Top 25+ API testing interview questions

According to a recent report, APIs (application programming interfaces) constitute nearly 71% of the entire web traffic. On the other hand, attacks targeting the APIs have witnessed a stark increase in recent times. Extensively testing APIs is the best solution to ensure their reliability and security of software testing projects.

The collection of API testing interview questions has been carefully crafted to include the functions of APIs, with a special focus on API testing, which covers core functionality testing, performance testing, security testing, and more. The testing interview questions have been categorized to assess one's domain knowledge on APIs and API testing.

API testing interview questions   - API basics

Irrespective of the role, every set of API testing interview questions starts off with testing your basic understanding of APIs. Here are some probable questions on API basics.

1. Can you briefly explain API and its uses?  

An application programming interface (API) enables faster and secure data transfer between software components and applications or services. APIs facilitate communication between different systems or components within the same application (internal APIs) or across different applications (public or private APIs). APIs allow integration without the need for applications to share all of their code. Once configured, each API request/call is responsible for fetching relevant information from the server, known as the API response, which is either displayed on the user interface of an application or is used for triggering the workflows and operations related to other functionalities.

Uses: APIs are generally used in the backend of applications; modern applications and services depend extensively on them for their functionalities. Their applications span different fields, including but not limited to banking, healthcare, ecommerce, and education.

2. What purpose do the different components of an API request and response serve?  

API request

API endpoint: An API endpoint is a URL that provides access to a specific resource. Each endpoint exposes a particular domain and its resources based on how it's configured.

Method: Each method is responsible for providing exact instruction to the server on the operation to be performed on the resource. Commonly used HTTP methods are GET, POST, PUT, PATCH, and DELETE.

Body: The request body carries the information and data needed for creating, modifying, or deleting an existing resource on the database. The body supports addition of raw data (JSON, XML, etc.), form data, URL encoded, binary data, or graphQL.

Header: A request header carries metadata, which helps provide the context related to the operation to be performed. The header can also carry the authentication or the details related to the type of response expected.

Parameters: The parameters are arguments that can assist with sorting, filtering, searching, and more to format the resource before it is sent back as a response.

API response

Header: An API response header carries various important information related to the content type present in the body, authentication status, and any message from the server.

Body: The response body contains the requested resource in one of the formats, like JSON and XML, based on the request. It can also show the relevant success or error message and other details related to the response.

Response code: The code is an indication of the state of API requests; each code clearly represents the success or failure of the request and the definitive response from the server.

3. How does an API work? Explain the concept in detail and provide an example.

An API request can be an instruction in a command-line interface or operations performed on the user-interface of the application that are transformed in the backend. The components of each request specify the authentication and requirements responsible for fetching the specified resource from the server or database.

After verifying the authenticity and feasibility of the request, in ideal cases, the server provides access to the resource along with a success message as the API response. In unforeseen cases, when there is an error or if the authentication fails, an error message is sent back. The API response is always supplemented with a status code, which is readily discernible.

Example

Inside a banking application, when a customer wants to check their transaction details for the past 30 days, they select the relevant filters in the UI and click on the respective button. The unique authentication method for the customer's session, along with the embedded requirement, is sent to the highly-secure servers as an API request.

Even though the servers are connected to the database housing the information for millions of customers, the unique authentication narrows down to the customer-specific data. The body of the request provides the details of the exact records to be pulled. The particular data from the server, along with other parameters, is sent back and verified once again before displaying the requested data to the customer.

4. What are the different types of APIs? Can you provide some examples?  

The different types of commonly used APIs are public, private, internal, and composite APIs.

Public APIs - Anyone can access public APIs without any restrictions, but there can be a limit on the number of requests within a given period of time.

Private APIs - Private APIs are protected with authentication, and are either part of services or are only available to the users based on subscription. If not part of the subscription model, they are generally billed based on the number of requests.

Internal APIs - Internal APIs are accessible only within the organization and are used for accessing sensitive information and performing crucial operations based on user roles.

Composite APIs - These merge multiple APIs in a single request to perform multiple operations simultaneously.

API testing interview questions - API advanced

The following questions are meant to test the depth of your knowledge of APIs—different types, protocols, authentication, and more.

5. What are the different ways to authenticate an API?

Some of the popular API authentication methods are Basic authentication, OAuth, JWTs (JSON web tokens), and API keys.

Basic authentication - The HTTP basic authentication type involves embedding the username and password into the authorization header field of every API call. Both the username and password are encrypted using Base64. Since credentials are shared during every call, there is no need for handshakes.

