API design interview questions

According to Postman’s 2023 State of the API report, over 75% of respondents agree that developers at API-first companies are more productive, create better software, and integrate faster with partners. With this in mind, it’s no surprise that so many people want to be a part of an API-first organization. Whether you’re applying to be a developer, QA engineer, data scientist, or technical product manager, these answers to some of the most common API design interview questions will help you navigate the interview process with confidence.

API design interview questions and answers: beginner

In this section, we’ll go over some of the most common beginner-level questions and answers about API design. These are questions you might be asked if you’re applying for a role as a product manager, technical writer, UX/UI designer, or sales and marketing professional at an API-first organization.

What is API design?

API design is the process of making intentional decisions about how an API will allow different software components to interact and exchange data with one another. These decisions, which are captured in a specification format such as OpenAPI or AsyncAPI, help ensure that the API is user-friendly and able to meet both present and future needs.

What is API-first design?

API-first design involves designing an API and its functionality at the beginning of the application development process. When following an API-first design approach, stakeholders create APIs that will serve as the application’s foundation and the contract between various software components, allowing for seamless integration and collaboration.

Why are APIs important in software development?

APIs enable different software components, services, or applications to interact and share data. APIs that are well-designed promote interoperability, efficiency, and reuse while also putting the user experience first. This increases adoption and allows developers to build on existing solutions to quickly and easily create modern and complex applications.

What is a REST API?

REST, which stands for Representational State Transfer, is a set of principles for creating simple, scalable, and flexible systems that can interact and share data over a network. REST APIs are stateless, resource-based, and leverage a standardized set of HTTP methods for client and server communication. REST is the most popular API architectural style and the foundation of the web.

What are the different components of an HTTP request?

The most basic REST APIs use standard HTTP methods like POST, GET, PUT, and DELETE to perform Create, Read, Update, and Delete (CRUD) operations on resources that are represented by URLs. An HTTP request includes four major pieces of information:

1. Method: A standard verb that describes the action being applied to the resource, such as POST, GET, PUT, or DELETE.
2. Uniform Resource Identifier (URI): Identifies the resource on the server. A URI, which is also known as an API endpoint, can be either a relative or absolute path, and it may contain data like path or query parameters. For example, /products might identify a list of products, /products?type=book might identify books within the product list, and /products/1234 might identify a specific book.
3. Request headers: Contain metadata about the request as key-value pairs. For instance, these HTTP headers might include the type of client or browser, the client-supported format, the message body format, and cache settings.
4. Request body: The payload, which is usually JSON or XML data, that is sent to the server. For example, if you send a POST request to /products, the request body will contain the data for the product you want to create.

What are the different components of an HTTP response?

An HTTP response includes three major components:

1. HTTP status code: Indicates the outcome of the request, such as 200 OK for success.
2. Response headers: Contain metadata about the response, including information like the content type, server details, and caching directives.
3. Response body: The payload that the server sends in response to the request. For example, this could be HTML from a web page, or it might be JSON or XML data from an API.

What is an HTTP status code?

An HTTP status code is a three-digit numeric code that a server returns as part of an HTTP response. The code provides information about the result of the request. For example, a successful request usually returns a 200 OK status code, while an unsuccessful request might return a 404 Not Found status code. HTTP status codes are organized into classes: codes in the 200s are successful, 300s indicate redirection, 400s signify a consumer or client error, and 500s point to a provider or server error.

What are the most common HTTP status codes you see when working with REST APIs?

These are some of the most common HTTP status codes:

• 200 OK
• 201 Created
• 204 No Content
• 400 Bad Request
• 401 Unauthorized
• 403 Forbidden
• 404 Not Found
• 500 Internal Server Error
• 503 Service Unavailable

What is a payload?

In the context of APIs, the payload is what goes in the body of the request or response. It contains the data that is sent as part of an API request or response using a POST or GET method. The payload contains the actual information being sent, such as JSON or XML data.

Why is versioning important in API design?

Versioning is crucial to API design because it helps maintain compatibility, offer stability, and reduce disruptions. Versioning also enables incremental updates, supports a wide range of clients, and fosters user and developer confidence. By isolating changes, versioning ensures that existing clients won’t break when improvements or fixes are introduced, which allows for smooth and controlled evolution of the API while encouraging adoption and clear communication about updates and deprecation plans.

API design interview questions and answers: intermediate

This section includes some common API design questions and answers at the intermediate level. These are some of the questions you might be asked in an interview if you’re applying for a role as a full-stack developer, QA engineer, data scientist, or technical product manager.

What are the key principles of good API design?

Good API design prioritizes the needs of API consumers while also being clear and consistent in its naming and behavior, offering helpful error feedback, and using standard and interoperable data formats. A well-designed API should have logical naming conventions, predictable behavior, and be easy to understand and use. It should also evolve gradually and thoughtfully, maintaining backward compatibility to support existing clients while allowing for incremental improvements to meet changing requirements.

How do you ensure consistent API design?

