10 critical REST API interview questions for 2024—answered

As you apply to engineering roles that require some level of background on REST APIs, you’ll likely be asked certain questions that test your familiarity with them.

To help you manage this part of the interview process, we tasked an engineering manager who routinely assesses candidates’ proficiency with REST APIs to identify the most important interview questions on the subject and to provide the answers an interviewer is likely looking for.

You can read on to see what they shared.

What is the concept of idempotency in REST APIs and how is it implemented?

Idempotency ensures that multiple identical requests have the same effect as a single request. In REST APIs, GET, PUT and DELETE methods are idempotent, meaning if you call them multiple times, the result is the same as making a single call. For instance, deleting a resource with DELETE is idempotent because, after the initial deletion, subsequent deletion requests have no object to act upon, so the state remains unchanged.

Related: A guide to understanding REST API integration

What are the main differences between PUT and PATCH requests in REST APIs?

PUT is used to update a resource entirely, while PATCH partially updates the resource. With PUT, you replace the entire resource with a new version, regardless of how many fields you provide. In contrast, PATCH applies a partial update, only modifying the fields specified in the request.

How do REST APIs handle versioning, and why is it important?

Versioning in REST APIs ensures backward compatibility and helps manage changes in the API without disrupting the clients. In other words, it allows clients to continue using an existing API version without being forced to adapt to new changes immediately.

Common strategies include URI versioning (e.g., /v1/resource), versioning through custom request headers, or using media types in Accept headers. 

What is the role of HTTP status codes in REST API design?

HTTP status codes provide a standardized way to indicate the success or failure of an HTTP request. In REST APIs, they inform the client about the result of their request, such as success (2xx codes), client errors (4xx codes), or server errors (5xx codes). Proper use of these codes enhances API usability and debugging.

How is caching utilized in REST APIs?

Caching in REST APIs improves performance by storing copies of frequently accessed data. It reduces the need to repeatedly fetch the same data from the server, thereby decreasing latency and server load. Caching can be implemented at various levels, including the client side, server side, or intermediary proxies, and is often controlled using HTTP headers like Cache-Control.

What is OAuth, and how is it used in the context of REST APIs?

OAuth is an authorization framework that enables third-party services to exchange web resources on behalf of a user. In REST APIs, OAuth is commonly used to authorize and authenticate users without sharing their credentials, using access tokens instead. It's essential for scenarios where an application accesses resources from another service on the user's behalf.

What is the concept of rate limiting in REST APIs and why is it used?

Rate limiting controls the number of requests a client can make to an API within a certain timeframe. It's crucial for maintaining the stability and reliability of an API by preventing abuse. This ensures that the API remains available and responsive for all users.

How do REST APIs utilize headers, and what are some common headers involved?

Headers in REST APIs provide additional context about the request or response, such as content type, authentication, caching policies, and more. Common headers include Content-Type (describes the media type of the resource), Authorization (carries credentials for authenticating the client), and Accept (specifies the media types that the client can process).

Explain the difference between stateful and stateless API design. How does REST fit into this?

A stateful API retains client state or session information across multiple requests. In contrast, a stateless API, like REST, doesn’t store any client state on the server. Each request from the client must contain all the necessary information for the server to fulfill the request. REST's stateless nature simplifies scalability and reliability, as the server doesn't need to maintain or synchronize session states.

What are the key differences between REST and SOAP?

REST and SOAP are two different approaches to web service communication. REST is an architectural style that uses standard HTTP methods and is considered more lightweight and flexible. It can return data in various formats, such as JSON or XML. SOAP, on the other hand,  is a protocol that defines a strict set of rules with a specific XML format, making it more standardized and formal.