API keys - Whenever a new user is created, a unique identifier key is also generated. This key is shared along with every API call to verify the user's identity.

JWTs - Once the user logs into the application, an encrypted JSON web token (JWT) is created and it is shared with every API call. If it matches with the JWT available on the server, access to the resource is granted and relevant operations are carried out successfully.

OAuth - Currently, OAuth 2.0 is considered the industry-standard, and is the most secure API authentication method in use. When a user logs into the application, an authorization grant is generated, which is necessary for obtaining the access tokens. Once the access tokens are available, each subsequent API call makes use of these tokens to gain access to the resource or server.

6. Can you discuss the different API protocols? 

The commonly used API protocols are REST, SOAP, GraphQL, gRPC, JSON-RPC, and XML-RPC

SOAP - Simple object access protocol (SOAP) is the oldest protocol in use that makes use of XML in the request and response body. It follows a predefined structure for messages and is highly secure.

REST - Representational state transfer (REST) follows a structured system that makes use of standard HTTP methods to carry out operations. Unlike SOAP, this architectural style can work with JSON, XML, and more.

GraphQL - This is a query language that provides the user with the exact data instead of the entire resource. GraphQL functions with just one endpoint containing multiple data schemas that provide accurate responses based on types and queries.

gRPC - A remote procedure call (RPC) focuses on carrying out operations on a remote system more efficiently, emulating an internal operation. Google remote procedure call (gRPC) is an open source framework created by Google for faster cross-platform communication between applications and services.

JSON-RPC and XML-RPC are remote procedure calls encoded in JSON and XML, respectively.

7. Can you explain payment APIs?

Payment APIs make it easier for businesses and services to accommodate a wider customer base that transcends international borders. A payment API connects the business with a payment processor to verify the customer's payment details and process the transaction. The details of the successful or unsuccessful transaction are shared with the business to manage the purchase workflow.

The payment API takes care of refunds if there's any difficulty fulfilling the order; recurring payments are also managed easily without any hiccups. The transaction details and real-time order processing is also monitored by the API for hassle-free payment management.

8. What are the different HTTP methods used in a REST API? 

The different HTTP methods used in a REST API are GET, POST, PUT, PATCH, DELETE, HEAD, TRACE, OPTIONS, and CONNECT.

GET - This HTTP method is responsible for fetching the records from the database, but the data retrieved is read-only.

POST - The POST method is used for creating a new record in the database. The authentication ensures data can be added only by authorized personnel.

PUT - Whenever an existing data record needs to be updated in the database, the PUT method is used. If the existing record is unavailable, a new record is created.

PATCH - Unlike the PUT method, PATCH is used for partially updating only specific fields of an existing data record.

DELETE - In case of scenarios when an existing record needs to be removed, the DELETE method is best.

HEAD - The HEAD method returns the headers associated with the resource, which delivers the information without the resource-intensive body.

TRACE - For diagnostics associated with testing or debugging, the TRACE method performs a loop-back test along the path to the resource.

OPTIONS - This method is used to gather information regarding the resource or server with the list of permitted HTTP methods and their actions.

CONNECT - The CONNECT method can help establish a tunnel to enable a secure connection with a remote server.

9. What is the difference between URI and URL?  

10. What are the differences between REST, SOAP, and GraphQL API?

API testing interview questions   - API testing basics

Here is a curated list of API testing interview questions designed to challenge your mastery over the theory related to software testing, specifically API testing.

11. What do you know about white box testing and black box testing?

Both of these software testing types differ depending on how much information is provided to the testing team on the backend of the functionality or application.

White box testing refers to the type of software testing in which the tester knows exactly how the functionality works, along with all of its dependencies. In terms of API testing, the tester has an understanding of the API and an overview of the server's behavior. This transparency directly contributes to the quality of test cases drafted, facilitating optimal test coverage.

In black box testing, though, the tester has no knowledge about the inner workings of the functionality and is limited by the information on expected behavior. In API testing, the scope of testing is restricted to checking if the API can retrieve data or perform the specific operation under the different acceptable scenarios outlined in the API document.

There is also gray box testing, which falls in between the extremities of white box testing and black box testing.

12. Can you explain the importance of API documentation?

Testing application programming interfaces requires comprehensive API documentation that clearly specifies the list of available API endpoints while extensively discussing the operations, parameters, and limitations of APIs. In most cases, once the API is created and the initial testing is completed, the different teams collaborate to focus on writing the API document, which is constantly updated to reflect the changes.

During the different stages of testing, the API documentation is helpful to check if the anomalies discovered are actually deviations from the expected behavior. Without the documentation, especially in an application that largely relies on APIs, it is difficult for the testers to conduct efficient API testing.

