Top 40 API Interview Questions and Answers

The Application Program Interface, or API as it is now more often known, was created with security in mind, but it was also extremely difficult to create and even more difficult to maintain. The most frequently requested API interview questions and answers will be covered in this section. Hone your skills with our online API course program.

API-Related Interview Questions on Fundamental Concepts

1. What is an API?

A collection of guidelines and procedures for creating and integrating application software is known as an Application Programming Interface (API).

2. Which are the primary categories of APIs?

REST (Representational State Transfer): Works with resources via the GET, POST, PUT, and DELETE HTTP methods.

SOAP (Simple Object Access Protocol): XML is used by this protocol to exchange data.

RPC (Remote Procedure Call): It enables a program to invoke a function on a different computer.

3. What distinguishes SOAP from RESTful APIs?

SOAP: Complex, stateful, utilizes XML, more secure, dependable for complex transactions; * REST: Simpler, stateless, uses HTTP, usually JSON, and flexible. 

REST API Interview Questions and Answers

4. What are the core principles of REST?

The core concepts of REST API are:

• Stateless: Every request is separate and has all the data it requires. Every request is handled independently by the server.
• Layered System: Each component of a layered system that employs REST’s hierarchical architecture can only view the layer with which it is interacting.
• Client-server: The server stores the resources, and the client requests them. Different languages and technologies can be used to construct the client and server separately.
• Cacheable: Response data can be classified as either cacheable or non-cacheable. The client can utilize the data for subsequent queries if it is cacheable.
  • As a result, there are fewer exchanges between the client and server. 
• Code on Demand: This optional feature enables an API to extend its functionality by sending snippets of code or applets to a client.

5. Describe the HTTP methods that REST APIs employ.

In REST APIs, the most popular HTTP methods are:

• GET: It pulls a resource or group of resources off the server. It is the HTTP method that is most frequently utilized.
• POST: It makes a fresh resource available on the server. It’s frequently used to upload files or submit form input.
• PUT: It uses the updated information in the request body to replace an existing resource on the server.
• DELETE: It eliminates a particular server resource.
• PATCH: It modifies specific fields or properties to update a resource partially.  

Get started with our API course syllabus. 

API Testing and Debugging Interview Questions and Answers

6. What tools can be used to test APIs?

APIs can be tested using a variety of tools, such as:

• Postman: An easy-to-use tool for developing, testing, and recording APIs. Both automated and manual testing are supported.
• Apache JMeter: A useful tool for functional and performance testing of REST and SOAP services is Apache JMeter. To assess the performance of APIs, it can mimic heavy requests.
• Rest Assured: An open-source Java library that makes it easier to test APIs from beginning to end. REST service testing in Java applications is made easier by it.
• Apigee: It is an application for testing cross-cloud APIs that works with APIs that hold a lot of data.
• Karate DSL: An open-source framework for testing APIs that doesn’t require knowledge of programming languages. It is designed for automated testing of APIs.
• Swagger: An open-source program for standardizing the documentation and testing of REST APIs. It offers an interactive user interface.
• Assertible: A service that focuses on monitoring and testing APIs. It provides a graphical user interface (GUI) for every feature.

7. How do you debug API issues?

Use debugging tools, examine request and response headers and bodies, look at HTTP status codes, and refer to API documentation.

• Identify the problem with the API.
• Examine the status updates.
• Examine the data in greater detail.
• Using a brute force approach.
• Strategy of backtracking.
• The strategy of division and conquest.

API Security Interview Questions and Answers

8. What are common API security threats?

Typical risks to API security include:

• Broken authentication.
• Function-level authorization is broken.
• Overexposure to data.
• Property-level authorization (BOLA) for broken objects.
• Misconfigured security.
• Ineffective inventory control.
• Injection.