From the beginning, it’s important to establish clear design guidelines and best practices. For instance, naming conventions, endpoint structures, HTTP methods, and response formats should all be standardized. Additionally, conducting regular design reviews, using linting to validate that design conventions are followed, providing clear API documentation, and fostering communication among the development team contribute to maintaining consistency throughout the API’s lifecycle.

How do you handle errors and exceptions in API responses?

When handling errors and exceptions in API responses, it is important to use standard error messages that have meaningful status codes and human-readable descriptions of the problem to help users fix the issue. For client errors (4xx), provide comprehensive error feedback all at once. For server errors (5xx), avoid revealing sensitive system details, like OS versions, databases, or stacktraces, while still offering clear information to help users or API developers troubleshoot.

What is the purpose of pagination in API responses, and how would you design it?

Using pagination in API responses helps clients retrieve and display large amounts of data in smaller, more manageable chunks. Use cursor-based pagination for optimal performance, where an API consumer navigates the data by making a request, then uses the opaque cursor from the response to make their next request with a “next” parameter. They can also leverage a “previous” cursor to retrieve prior data.

If it’s necessary for users to directly access a specific page within your dataset, you can implement index-based pagination. This method separates the data into discrete pages, which clients can request by passing in the number of the “page” as a parameter. Although it can negatively affect performance, index-based pagination can be useful when users require exact control over page navigation within the dataset.

What’s the difference between REST and RESTful?

The word “RESTful” describes APIs or services that adhere to REST principles. To be RESTful, a service or API must follow the rules and best practices defined by REST, such as properly using HTTP methods and representing resources as URLs. Being RESTful also entails hiding server implementation details from clients, which promotes flexibility and scalability. In a RESTful design, efficient caching techniques can also minimize unnecessary data transfers and boost performance.

What are the advantages of RESTful APIs?

RESTful APIs are known for their simplicity and scalability. They map operations to resources using standard HTTP methods, promoting clarity and ease of use. RESTful APIs also encourage loose coupling between clients and servers, making it easier to evolve systems over time, and they benefit from a well-established ecosystem of tools and libraries. In addition, a REST API with a consistent interface and smart design is inherently easy to find and use, even without extensive documentation.

What are the disadvantages of RESTful APIs?

RESTful APIs can encounter issues with data over-fetching or under-fetching. These problems can often be reduced by fine-tuning the API structure for more precise data control, but they can also be an indication of an unsuitable design. Sometimes this issue can be solved by using a different REST API format, such as JSON:API, which allows the retrieval of specific subsets of available fields. If exact control over data fetching is crucial, GraphQL might be a better fit.

Another disadvantage of REST is that its request-response model isn’t built for applications that require live data. Because the client makes a request and then waits for the server to respond, applications can’t fetch real-time data in an efficient way. RESTful APIs must also encode and decode binary files like photos and videos before they can be sent. As a result of this overhead, timeouts may occur when transfers become slower and demand more bandwidth.

How do you handle a long-running operation with a REST API?

Long-running operations are typically handled asynchronously with REST APIs. First, the client initiates the operation and receives an identifier to track its progress. The client can then check the status of the operation until it is finished, as the API server processes it in the background and sends status updates or the final result via the identifier. This approach ensures that long operations do not block the client and provides a scalable and responsive API.

How do you represent a “do something” operation in a REST API?

“Do something” operations are those that need to be performed on a system or application but don’t correspond to a typical CRUD operation. To perform a “do something” operation via a REST API, you can define a resource with a noun that matches the action or the results of the action. You can then use this resource like any other resource—for example, to perform an operation that results in a state change or the creation of a resource, simply execute an action with the POST method and include the action’s input data in the request body. The response should include the action’s output data and the appropriate HTTP status code.

What tools do you consider essential for the API design process?

First and foremost, it’s essential to have a robust API design, documentation, mocking, and testing tool such as Postman. Version control systems, such as Git, are also critical in managing OpenAPI definitions, as they allow for efficient versioning and synchronization to track changes over time. Additionally, linting tools help maintain code quality and consistency throughout your API development process. Together, these tools collectively empower API developers to create well-structured, comprehensively documented, and thoroughly tested APIs, which enhances their usability and ease of integration.

API design interview questions and answers: advanced

In this section, we’ll go over some advanced API design questions. If you’re applying for a role as a backend developer, API designer or architect, DevOps engineer, or solutions architect, you might be asked some of the more in-depth questions in this section.

What is the process of designing an API from scratch?

Define the API’s goals, scope, and purpose first, taking into account the requirements of both developers and end users. This process usually occurs during the initial “Define” phase of the API lifecycle. You should also consider the subject matter, the providing system, and the limitations and preferences of API consumers. These constraints might include factors like supported HTTP methods, industry-specific data formats, or system availability windows.

Next, create a programming interface that is clear, consistent, versatile, evolvable, and user-friendly. To do this, decide which features and capabilities you want your API to have—and then derive actions and resources from those features. Next, define resource paths, choose the appropriate methods, and establish meaningful status codes. As you go, pay attention to the fine-grained modeling of input and output data. Finally, continuously gather feedback to refine and improve the API’s design and functionality over time.