13. When should you use an API automation testing framework? 

An API automation testing framework helps ensure that the testers do not waste time configuring the setup for creating the test scripts. The framework has prebuilt options for performing automated API testing by facilitating the connections between applications and services or backend servers. It can also readily support testing different API protocols and provides further customizations for the individual components.

Creating a framework is a tedious task that involves coordinating with multiple teams and configuring the setup for integrating it with the existing CI/CD and project management tools. Some of the common API testing tools with the framework are Postman, Apigee, JMeter, REST-assured, and Zoho QEngine.

14. What should you remember while drafting API test cases?

Ideally, API test cases should incorporate the following:

Optimal test coverage - The API test cases, just like the other tests in software testing, should focus on maximum test coverage to ensure there are fewer instances of failure in the production phase. Thorough understanding of the updated API documentation is essential to create test scripts that cover the maximum number of scenarios.

API test automation - Each API test case has to be created with the ability to support automated testing for scalability. Automated API testing brings about a lot of improvements compared to manual testing, especially in a larger project involving hundreds of APIs.

Test management - To manage automated test cases, the tester should focus on test data management and also on how to sort and filter API test cases based on their functionalities.

15. How easy or difficult is it to draft API tests?

It is relatively easy to perform API testing when compared to testing native web or mobile applications. The absence of a graphical user interface could prove beneficial in creating test scripts as there are no element locators, UI testing or other complex scenarios. API testing just requires a general understanding of the APIs and an in-depth understanding of the operations as outlined in the API documentation.

16. Should you implement a test environment for API testing?

Implementing a test environment for API testing is a good idea for the following reasons:

• A test environment isolates the possibility of breaking or affecting the actual database or server powering the application.
• A test environment reduces the time taken to perform API testing, as the entire database is not used, thereby greatly reducing the load on the API system.

17. What are the different steps involved in performing API testing? 

The API testing process follows a structured method:

1. API requirements - Before the testing can begin, the testing team needs to thoroughly understand the purpose and functions of the API to be tested. The documentation can provide a clear understanding of the testing requirements.
2. Test strategy - Discuss how to formulate an optimal strategy for test creation and execution with the relevant teams.
3. Test environment - The next step in API testing is to set up a test environment for assessing the APIs; this helps with efficient and secure testing.
4. Parameters - Every API created for an application comes with multiple parameters that provide information during API calls that augment the existing actions. Understanding these parameters can help with efficiently testing them.
5. API requests - The individual API requests, along with the relevant parameters, need to be tested in succession, one after the other to check if the APIs are working properly.
6. API responses - Each API response has to be checked to see if the retrieved data conforms with the API’s functionality. Furthermore, the QA team assesses if each API request returns relevant success and error messages with status codes.
7. Scenarios - It is important that APIs are tested by emulating real-world scenarios—multiple API calls executed in a particular order to fetch data and pass instructions between the client system and the servers—related to the application’s functionalities.
8. Analyze results - The final step is to document the results of the API tests for collaborating with the different teams and to work on fixes.

18. What is error handling in APIs and how do you address it?

Error handling is a crucial part of software development, specifically testing. When it comes to APIs, the errors could range from invalid input data to difficulty in accessing the servers altogether.

Extensive testing - It is a good practice to write test cases anticipating the different real-world scenarios; this can help address all possible errors that might arise.

Customized error message - If programmed, each API error corresponding to a particular status code throws a specific and descriptive error message.

Regular maintenance - APIs, just like software applications, are subject to changes when the database or server is updated. Running the existing test cases at regular intervals can help verify that the existing functionalities work as intended.

19. What are the advantages and disadvantages of API test automation?

Existing test cases have a better chance of uncovering the errors in APIs when automated since test automation does not require any human intervention, thereby reducing the time and resources needed. It is particularly useful in scenarios that require continuous testing, as they pass part of the response from previous API calls inside the body of subsequent API calls.

Automated test cases are executed per the instructions set by the team inside the testing tools; this could prove fatal when there is an error on the server side. Automation could ignore successive error messages and continuously trigger API calls as planned, especially if parallel testing is available.

20. Please list at least five HTTP status codes for REST API.

While there are many standard HTTP status codes for REST API, the following six are the most commonly used:

200 - Successful HTTP request

400 - Bad request

404 - Not found

429 - Too many requests

500 - Internal server error

504 - Gateway timeout

API testing interview questions   - API testing advanced

21. What are some tips to improve API testing?

Planning - Unlike functionalities, multiple APIs can be executed in succession; therefore, they need to be segregated based on their operations and dependencies. The order of test script executions equates to the usage of resources for testing.