9. How can you secure APIs?

Here are a few strategies for protecting APIs:

• Monitoring and logging: To identify and address security incidents and record API requests, answers, and failures.
• Use API Gateway: API gateways authenticate traffic and lessen vulnerabilities by serving as go-betweens for clients and backend services.
• API Testing: To find security vulnerabilities, test API endpoints on a regular basis from the viewpoint of an attacker.
• Data Encryption: Employ Transport Layer Security (TLS) for encryption in transit and encryption at rest to stop hackers from exploiting a hacked API server.
• Input Validation: Verify that user input complies with the required format, type, length, and value range.
• Sanitize User Input: To stop attacks like SQL injection, cross-site scripting, and command injection, validate and clean inputs and outputs.
• Object-level authorization: Limit the amount of data that is exposed and safeguard sensitive information by using encryption. 

If you are a serious job seeker living around OMR, check out our API training institute in OMR. 

API Design and Architecture Interview Questions and Answers

10. What is API versioning and why is it important?

API versioning is the process of managing API changes to alert its users. It’s crucial since it enables developers to maintain the functionality of current clients while gradually evolving an API. 

The following are some advantages of versioning APIs:

• Preserve Consumer Trust: Versioning an API makes it possible to successfully roll out updates without upsetting clients.
• Maintains the API Secure: Developers can issue security fixes and upgrades through API versioning without affecting users who are still using earlier versions.
• Builds Trust: A solid API versioning plan enhances an organization’s reputation and fosters confidence.
• Keeps the API’s Performance: Versioning an API keeps it safe, error-free, and efficient.

URL versioning, query parameter versioning, header versioning, and consumer-based versioning are a few methods for API versioning. 

Particularly when an API has several versions, testing is a crucial component of both development and maintenance. 

11. Explain the concept of API rate limiting.

A control method called API rate restriction sets a limit on how many requests an API may process in a specific amount of time. It’s an essential procedure to guarantee that APIs function properly, offer a satisfying user experience, and shield the system from overburden.  

12. How does the API rate limit work?

API rate restriction operates in the following ways:

• Set Limits: The maximum number of requests that can be made in a given amount of time is controlled by API rate limitation. Usually, this duration is a few seconds or minutes.
• Returns Error Messages: An error message will be returned by the API system if an excessive number of queries are performed. The client has sent too many requests, as indicated by the HTTP 429 Too Many Requests client error response status code.
• Throttles Connections: A client’s connection is throttled if they make more requests than they are permitted to. To minimize errors, processing slows down while the connection stays open.  
• Prevents DDoS Attacks: DDoS attacks, which entail bombarding a server or service with requests in an attempt to overload its capacity, can be avoided via rate limiting. 

There are several approaches to implementing API rate limiting, such as throttling, request queues, and algorithm-based techniques. 

13. What are some common API design best practices?

The following are recommended approaches for designing APIs:

• Employ consistent naming: To make the API easier for developers to understand, use descriptive and consistent naming standards.
• Observe the RESTful principles: To preserve consistency and adhere to REST standards, use HTTP methods appropriately.
• Maintain light payloads: Request and response payloads should only contain the most important information.
• Update your API version: When making modifications to the API, use versioning to keep backward compatibility intact.
• Make use of HTTP response status codes: To indicate whether a request was successful, unsuccessful, or something else entirely, use standard HTTP status codes in your answers.
• Make use of caching: To enhance performance and response times, keep frequently used material in memory.
• Provide pagination and filtering: To control the volume of data returned, provide pagination and filtering capabilities.
• Implement error handling: Record errors and deliver insightful error messages.
• Conduct a comprehensive test: To guarantee the API’s dependability and functionality, test and keep an eye on it.
• Complete the document: To aid developers in comprehending and utilizing the API, provide thorough and understandable documentation.
• Use appropriate authorization and authentication: Put in place appropriate authorization and authentication. 

14. Explain what RESTful web services mean by addressing.

The process of identifying one or more resources on the server is called addressing. The Uniform Resource Identifier, or URI, is used to achieve this operation. The URI’s general format is, 

<protocol>://<application-name>/<type-of-resource>/<id-of-resource>

API Testing and Performance Interview Questions and Answers

15. What is API performance testing, and why is it important?

API performance testing is the practice of assessing an Application Programming Interface’s (API) functionality under various circumstances. It’s crucial since it guarantees that APIs can withstand the demands made of them and satisfy user expectations.  

