Top 35 API Testing Interview Questions & Expert Answers

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Boost your chances of interview success with this comprehensive guide featuring the top 35 API Testing interview questions and answers. This resource covers a wide range of topics, including key tools, techniques, and best practices to help you stay ahead in your preparation. Let me know if you’d like help formatting the actual Q&A list or turning this into a downloadable document or article!

1) What is an API?

API (Application Programming Interface) helps in communication and data exchange between two software systems. API act as an interface between two applications and allows the two software systems communicate with one another. API is a collection of functions which can be executed by another software program.

API works as: it takes a request from the source, takes that request to the database, fetches the request data from the database, and returns a response to the source. API takes the requests from the user and gives the response without exposing the internal details. API acts as an Abstraction.

Example: Amazon API, Google Map API

2) What is API testing?

API testing is a type of software testing that involves testing APIs directly. API is a part of integration testing to check whether the API meets expectations in terms of functionality, reliability, performance, and security of applications. Multiple API system can performed API testing. In API testing, our primary focus is on Business Logic Layer of the software architecture.

3) What are the types of API testing?

API testing involves the following types of testing:

Unit Testing
Functional Testing
Load Testing
Runtime/Error Detection
Security Testing
UI Testing
Interoperability and WS compliance Testing
Penetration Testing
Fuzz Testing

4) What are the protocols used in API Testing?

Protocols used in API testing are:

HTTP
REST
SOAP
JMS
UDDI

5) What are the tools used for API Testing?

The tools used for API testing are:

Parasoft SOAtest
PostMan
AlertSite API monitoring

6) What is an API test environment?

For API the test environment is a quite complex method where the configuration of server and database is done as per the requirement of the software application. API testing does not involve graphical user interface (GUI).

API is checked for its proper functioning after installation.

7) What is an API framework?

API framework is described by the config. A file that consists of the list of all APIs that are required to be activated and are activated for any particular program run. This is essential as every test run does not requires all APIs.

8) What are the limits of API usage?

Many APIs have certain limits set up by the provider. Hence, try to estimate our usage and understand how that will impact the overall cost of the offering.

9) What are the advantages of API testing?

Advantages of API testing are:

Test for core functionality: API testing provides access to the application without the user interface. The core functionality of the application will be tested before the GUI tests. This will help to detect the minor issue which can become bigger during the GUI testing.
Time effective: API testing is less time consuming than GUI testing. Particularly, API test requires less code so it can provide better and faster test coverage compare to GUI test automation. This will reduce the cost for the testing project.
Language Independent: In API testing, data is exchanged using XML or JSON. These transfer modes are completely language-independent, which allows users to select any code language when adopting the automation test service for the project.
Easy Integration with GUI: API tests provide highly integrable tests, which are useful to perform functional GUI tests after GUI tests. Simple integration would allow new user accounts to be created within the application before the GUI starts.

10) What are the principles of an API test design?

Here are the seven principles of API test design.

Exhaustive Testing: Exhaustive testing is not possible. Instead we need optimal amount of testing which is based on the risk assessment of the application.
Defect Clustering: Defect Clustering states that a small number of modules contain the most of the defect detected. Approximately 80% of the defect found in 20% of the modules. By experience we can identify such risky modules. But this approach has its own problems. If the same tests are repeated over and over again, eventually the same test case will no longer find new bugs.
Pesticide Paradox: Testers cannot depend on existing technique. They must have to look continually to improve the existing method to make testing more effective. But even all these hard work in testing we can never claim our product is bug free. To overcome this, test cases need to be regularly reviewed and revised add new and different test cases to help find more defects.
Testing shows presence of defects: Testing principle states that- testing talks about the presence of defects not about the absence of defect. Software testing reduces the probability of undiscovered defects remaining in the software but even if no defects found, it is not a proof of correctness.

But if we work hard, taking all precautions and make our software products 99% bug free. The software does not meet the needs and requirements of the client.

Absence of error -fallacy: This can be possible the software which is 99% bug free is still unusable. The case can be if the system is tested for the wrong requirement. Software testing is not finding the defects but also to check that software addresses the business needs. The absence of error is fallacy i.e. finding and fixing defects does not help if the system build is unusable and doesn’t fulfill the user’s needs and requirements.
Early Testing: Testing should start as soon as possible in the software development lifecycle. So that defects in the requirement or design phase are captured in the early stages. It is cheaper to fix defects in the early stages of testing. We should start finding the bug at the moment the requirements are defined.
Testing is context dependent: Testing is context dependent that we test an e-commerce site will be different from the way we test a commercial. All the developed software is not identical. We will use different methodologies; techniques and types of testing depend on the application type.

