CQRS pattern in Microservices - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the main components of CQRS.

Microservices

In CQRS, the [1] handles commands that change data.

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

AAPI gateway

BQuery side

CCommand side

DEvent store

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the query side with the command side.

Thinking the event store handles commands.

The Command side is responsible for handling commands that change the system state.

2fill in blank

medium

Complete the code to explain the role of the query side in CQRS.

Microservices

The [1] is optimized for reading data and answering requests.

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

ACommand side

BQuery side

CEvent bus

DLoad balancer

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the command and query sides.

Assuming the event bus handles queries.

The Query side is designed to efficiently handle read operations in CQRS.

3fill in blank

hard

Fix the error in the description of CQRS event handling.

Microservices

Events are published to the [1] to notify other parts of the system.

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

AEvent bus

BQuery side

CCommand side

DDatabase

Attempts:

3 left

💡 Hint

Common Mistakes

Thinking events are sent directly to the database.

Confusing the command side with the event bus.

The event bus is the component that distributes events to interested subscribers in CQRS.

4fill in blank

hard

Fill both blanks to complete the CQRS request flow.

Microservices

Client sends a [1] to the [2], which updates the data and publishes events.

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

Acommand

Bquery

Ccommand side

Dquery side

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping command and query terms.

Sending commands to the query side.

Clients send commands to the command side to change data in CQRS.

5fill in blank

hard

Fill all three blanks to complete the CQRS read model update process.

Microservices

Events from the [1] are received by the [2], which updates the [3] for fast queries.

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

Acommand side

Bevent handler

Cread model

Ddatabase

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the read model with the command side.

Thinking the database directly handles events.

The command side publishes events, the event handler processes them, and the read model is updated for queries.

Practice

1. What is the main purpose of the CQRS pattern in microservices architecture?

easy

A. To separate read and write operations for better scalability

B. To combine all database operations into a single service

C. To encrypt data during transmission between services

D. To cache all data on the client side for faster access

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 side to be optimized and scaled independently, improving performance and maintainability.

3. Final Answer:

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

4. Quick Check:

   CQRS = Separate reads and writes [OK]

Hint: CQRS splits commands and queries separately [OK]

Common Mistakes:

• Thinking CQRS merges all operations into one service
• Confusing CQRS with encryption or caching
• Assuming CQRS only applies to database encryption

2. Which of the following is the correct way to describe the command side in CQRS?

easy

A. Handles read-only queries to fetch data

B. Manages user authentication and sessions

C. Processes write operations that change state

D. Caches data for faster retrieval

Solution

1. Step 1: Define command side role

   The command side in CQRS is responsible for handling commands, which are operations that change the system's state (writes).

2. Step 2: Eliminate incorrect options

   Read-only queries belong to the query side, caching is a separate concern, and authentication is unrelated to CQRS commands.

3. Final Answer:

   Processes write operations that change state -> Option C

4. Quick Check:

   Command side = writes [OK]

Hint: Commands change data, queries read data [OK]

Common Mistakes:

• Confusing command side with query side
• Thinking command side handles caching
• Mixing authentication with CQRS commands

3. Given the following simplified CQRS flow:

1. User sends a command to update an order.
2. Command handler updates the write database.
3. An event is published.
4. The read model updates asynchronously.
What is the main reason for step 4?

medium

A. To validate the command before processing

B. To keep the read database in sync with the write database

C. To rollback the write operation if needed

D. To encrypt the data before sending to the client

Solution

1. Step 1: Understand event role in CQRS

   After the write database updates, an event signals that data changed.

2. Step 2: Purpose of read model update

   The read model updates asynchronously to reflect the latest data for queries, keeping it consistent with writes.

3. Final Answer:

   To keep the read database in sync with the write database -> Option B

4. Quick Check:

   Event updates read model = sync reads [OK]

Hint: Events update read model after writes [OK]

Common Mistakes:

• Thinking event validates or rolls back commands
• Confusing encryption with event handling
• Assuming read model updates happen synchronously

4. In a CQRS system, a developer notices that the read model sometimes shows stale data after a write. What is the most likely cause?

medium

A. The client is caching old data aggressively

B. The command handler failed to update the write database

C. The write database is not replicated properly

D. The event to update the read model is delayed or lost

Solution

1. Step 1: Identify cause of stale read data

   In CQRS, the read model updates asynchronously via events. If events are delayed or lost, the read model lags behind.

2. Step 2: Rule out other causes

   If the write database failed, writes wouldn't succeed. Client caching or replication issues are less likely to cause this specific CQRS symptom.

3. Final Answer:

   The event to update the read model is delayed or lost -> Option D

4. Quick Check:

   Stale reads = delayed event update [OK]

Hint: Stale reads usually mean event delay or loss [OK]

Common Mistakes:

• Blaming write database failure without evidence
• Ignoring event delivery reliability
• Assuming client caching is always the cause

5. You are designing a high-traffic e-commerce system using CQRS. Which approach best handles the challenge of scaling the read side independently from the write side?

hard

A. Use separate databases for read and write models with event-driven synchronization

B. Use a single database for both reads and writes with strong locking

C. Cache all writes on the client and batch update the database later

D. Directly query the write database for all read requests

Solution

1. Step 1: Understand scaling needs in CQRS

   Separating read and write databases allows independent scaling and optimization for each workload.

2. Step 2: Evaluate options for scaling reads

   Event-driven synchronization keeps the read database updated asynchronously, enabling fast, scalable queries without locking.

3. Step 3: Reject unsuitable options

   Single database with locking limits scalability; client caching risks data loss; querying write DB for reads causes contention.

4. Final Answer:

   Use separate databases for read and write models with event-driven synchronization -> Option A

5. Quick Check:

   Separate DBs + events = scalable CQRS [OK]

Hint: Separate read/write DBs with events scale best [OK]

Common Mistakes:

• Using one DB with locking reduces scalability
• Relying on client caching risks consistency
• Reading directly from write DB causes contention