The following are some advantages of testing API performance:

• Identify bottlenecks: Performance bottlenecks can be found with the aid of API performance testing.
• Measure response times: Response times can be measured with the aid of API performance testing.
• Verify scalability: System scalability can be verified with the aid of API performance testing.
• Assure dependability: API performance testing can assist in guaranteeing the API’s general stability and dependability.
• Improve user experience: Testing the performance of APIs can improve user experience. 

16. How do you test API security vulnerabilities?

There are several techniques you may use to check for API security flaws, such as:

• Static and dynamic analysis: It aids in locating vulnerabilities.
• Fuzz testing: It evaluates the system’s ability to withstand unforeseen input.
• Penetration testing: It evaluates the security of the API by simulating actual attacks.
• Automated Security Testing: Automate vulnerability scanning, fuzz testing, and security checks with programs like Burp Suite or OWASP ZAP.
• Security misconfiguration testing: Verifies that the API’s settings, permissions, and access controls are configured correctly.

API and Microservices Interview Questions and Answers

17. How do APIs enable microservices architecture?

Microservices architecture is made possible by APIs (Application Programming Interfaces) through:

• Supplying a layer of secure abstraction.
• Facilitating communication.
• Centralizing points of entrance.
• Providing an overview of the internal system architecture.
• Request routing.
• Making functionality visible.
• Constructing applications as separate parts.

18. Define REST Resources.

In the REST architecture, each piece of content is seen as a resource. In the context of object-oriented programming, the resource is comparable to the object. Text files, HTML pages, photos, or any other type of dynamic data can be used to represent them.

While the REST client uses (accesses and alters) these resources, the REST server grants access to them. A URI is used to globally identify each resource.

19. What are the challenges of API-driven development?

There are several obstacles that API-driven development may encounter, such as:

• Security: As sensitive data is sent between apps, security is a major consideration while developing APIs.
• Inadequate API discovery: Compatibility problems, obsolete or insecure APIs, and inadequate API structure might result from this.
• Putting scalability last: Ignoring scalability can have major repercussions as the number of users and data being processed increases.
• Documentation: Testers may find it challenging to comprehend the API due to inadequate documentation, which may result in less successful testing.  
• API Integration: In addition to involving maintenance, upgrades, security threats, system variety, and complicated technical code, API integration can be costly and time-consuming.
• Tooling Support: Conventional manual methods for designing APIs can be laborious and error-prone.  

20. Why is it easy to scale REST services?

The idea behind REST services is statelessness, which has no data is stored on the server between requests. Because the servers don’t have to talk to each other as often when processing requests, horizontal scaling is made simpler.

Rebuild your career with our wide range of IT training course options. 

API Interview Questions for Experienced

21. Explain an API Gateway.

Based on predetermined policies, such as authentication, authorization, access control, SSL/TLS offloading, routing, and load balancing, API gateways process requests.

API gateways are useful for:

• Developing, releasing, preserving, keeping an eye on, and safeguarding APIs
• Overseeing, keeping an eye on, and managing APIs for various backend services
• Making the process of development and deployment simpler
• Lowering the system’s overall complexity
• Cloud-based and on-premise API gateways are the two primary varieties of API gateways.  

22. List some examples of API gateways.

Here are a few examples of API gateways:

• Amazon API Gateway: Developers may construct, publish, maintain, monitor, and secure APIs with Amazon API Gateway, a fully managed service.
• Azure API Gateway: Perfect for multi-cloud and hybrid settings
• Tyk: An open-source tool for managing application program interfaces (APIs) that links RESTful APIs.

23. Define OAuth and OpenID Connect.

Two protocols that support safe user authentication and authorization are OAuth and OpenID Connect (OIDC):

• OAuth: A protocol that creates access tokens to grant access to resources that are protected. OAuth is frequently used to enable third-party apps or grant access to APIs.
• OpenID Connect: An authentication protocol that expands upon OAuth 2.0 by incorporating single sign-on (SSO) and user authentication features. OIDC enables websites and apps to recognize users by requesting a login from a reliable source.  

24. What is API mocking?

A software development approach called API mimicking allows testers to continue testing in situations where the real API is unavailable by simulating it. Although it doesn’t handle data or interact with external systems, a mock API imitates the behavior of the real API.

