API gateway concept in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Role of an API Gateway in Microservices Architecture

Which of the following best describes the primary role of an API Gateway in a microservices architecture?

A It stores all the data for microservices and manages database transactions.

B It acts as a single entry point that routes client requests to appropriate microservices and handles cross-cutting concerns like authentication and rate limiting.

C It is responsible for deploying microservices to cloud infrastructure automatically.

D It replaces the need for load balancers by distributing traffic evenly across servers.

💡 Hint

Think about what component manages client requests and common tasks like security in microservices.

An API Gateway serves as a single entry point for clients. It routes requests to the correct microservice and handles tasks like authentication, logging, and rate limiting, which are common across services.

❓ Architecture

intermediate

API Gateway Request Flow

In a system using an API Gateway, what is the correct sequence of steps when a client makes a request?

A 1,2,3,4,5

B 1,3,2,4,5

C 2,1,3,4,5

D 1,2,4,3,5

💡 Hint

Consider the logical order of authentication before routing and response flow.

The client first sends a request to the API Gateway. The gateway authenticates the request, then routes it to the correct microservice. The microservice processes it and returns the response to the gateway, which then sends it back to the client.

❓ scaling

advanced

Scaling API Gateway for High Traffic

Your system uses an API Gateway that is becoming a bottleneck under high traffic. Which approach best improves scalability while maintaining reliability?

A Remove the API Gateway and let clients call microservices directly to reduce overhead.

B Increase the CPU and memory of a single API Gateway instance to handle more requests.

C Deploy multiple instances of the API Gateway behind a load balancer to distribute incoming requests evenly.

D Use a single-threaded API Gateway instance to avoid concurrency issues.

💡 Hint

Think about horizontal scaling and fault tolerance.

Deploying multiple API Gateway instances behind a load balancer allows the system to handle more requests by distributing load and provides fault tolerance if one instance fails.

❓ tradeoff

advanced

Tradeoff of Using API Gateway for Client Requests

What is a common tradeoff when introducing an API Gateway in front of microservices?

A It guarantees zero downtime during deployments without additional effort.

B It eliminates the need for microservices to handle any business logic.

C It reduces security risks by removing authentication from the system.

D It adds an extra network hop which can increase latency but centralizes cross-cutting concerns.

💡 Hint

Consider the impact on request time and system management.

Using an API Gateway adds an extra step in the request path, which can increase latency slightly. However, it centralizes tasks like authentication and logging, simplifying microservices.

❓ estimation

expert

Estimating API Gateway Capacity for Peak Load

Your API Gateway must handle 10,000 requests per second at peak. Each request takes 5 milliseconds of processing time on the gateway. How many API Gateway instances are needed to handle the load without queuing?

A At least 50 instances

B At least 20 instances

C At least 10 instances

D At least 5 instances

💡 Hint

Calculate how many requests one instance can handle per second, then divide total requests by that number.

One instance can handle 1000 ms / 5 ms = 200 requests per second. To handle 10,000 requests per second, 10,000 / 200 = 50 instances are needed.

Practice

1. What is the primary role of an API gateway in a system architecture?

easy

A. It replaces the need for backend services.

B. It stores all the data for the application.

C. It directly manages the database connections.

D. It acts as a single entry point for client requests to backend services.

Solution

1. Step 1: Understand the role of API gateway

   An API gateway serves as the main entry point for clients to access multiple backend services, simplifying communication.

2. Step 2: Eliminate incorrect roles

   Storing data, managing database connections, or replacing backend services are not responsibilities of an API gateway.

3. Final Answer:

   It acts as a single entry point for client requests to backend services. -> Option D

4. Quick Check:

   API gateway = single entry point [OK]

Hint: API gateway = one door for all client requests [OK]

Common Mistakes:

• Confusing API gateway with database or storage
• Thinking API gateway replaces backend services
• Assuming API gateway manages data directly

2. Which of the following is the correct function of an API gateway in handling client requests?

easy

A. Directly querying the database for each request.

B. Serving static files like images and videos.

C. Routing requests to appropriate backend services.

D. Replacing the client application.

Solution

1. Step 1: Identify API gateway functions

   An API gateway routes incoming client requests to the correct backend services based on the request type or URL.

2. Step 2: Remove unrelated functions

   Serving static files, querying databases directly, or replacing clients are not typical API gateway roles.

3. Final Answer:

   Routing requests to appropriate backend services. -> Option C

4. Quick Check:

   API gateway routes requests = C [OK]

Hint: API gateway directs traffic to backend services [OK]

Common Mistakes:

• Thinking API gateway queries databases directly
• Confusing API gateway with CDN or static server
• Assuming API gateway replaces client apps

3. Consider this simplified API gateway flow:

Client -> API Gateway -> Service A (auth) -> Service B (data)

If the API gateway receives a request and Service A rejects it due to failed authentication, what will the client receive?

medium

A. Authentication error response

B. Data from Service B

C. No response (timeout)

D. Success response with empty data

Solution

1. Step 1: Trace request through services

   The request first goes to Service A for authentication. If Service A rejects it, the request does not proceed further.

2. Step 2: Determine client response

   Since authentication failed, the API gateway returns an authentication error to the client instead of data or success.

3. Final Answer:

   Authentication error response -> Option A

4. Quick Check:

   Failed auth = error response [OK]

Hint: Failed auth stops flow, returns error [OK]

Common Mistakes:

• Assuming data is returned despite failed auth
• Thinking client gets no response (timeout)
• Believing success with empty data is sent

4. An API gateway is configured to route requests to multiple services. However, clients report 404 errors for valid endpoints. What is the most likely cause?

medium

A. Backend services are down.

B. API gateway routing rules are misconfigured.

C. Client requests are malformed.

D. API gateway is overloaded.

Solution

1. Step 1: Analyze 404 error meaning

   A 404 error means the requested resource was not found, often due to incorrect routing or URL mapping.

2. Step 2: Identify likely cause in API gateway

   If backend services are up and clients send correct requests, misconfigured routing rules in the API gateway cause 404 errors.

3. Final Answer:

   API gateway routing rules are misconfigured. -> Option B

4. Quick Check:

   404 = routing misconfiguration [OK]

Hint: 404 usually means wrong routing path [OK]

Common Mistakes:

• Assuming backend services are down without checking
• Blaming client requests without validation
• Thinking overload causes 404 errors

5. You want to design an API gateway that aggregates data from three backend services before responding to the client. Which approach best fits this requirement?

hard

A. API gateway calls all three services, combines their responses, then sends one response to client.

B. API gateway caches data and never calls backend services.

C. API gateway sends separate responses from each service to the client.

D. API gateway forwards client requests directly to one backend service only.

Solution

1. Step 1: Understand aggregation requirement

   The API gateway must collect data from multiple services and combine it into a single response for the client.

2. Step 2: Evaluate options for aggregation

   Forwarding to one service or sending separate responses does not aggregate data. Caching alone does not fulfill aggregation if fresh data is needed.

3. Final Answer:

   API gateway calls all three services, combines their responses, then sends one response to client. -> Option A

4. Quick Check:

   Aggregation = combine multiple service responses [OK]

Hint: Aggregate by combining multiple service responses [OK]

Common Mistakes:

• Thinking forwarding to one service aggregates data
• Sending multiple responses instead of one combined
• Assuming caching replaces aggregation