fill in blank

medium

Complete the code to describe the Query side responsibility in CQRS.

HLD

The Query side in CQRS is optimized for [1] data efficiently.

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Areading

Bvalidating

Cdeleting

Dupdating

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up reading and writing operations.

The Query side focuses on reading data efficiently without modifying it.

3fill in blank

hard

Fix the error in the description of CQRS architecture.

HLD

In CQRS, the [1] and Query sides share the same database schema.

Drag options to blanks, or click blank then click option'

ACommand

BQuery

CCache

DAPI

Attempts:

3 left

💡 Hint

Common Mistakes

Assuming both sides use the same database schema.

Command and Query sides usually have separate data models or databases to optimize their tasks.

4fill in blank

hard

Fill both blanks to complete the CQRS flow description.

HLD

The [1] side processes requests to change data, while the [2] side handles requests to fetch data.

Drag options to blanks, or click blank then click option'

ACommand

BQuery

CCache

DLoad Balancer

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping Command and Query roles.

Commands change data; Queries fetch data.

5fill in blank

hard

Fill all three blanks to complete the CQRS architecture components.

HLD

In CQRS, the [1] handles data changes, the [2] serves read requests, and the [3] ensures data synchronization between them.

Drag options to blanks, or click blank then click option'

ACommand side

BQuery side

CEvent bus

DDatabase

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the event bus with the database.

The Command side handles writes, the Query side handles reads, and the Event bus syncs data changes.

Practice

1. What is the main purpose of using CQRS in system design?

easy

A. To encrypt data for security purposes

B. To combine all database operations into a single service

C. To separate read and write operations for better scalability

D. To reduce the number of servers needed

Solution

1. Step 1: Understand CQRS concept

   CQRS stands for Command Query Responsibility Segregation, which means separating commands (writes) from queries (reads).

2. Step 2: Identify the main benefit

   This separation allows each part to be optimized and scaled independently, improving performance and maintainability.

3. Final Answer:

   To separate read and write operations for better scalability -> Option C

4. Quick Check:

   CQRS = Separate reads and writes [OK]

Hint: CQRS splits commands and queries for scaling [OK]

Common Mistakes:

• Thinking CQRS combines operations into one service
• Confusing CQRS with security encryption
• Assuming CQRS reduces server count directly

2. Which of the following is the correct way to describe the role of the 'Command' in CQRS?

easy

A. It processes write operations that change system state

B. It handles read-only queries to fetch data

C. It stores cached data for faster access

D. It manages user authentication and authorization

Solution

1. Step 1: Define Command role in CQRS

   Commands are responsible for write operations that modify the system's state.

2. Step 2: Differentiate from Query

   Queries only read data without changing it, so they are not commands.

3. Final Answer:

   It processes write operations that change system state -> Option A

4. Quick Check:

   Command = Write operations [OK]

Hint: Commands change data; queries only read [OK]

Common Mistakes:

• Confusing commands with queries
• Thinking commands handle caching
• Assuming commands manage security

3. Consider a system using CQRS where the write side updates a user profile and the read side serves user data. If the write side updates the user's email, what is the expected behavior on the read side immediately after the update?

medium

A. The read side instantly shows the updated email without delay

B. The read side deletes the user data until refreshed

C. The read side rejects the query until the write completes

D. The read side may show the old email briefly due to asynchronous update

Solution

1. Step 1: Understand asynchronous update in CQRS

   In CQRS, the read side is often updated asynchronously via events after the write completes.

2. Step 2: Identify read side behavior after write

   Because of this delay, the read side may temporarily show stale data until it receives the update event.

3. Final Answer:

   The read side may show the old email briefly due to asynchronous update -> Option D

4. Quick Check:

   Read side updates asynchronously = possible stale data [OK]

Hint: Reads update asynchronously, so data may lag briefly [OK]

Common Mistakes:

• Assuming immediate read consistency
• Thinking reads block until writes finish
• Believing read data is deleted during update

4. A developer implemented CQRS but notices that the read model is not updating after commands execute. What is the most likely cause?

medium

A. The read model database is corrupted and cannot be read

B. The command handler is not sending events to update the read model

C. The query side is trying to write data instead of reading

D. The system is using synchronous updates causing deadlocks

Solution

1. Step 1: Identify how read model updates in CQRS

   The read model updates via events sent by the command handler after state changes.

2. Step 2: Diagnose missing updates

   If the read model is not updating, likely the events are not being sent or processed properly.

3. Final Answer:

   The command handler is not sending events to update the read model -> Option B

4. Quick Check:

   Missing events cause read model stale data [OK]

Hint: Check if events are sent after commands [OK]

Common Mistakes:

• Blaming database corruption without evidence
• Confusing query side roles
• Assuming synchronous updates cause deadlocks here

5. You are designing a high-traffic e-commerce system using CQRS. Which approach best ensures that the read side remains highly available and scalable while keeping data reasonably fresh?

hard

A. Use event sourcing to asynchronously update read models and deploy multiple read replicas

B. Use a single database for both reads and writes to avoid data duplication

C. Synchronously update the read model within the command transaction to ensure consistency

D. Disable caching on the read side to always fetch fresh data from the write database

Solution

1. Step 1: Understand scalability needs in CQRS

   Separating reads and writes allows scaling read replicas independently to handle high traffic.

2. Step 2: Use event sourcing for asynchronous updates

   Event sourcing helps keep read models updated asynchronously, balancing freshness and availability.

3. Step 3: Evaluate other options

   Single database limits scalability; synchronous updates reduce availability; disabling caching hurts performance.

4. Final Answer:

   Use event sourcing to asynchronously update read models and deploy multiple read replicas -> Option A

5. Quick Check:

   Event sourcing + read replicas = scalable, fresh reads [OK]

Hint: Event sourcing + replicas = scalable reads with freshness [OK]

Common Mistakes:

• Using single DB limits scalability
• Synchronous updates reduce availability
• Disabling cache hurts performance