Among the advantages of API mocking are:

• Quicker progress.
• Simpler testing.
• Cutting down on dependency.
• Parallel development.
• Managing partial or unavailable APIs.
• Error and edge case simulation.
• Testing integrations with third parties.

A mock API can be made by:

• Describe the methods and endpoints of the API.
• Determine the parameters of the request.
• Describe the answers.
• Create the mock API, test it, 
• Make adjustments. 

You can save time and minimize manual input by using automatic mock construction. This is very helpful if you need to make a lot of mocks.

Among the tools for API mimicking are:

• Postman: A free program for Linux, Mac, and Windows
• WireMock: An open-source Java utility
• Mountebank: A free program for Linux, Mac, and Windows
• Apigee: An online utility that requires a subscription  

25. What is API contract testing?

The software development technique known as API contract testing compares an API to a contract to ensure that it operates as intended. The contract outlines the intended behavior of the API and is typically a specification or documentation.

The following information relates to testing API contracts:

• What it does: Testing API contracts ensures that system-to-system requests and answers match the predetermined parameters.
  • Additionally, it confirms that several applications or services are interacting with one another under the terms of their contracts.  
• When it’s beneficial: In a microservices architecture, API contract testing is particularly useful for verifying API interactions. It offers great isolation and quick feedback.
• How it operates: Testing of API contracts records the exchanges between services and stores them in a contract. The adherence of both parties can then be confirmed using this contract. 

Explore various software testing courses at SLA. 

26. How do you handle errors and exceptions in APIs?

The following are some methods for managing exceptions and failures in APIs:

• Use HTTP status codes: To convey the type of issue, use normal HTTP status codes. For instance, 500 for Server Error and 404 for Not Found.
• Put in place a typical error response framework: Add details, a message, and an error code to the error response.
• Possess both general and specialized exception handling techniques: To guarantee consistent error answers, apply these techniques.
• Securely handle authentication errors: When dealing with login issues, don’t divulge private information.
• Record errors: Record errors for monitoring and debugging.
• Clearly state any validation errors: Provide explicit validation errors to assist users in resolving input problems.
• Apply exception filters: When a controller method in the ASP.NET Web API throws an unhandled exception, exception filters are run.  
• Try/catch/finally blocks: It can be used to release resources or recover from errors.
• Address common conditions: Avoid exceptions by managing common conditions.
• Create classes that don’t have exceptions: Create classes to avoid exceptions.
• Restore the state: When methods fail to finish because of exceptions, restore the state.

Verifying the URL, looking through the API documentation, and getting in touch with the API provider are further methods for resolving API problems.  

27. What is API documentation and why is it important?

A technical handbook that describes how to use an application programming interface (API) is called API documentation. For developers, it’s an essential resource because it aids in:

• Understand API: To assist developers in understanding how to use the API, documentation offers concise instructions and examples.
• Reduce Integration Time: Rapid API integration is facilitated by well-written documentation.
• Enhance the User Experience: A better user experience results from users understanding how to use the API efficiently due to API documentation.
• Build Trust: Ensure users don’t waste time attempting to use out-of-date features by providing them with up-to-date API documentation.  

28. How do you measure API performance?

The following methods can be used to gauge API performance:

• Response Time: One important metric that gauges how long it takes an API to reply to a request is response time, sometimes referred to as latency. Because it impacts the user experience, it’s a crucial measure of API performance.
• Error Rate: This statistic shows what proportion of queries are unsuccessful. It may indicate possible problems with system stability and user confidence.
• Load Testing: Sending several requests to an API at once in order to assess its performance under high load is known as load testing.
• Continuous Performance Testing: It makes sure that any modifications or enhancements to APIs are examined for potential effects on performance.  
• Compress API responses: You can save bandwidth and speed up data transfers by turning on compression on both the server and client sides.
• Track usage: Usage may be monitored with analytics tools.
• Scalability testing is an additional technique to gauge API performance.

29. What is API observability?

The process of gathering, evaluating, and comprehending data from APIs in practical is known as API observability. It offers a single location to examine all facets of API adoption, performance, and error rates.