What are some best practices for RESTful API design?

There are many best practices for designing effective RESTful APIs. Start by designing resource paths with meaningful names and clear structures that reflect the relationships between resources. Next, standardize the API’s actions and capabilities by representing them as CRUD operations applied to resources. It’s also important to return meaningful status codes to indicate operation success or failure, maintain overall consistency and predictability, and adhere to established standards in URLs, operation behavior, data organization, naming conventions, and data typing.

To increase performance, make the API cacheable and stateless. Additionally, make sure the right versioning mechanisms are in place to handle changes without causing disruptions. These guidelines bring clarity, consistency, and scalability to API design, which in turn promotes user satisfaction and developer adoption.

How would you handle the versioning and deprecation of an API?

There are several steps you can take to gracefully manage API versioning and deprecation. For instance, use version identifiers in URLs or headers to let clients choose between versions, and introduce new, non-backward-compatible versions only when absolutely necessary. Give each version a long lifespan to identify weaknesses in its design, making only backward-compatible changes, even if it delays some updates and fixes. You can then leverage new versions to introduce major features alongside necessary improvements. If users are eager to use the new features, they’re more likely to accept breaking changes. It’s also important to make sure that clients are aware of changes, which involves providing thorough documentation and precise deprecation timelines. Deprecated APIs can be safely retired after the migration period ends.

How do you handle backward compatibility in API evolution?

Preserving the features and data structures that current clients depend on is essential for maintaining backward compatibility when developing an API. Although it is usually possible to add new features or output data while maintaining compatibility with previous versions, breaking changes may result from things like altering data types or formats, adding or removing values in enumerations, or making an existing query parameter or request body property required.

Avoid breaking client code by not removing or changing any currently supported endpoints or fields without first clearly marking them as deprecated and providing an explanation of how to migrate away from them. Selecting extensible data types and formats early on—such as objects rather than strings or arrays—can also be helpful. With a versioned approach, clients can choose which API version they want to use, so older clients continue to work while newer clients can be safely upgraded at their own pace.

Is it always a problem to introduce a breaking change?

A breaking change may not be a problem if its benefits significantly outweigh the potential disruption to current clients. For instance, releasing a new version with improved performance, security, or features may be essential for the API’s long-term viability. The transition to a new version may be relatively smooth if the API consumer base is small or easily adaptable. Additionally, if the API provider has a well-established deprecation strategy and clear communication channels with consumers, the impact of breaking changes can be managed effectively.

What are caching best practices?

Caching can improve API performance by minimizing redundant data requests and leveraging conditional requests. Using cache keys that specifically identify resources, using the right cache-control headers, and choosing cache expiration strategies that take data volatility into account are some of the most important caching best practices. Effective caching can reduce server load, minimize latency, and improve the overall responsiveness of APIs.

What is content negotiation in HTTP, and how does it relate to API design?

When sending and receiving data over HTTP, the client and server must first agree on a common format and language through a process known as “content negotiation.” Content negotiation enables clients to specify their preferred content type (for example, JSON, XML, CSV, or PDF) and language (for example, English or French), and the server responds accordingly. Content negotiation is crucial because it helps account for the various client preferences and requirements while improving the API’s usability and versatility.

How do you ensure security in API design, especially when handling sensitive data?

To ensure security at the API design level, take a proactive approach. Begin by carefully evaluating the necessity of certain features and data. If a feature or piece of data isn’t absolutely necessary, leaving it out can help prevent security issues. Additionally, use different APIs or operations for sensitive and non-sensitive data and operations. Avoid including sensitive information in URLs, and substitute raw sensitive data with processed, less sensitive alternatives.

You should also consider API authentication and authorization during the API design process. For instance, create and apply OAuth scopes to restrict access, ensuring users only access operations they are authorized for. Additionally, always include access controls in descriptions to help guide implementation, such as indicating which data is accessible to identified consumers. This approach improves API security by reducing unnecessary exposure and effectively enforcing access controls.

How do you ensure you’re creating the “right” API?

Creating the “right” API requires a comprehensive understanding of the problem the API is meant to solve. Begin with a clear definition of the API’s goals, scope, and purpose, taking into account both developer and end-user perspectives. Involve stakeholders on a regular basis, collect feedback, and adapt the API design to changing requirements. Usability testing and user research can help ensure that the design works as intended.

In addition to meeting the initial requirements, the API should be versatile enough to be reused in other contexts. Creating an API that meets the criteria is “doing it right,” but the “right” API is one that does so and is flexible enough to be reused. Make sure the API design continues to be the “right” solution over time by revisiting it on a regular basis and making adjustments based on actual usage and evolving demands.

Conclusion

API design plays an essential role in today’s API-first world. In this article, we’ve gone through numerous API design topics, from the fundamental building blocks to advanced strategies and concepts. The range of questions we’ve answered show how API design is both an art and a science. Whether you’re a developer, a product manager, or a user experience designer, API design knowledge is critical to your success in the software industry today.

Technical review by Arnaud Lauret.