API endpoints - Each API endpoint is mapped onto a collection of resources. Keeping an up-to-date account of the endpoints can help address scenarios that could arise from real-world use.

Testing tool - Choosing an all-in-one platform, like Postman, for testing APIs can help ensure all aspects of the individual APIs are assessed thoroughly.

22. What are the different challenges faced during API testing?

Some of the common challenges faced during API testing are:

Different architecture - Based on the requirement, each API belongs to a different architecture. In such cases, it is not ideal to proceed with the generic set of tests which were drafted for an API belonging to different architecture. In some cases, testing has to be carried out with multiple testing tools or a unified testing tool.

Insufficient API documentation - Each API is customized and developed to serve a specific purpose. In the absence of API documentation that clearly outlines its workflow along with a list of sample inputs and outputs, it is difficult to perform efficient testing.

23. What parameters are used in API performance testing?

Response time - Response time refers to the time taken for the API to fetch the relevant data from the server based on the request. It is an indication of how fast or slow the API system takes to perform a particular operation.

Status code - Each status code corresponds to a particular response from the database server. Analyzing the status code reveals the exact cause of error in case of failure in fetching the records.

Accuracy - Each API request based on its parameters is mapped to a specific operation; if it works as intended, the operation is completed successfully and the requester receives the configured success message. Accuracy denotes how close or different the actual response is from the expected response.

24. Please discuss web API testing briefly.

Modern web services can perform advanced functionalities swiftly and efficiently thanks to the thousands of API calls triggered every second. Progressive web applications depend on APIs for fetching various assets and relevant information essential for their workflows. Web API testing checks if a connection between the API and web service is established for secure and reliable data transfer.

In a web application, UI testing basically focuses on whether the UI elements are displayed correctly on different devices. However, web API testing focuses on the individual API calls made from inside the application to perform operations in the backend. It also checks if the API calls facilitating the web services are secure.

25. What are some of the common vulnerabilities addressed during API security testing?

Extensive API security testing involves emulating the following attacks and other real-world scenarios that could provide potential backdoor access to the application/servers for nefarious purposes.

Injection attacks - This type of attack can be destructive as it deals with manipulating the API requests to inject malicious code into the server. One of the most common examples is SQL injection.

Denial of Service (DoS) attacks - The attacks are aimed at overwhelming the security system by disarming it with a barrage of consecutive API requests.

Inefficient user authentication - The authentication used for APIs is expected to be secure since a vulnerability associated with a single endpoint can be used for gaining unauthorized access to the server.

26. What are the different types of tests carried out as part of API testing?

API testing follows the general set of tests and the advanced testing techniques that are part of functional and non-functional testing. Functional testing involves a slew of testing types like unit testing, smoke testing, sanity testing, integration testing, regression testing, and much more. The notable API testing techniques are:

Unit testing - Unit testing is a set of basic tests to check if the API created works as intended. The API call accepts the relevant parameters to provide the required data through the response.

Regression testing - The creation of the new API and the corresponding changes to the database schemas can affect the existing APIs or functionalities of the application. Tests are drafted specifically to address that.

Integration testing - API tests are particularly useful to check if an integration with a third-party service or the integration with the database or server is working efficiently.

Non-functional testing also plays an important role in performing API testing. A few important testing types are:

Security testing - The APIs associated with the application enable easy access to the sensitive records stored on the server, which makes them a target for hackers. Security testing ensures data transfer is made possible without compromising the security of the system.

Performance testing - One of the main reasons why APIs are preferred is for the reliability and speed at which data is retrieved from the servers. API performance testing checks different parameters like response time, accuracy, error handling, and more. There are specific tests to perform API load testing to check if the system can handle a barrage of API requests.

Usability testing - The APIs can work without any errors during the different tests, but the ease of using them and other aspects that could affect the user experience can’t be assessed using traditional testing methods. Hence, usability testing is carried out to check the application from end user’s perspective.

There are certain testing types that do not fall under the purview of either function or non-functional testing, even though they are included in API testing. One such testing type is:

System testing - System testing specifically checks if the collection of APIs that enable different functionalities within the application integrate and work together as one entity.

Conclusion

Cracking an API testing interview can be challenging for various reasons since API testing is needed irrespective of the platform or the type of application. The best approach would be to not just focus on API testing questions, but to familiarize yourself with API testing interview questions that cover common concepts in software development as a whole. Starting from unit testing and building, to maintaining and testing an API framework, having an understanding of everything could prove beneficial towards your performance in an interview.