The following are some advantages of observable APIs:

• Identify and defend against API dangers: Platforms for API observability can monitor API requests and their origins, which can aid in identifying and thwarting API threats.
• Keep an eye on the client experience: By monitoring user interactions with APIs, API observability can assist in uncovering any problems and pinpointing possible hotspots.
• Improved user experience: To customize API answers and enhance user experience, API observability can be used to evaluate usage trends.
• Enhance performance: Finding areas where APIs can be quicker and more effective is made easier with the aid of API observability.  
• Troubleshoot more quickly: When problems arise, API observability can aid in their diagnosis. 
• Make decisions based on data: Future API development and management projects can benefit from the information that API observability can offer. 

Log analytics and visualization, centralizing log data from several sources, utilizing advanced analytics for more profound insights, and working with third parties are a few ways for API observability. 

30. What is API governance?

A company’s use, development, and deployment of its application programming interfaces (APIs) are governed by a set of rules, guidelines, and practices known as API governance. It guarantees that APIs are dependable, safe, and in line with corporate goals.

From design to retirement, API governance encompasses the full API lifespan and consists of:

• Standardization: Defining rules for versioning, documentation, API design, and error management.
• Security Policies: Establishing guidelines and procedures to safeguard private information and guarantee legal compliance.
• Lifecycle management: Making sure APIs are reliable and practical across the course of their existence.
• Analytics and monitoring: Monitoring the efficacy and performance of APIs with real-time analytics.

Here are the various manual testing interview questions and answers for you.

31. What is API monetization?

The process of making money from application programming interfaces (APIs) is known as API monetization.

• Charging for access: Companies may impose a subscription fee on developers to get them access to their APIs.
• Offering a free basic version: Companies can charge for premium functionality and provide a free basic version of their API.
• Usage-based charging: Companies have the option to charge according to the number of times the API is used.
• Indirect monetization: Indirectly enhancing their primary services through APIs can help businesses draw in and keep customers.  

32. What is GraphQL?

GraphQL is a server-side runtime and query language for application programming interfaces (APIs) that provides the requested data to API clients. 

With a single API call, developers may use GraphQL as an alternative to REST to make queries to retrieve data from several data sources. 

33. What is gRPC?

An open-source, high-performance framework for remote procedure calls (RPCs) called gRPC makes it possible for services to communicate with one another:

Features of gRPC:

Protocol Buffers are the interface description language used by gRPC, which offers functionality such as:

• Verification
• Both-way streaming
• Control of flow
• Bindings that block or do not block
• Timeouts and cancellations
• Tracing, load balancing, pluggable auth, and health checking

Advantages that gRPC provides:

• Implementations independent of language
• decreased use of the network
• Eleven languages with idiomatic client libraries
• A basic foundation for defining services  

Use Cases:

• Many organizations use gRPC for a variety of purposes, such as:
• Using a microservices-style architecture to connect services
• Linking customers on mobile devices to backend services

History: Google was the original developer of gRPC, which was first made available in August 2016. The Cloud Native Computing Foundation is in charge of overseeing it.  

34. What is a serverless API?

Web services that operate on demand, in response to events or requests, as opposed to continually on a server, are known as serverless APIs. AWS, Google Cloud, or Azure are examples of third-party providers that host and administer them.

The following are some advantages of serverless APIs:

• Reduced Cost: Instead of paying for unused or idle capacity, you only pay for the time and resources your service uses.
• Increased agility: Without having to worry about server provisioning, scaling, or maintenance, developers can concentrate entirely on writing code.
• Faster time-to-market: API-based apps can reach the market more quickly through serverless APIs. 

Applications that are asynchronous, stateless, and do not store client data between sessions are best suited for serverless architecture. Chatbots, task schedulers, and Internet of Things applications are a few instances of serverless applications. 

35. What is API-first development?

A software development methodology known as “API-first development” places an emphasis on designing and developing APIs (Application Programming Interfaces) before other program components. APIs provide communication between various software systems.

