API gateway concept in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an API Gateway?

An API Gateway is a server that acts as a single entry point for all client requests to multiple backend services. It handles request routing, composition, and protocol translation.

beginner

Name two main functions of an API Gateway.

1. Routing client requests to the correct backend service.
2. Handling security like authentication and rate limiting.

intermediate

Why use an API Gateway instead of direct client calls to services?

It simplifies client communication by providing one endpoint, improves security by centralizing checks, and can optimize performance by aggregating responses.

intermediate

What is request composition in the context of an API Gateway?

Request composition means the API Gateway combines data from multiple backend services into a single response for the client.

intermediate

How does an API Gateway help with scalability?

By offloading tasks like authentication, rate limiting, and request routing, it reduces load on backend services and allows independent scaling of components.

What role does an API Gateway play in a microservices architecture?

A Stores all data for backend services

B Acts as a single entry point for client requests

C Replaces the database layer

D Runs client-side code

An API Gateway acts as a single entry point, routing requests to appropriate backend services.

Which of the following is NOT a typical feature of an API Gateway?

A Rate limiting

B Authentication and authorization

C Request routing

D Direct database queries

API Gateways do not perform direct database queries; they route requests to backend services that handle data.

How does an API Gateway improve security?

A By centralizing authentication and authorization

B By storing user passwords

C By encrypting backend databases

D By running client code securely

API Gateways centralize security checks like authentication and authorization before forwarding requests.

What is request composition in an API Gateway?

A Combining multiple backend responses into one

B Encrypting client requests

C Caching database queries

D Running backend services

Request composition means the API Gateway merges data from multiple services into a single response.

Which benefit does an API Gateway provide for scalability?

A Stores all client data

B Runs all backend services itself

C Offloads common tasks from backend services

D Eliminates the need for load balancers

By handling tasks like authentication and routing, the API Gateway reduces backend load, aiding scalability.

Explain the role and benefits of an API Gateway in a microservices system.

Think about how it simplifies client communication and protects backend services.

Describe how an API Gateway manages client requests and backend responses.

Consider the flow from client to backend and back.

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