Microservice communication basics in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding synchronous vs asynchronous communication

Which statement correctly describes the difference between synchronous and asynchronous communication in microservices?

A Synchronous communication waits for a response before continuing, while asynchronous communication does not wait and proceeds immediately.

B Synchronous communication never waits for a response, asynchronous always waits for a response.

C Asynchronous communication requires both services to be online at the same time, synchronous does not.

D Synchronous communication uses message queues, asynchronous uses direct HTTP calls.

💡 Hint

Think about whether the sender waits for the receiver to reply before moving on.

Synchronous communication means the sender waits for the receiver's response before continuing. Asynchronous communication allows the sender to continue without waiting.

❓ Architecture

intermediate

Choosing communication method for microservices

You have two microservices: one handles user authentication and the other processes payments. Which communication method is best when the payment service must confirm user identity before processing?

A Use synchronous HTTP calls so the payment service waits for authentication response before proceeding.

B Use asynchronous messaging with a message queue to send user identity and wait for confirmation.

C Use asynchronous event broadcasting without waiting for any response.

D Use direct database access by the payment service to check user identity.

💡 Hint

Consider if the payment service needs immediate confirmation before continuing.

Synchronous HTTP calls are best when the payment service must wait for user authentication confirmation before processing payments.

❓ scaling

advanced

Scaling microservices with asynchronous communication

Which benefit does asynchronous communication provide when scaling microservices?

A It requires all services to be tightly coupled, improving coordination.

B It forces all services to process requests in a strict order, reducing complexity.

C It allows services to handle requests independently without waiting, improving throughput and resilience.

D It eliminates the need for message brokers or queues.

💡 Hint

Think about how asynchronous communication affects service independence and load handling.

Asynchronous communication lets services work independently and not wait for each other, which helps handle more requests and recover from failures.

❓ tradeoff

advanced

Tradeoffs of using message queues in microservices

What is a common tradeoff when using message queues for microservice communication?

A Guaranteed real-time responses but no fault tolerance.

B Improved decoupling and reliability but increased complexity and potential message delays.

C No need for service discovery but requires synchronous calls.

D Simpler architecture but no support for retrying failed messages.

💡 Hint

Consider what message queues add and what challenges they introduce.

Message queues help decouple services and improve reliability but add complexity and can cause delays due to asynchronous processing.

❓ estimation

expert

Estimating message queue capacity for microservices

You expect 10,000 requests per minute between two microservices communicating asynchronously via a message queue. Each message is about 1 KB. What is the minimum required throughput of the message queue in MB per second?

A Approximately 1.67 MB/s

B Approximately 0.017 MB/s

C Approximately 16.7 MB/s

D Approximately 0.17 MB/s

💡 Hint

Calculate total KB per minute, convert to MB, then divide by 60 seconds.

10,000 messages/min × 1 KB = 10,000 KB/min = 10 MB/min. 10 MB ÷ 60 sec ≈ 0.167 MB/s.

Practice

1. Which of the following is a common way for microservices to communicate synchronously in an Express-based system?

easy

A. Sharing global variables in memory

B. Direct database access by all services

C. Using HTTP REST API calls between services

D. Using local file system for message passing

Solution

1. Step 1: Understand synchronous communication in microservices

   Synchronous communication means one service waits for a response from another before continuing.

2. Step 2: Identify common synchronous methods in Express

   HTTP REST API calls are widely used for synchronous communication between microservices in Express.

3. Final Answer:

   Using HTTP REST API calls between services -> Option C

4. Quick Check:

   Synchronous communication = HTTP REST API calls [OK]

Hint: Sync communication usually uses HTTP requests [OK]

Common Mistakes:

• Thinking direct DB access is common for microservices
• Assuming shared memory works across services
• Using file system for communication

2. Which Express code snippet correctly sends a JSON response from one microservice to another?

easy

A. res.json({ data: 'value' })

B. res.write({ data: 'value' })

C. res.send("{ data: 'value' }")

D. res.render({ data: 'value' })

Solution

1. Step 1: Identify Express method for JSON response

   Express provides res.json() to send JSON formatted responses easily.

2. Step 2: Compare other methods

   res.send() can send JSON but res.json() is preferred for clarity; res.write() is low-level and res.render() is for templates.

3. Final Answer:

   res.json({ data: 'value' }) -> Option A

4. Quick Check:

   Express JSON response = res.json() [OK]

Hint: Use res.json() to send JSON responses in Express [OK]

Common Mistakes:

• Using res.send() without JSON stringification
• Confusing res.render() with JSON response
• Using res.write() incorrectly

3. Consider two microservices where Service A calls Service B using HTTP GET. What will Service A receive if Service B responds with res.status(200).json({ message: 'Success' })?

medium

A. A JSON object { message: 'Success' }

B. A plain text 'Success' string

C. An HTTP 404 error

D. An empty response

Solution

1. Step 1: Understand Service B's response

   Service B sends HTTP status 200 with JSON body { message: 'Success' }.

2. Step 2: Determine what Service A receives

   Service A receives the JSON object exactly as sent, since status 200 means OK and body is JSON.

3. Final Answer:

   A JSON object { message: 'Success' } -> Option A

4. Quick Check:

   HTTP 200 + res.json() = JSON object response [OK]

Hint: HTTP 200 with res.json() returns JSON object [OK]

Common Mistakes:

• Assuming plain text instead of JSON
• Confusing status codes with response body
• Expecting empty or error response

4. In an Express microservice, the following code is used to call another service asynchronously. What is the main issue?

fetch('http://service-b/api/data')
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.log(error))

medium

A. Missing await keyword for fetch call

B. fetch is not defined in Node.js by default

C. No error handling for non-JSON responses

D. Using console.log instead of res.send

Solution

1. Step 1: Check environment support for fetch

   Node.js versions before 18 do not have fetch globally defined by default.

2. Step 2: Identify the main issue in Express microservice

   Without importing or enabling fetch, this code will cause a ReferenceError in Node.js.

3. Final Answer:

   fetch is not defined in Node.js by default -> Option B

4. Quick Check:

   Node.js needs fetch polyfill or version 18+ [OK]

Hint: Node.js needs fetch polyfill or v18+ for fetch [OK]

Common Mistakes:

• Thinking await is mandatory with then() chains
• Ignoring fetch availability in Node.js
• Confusing console.log with response sending

5. You want to design communication between two Express microservices where Service A sends a task to Service B and does not wait for a response. Which approach is best to ensure reliable, scalable communication?

hard

A. Use WebSocket connection for real-time two-way communication

B. Use HTTP POST with immediate response from Service B

C. Use direct database writes by Service A for Service B to poll

D. Use a message queue like RabbitMQ for asynchronous messaging

Solution

1. Step 1: Understand asynchronous communication needs

   Service A sends a task and does not wait, so async messaging is needed.

2. Step 2: Evaluate options for reliability and scalability

   Message queues like RabbitMQ provide reliable, scalable async communication with delivery guarantees.

3. Final Answer:

   Use a message queue like RabbitMQ for asynchronous messaging -> Option D

4. Quick Check:

   Async + reliable + scalable = message queue [OK]

Hint: Async tasks use message queues for reliability [OK]

Common Mistakes:

• Using HTTP POST expecting async without waiting
• Relying on database polling which is inefficient
• Using WebSocket for one-way async tasks