The following are some advantages of developing with APIs first:

• Enhanced developer experience: It’s simple for developers to integrate particular services into their apps.
• Improved user experiences: without completely redesigning the program, APIs can be added or modified.
• Increased scalability: code and APIs can be applied to a variety of applications.
• Advantage over competitors: An organization can differentiate itself by implementing API-first development.
• Innovation: APIs may serve as the fundamental elements that direct the creation of additional application code.

A variety of stakeholders, including developers, business analysts, product owners, and consumers, contribute to the collaborative process of API-first development.  

36. What is API-led connectivity?

Using reusable APIs, API-led connectivity is a technique for tying data and apps together inside a company. It’s a cutting-edge strategy that can help companies increase data connectivity and optimize processes.

The following are some advantages of connectivity driven by APIs:

• Reduces complexity and cost: To meet various business goals, organizations can construct APIs once and use them again in diverse situations.  
• Increases the accuracy of data: Out-of-stock inventories and other data inconsistencies can be eliminated with the aid of API-led connectivity. 
• Expedites the retrieval of vital business data: Access to business-critical data, like pricing information, storing product data, and customer history, can be accelerated with the use of API led connection.  
• Builds an architecture that is separated: A decoupled architecture that abstracts away complexity and enables a more flexible reaction to change can be produced by API-led connection.  

37. What are the three layers of API-led connection?

Three layers are used in API-led connection, and each tier’s API has a specific, constrained role:

• System APIs: These APIs expose legacy systems and link users to the organization’s fundamental systems.
• Process APIs: These APIs work with and modify data both inside and between systems.
• Experience APIs: The goal of these APIs is to provide a positive end-user experience.  

38. What is API security testing?

The practice of assessing an application programming interface’s (API) security in order to find and address any vulnerabilities is known as API security testing. Making sure that APIs are safe and capable of withstanding harmful queries and attacks is the aim of API security testing.

The following techniques are employed in API security testing:

• Static and dynamic analysis: It examines the code of the API.
• Fuzz testing evaluates the API’s capacity to manage unforeseen inputs.
• Penetration testing: It evaluates the security of the API by simulating actual attacks.
• Software Analysis of Composition (SCA): checks a database of known vulnerabilities against the dependency graph of a program.  

Check out our students’ reviews and join SLA for a promising IT career. 

39. What is API lifecycle management?

The process of managing an API from inception to retirement is known as API lifecycle management. Designing, developing, deploying, maintaining, and retiring an API requires coordinating a number of teams and procedures.

The following are the phases of an API lifecycle:

• Planning: Planning and design are the first steps in the API development process.
• Development: A wide variety of programming languages can be used to write APIs.
• Testing: The API has undergone testing.
• Deployment: The API has been made available.
• Maintenance: The API is kept up to date.
• Retirement: The API has been shut down.

Businesses and developers should be aware of API lifecycle management. Building dependable, scalable, and secure APIs can be facilitated by adhering to the stages of the API lifecycle.  

40. What is API retirement?

The process of removing an API from usage, typically due to its obsolescence or lack of need, is known as API retirement.

• API is no longer required: If an API was created temporarily and the task is completed, it may be retired.
• API is no longer in use: If an API becomes obsolete as a result of retirement or application migration, it may be decommissioned.
• The API’s useful life is coming to an end: If an API has reached the end of its useful life, it may be retired.
• API is not used very much: If developers or end users aren’t using an API frequently, it may be decommissioned.

An API retirement procedure needs to be well thought out and organized. Consolidating APIs or replacing them with new ones are the most popular methods of retiring an API. 

Examples of API retirement include the following:

• Microsoft Azure API Management: After March 15, 2025, the direct management API for API Management will be deprecated and no longer supported.
• IBM API Connect: When a product is decommissioned in API Manager, all related APIs are taken down and all subscriptions are terminated.
• Sabre Dev Studio: Older Sabre Dev Studio API versions are removed from the system with a minimum of ninety days’ notice.  

Conclusion

We hope these API interview questions and answers will be helpful for freshers and experienced people on various topics. Keep up with the most recent developments, practice using real-world APIs, and be ready to share your expertise. Learn with hands-on exposure with our API training in Chennai.