11) What is an API framework?

A framework or software framework is a platform for developing software applications. An API framework is a foundation on which software developers can build applications for a specific platform.

Example: A framework can include predefined classes and functions that can be used to process input, manage hardware devices and interact with system software.

The framework is similar to an Application Programming Interface, technically framework includes an API. The framework serves foundation for programming, while the API provides access to the elements supported by the framework. The framework also includes code libraries, a compiler, and other programs used in the software development process. API framework is defined by the configuration file, which consists of a list of all APIs that are required to be activated and activated for a particular program run.

12) What are the common tests that are performed on the API?

Here are the common tests that are performed on the API:

The response of the API should be verified based on the request. We will verify that the return value is based on the request.
When the API is updating any data structure, we should verify that the system is authenticating the outcome.
We will verify whether the API triggers other events or requests another API.
We will verify the behavior of the API when no value is returned.

13) What exactly needs to be verified in API testing?

In API testing, we send a request to the API with the known data and then analyze the response.

We will verify the accuracy of the data.
Will see the HTTP status code.
We will see the response time.
Error codes in case the API returns any errors.
Authorization would be checked.
Non-Functional testing, such as performance testing, security testing.

14) What are the differences between API and Web Services?

Serial	API	Web Services
1.	API may or may not need a network for its operations.	Web Services always need a network for their operation.
2.	API can be communicated through SOAP, REST, XML-RPC, and CURL calls as well. API can also be exposed in number of ways like JAR, DLL, XML over HTTP, JSON over HTTP etc.	Web services can be communicated through SOAP, REST, AND RPC.
3.	API can perform all the operations that a web service can’t perform.	A web service can’t perform all the operations like an API.
4.	Not all APIs are web services.	All web services are API

15) What is API documentation?

A good documentation is must for any foundation. API documentation serves as quick reference for accessing library or working within a program. When we use any such documents, it must consists of proper plan, content source, proper layout, information related to each function etc.

There are various documentation tools like Doxygen and JavaDoc. Here are the functions that are documented, which revolve around the parameters like:

Function description
Type and syntax of error message that may occur
Syntax, elements, and sequence needed for each parameter
Links regarding functions

16) What is the most used template for API documentation?

Here are the various documentation templates that make the whole process simple and easy. They are:

Swagger
Miredot
Slate
FlatDoc
API blueprint
RestDoc
Web service API specification

17) What are the types of bugs that can be found during API testing?

API testing helps us to find many types of bugs, which are:

Stress
Security
Duplicate or missing functionality
Reliability
Unused flags
Performance
Incompatible error handling
Multi-threaded issue
Improper errors

18) What are the differences between API testing and UI testing?

UI (User Interface) testing means the testing of the graphical user interface. The focus of UI testing is on the look and feel of the application. In user interface testing, the main focus is on how users can interact with app elements, such as images, fonts, layout, etc. are checked.

API testing allows the communication between two software systems. API testing works on backend also known as backend testing.

19) What is SOAP?

SOAP (Simple Object Access Control) . It is an XML based protocol that helps in exchanging information among computers.

20) What is REST API?

REST API is a set of function helps the developers performing requests when the response is receiving. Through HTTP protocol interaction is made in REST API.

REST is defined as Representational State Transfer. It is an effective standard for API creation.

21) What are the differences between SOAP and REST API?

Serial	SOAP API	REST API
1.	SOAP stands for Simple Object Access Protocol.	REST stands for Representational State Transfer.
2.	SOAP is a protocol.	REST is an architectural pattern.
3.	SOAP can work with the XML format. In SOAP, all the data is passed in XML format.	REST permit different data format such as Plain text, HTML, XML, JSON etc. But the most preferred format for transferring data is in JSON.

22) What are the major challenges faced during API testing?

The major challenges faced during the API testing are:

Parameter Selection
Parameter Combination
Call sequencing
Output verification and validation
A major challenge is providing input values, which are very difficult because a GUI is not available.

23) What is the difference between API Testing and Unit Testing?

The difference between API testing and Unit testing is:

Serial	API Testing	UNIT Testing
1.	API testing is a form of black box testing.	Unit testing is a form of white box testing.
2.	API testing is performed after the project’s completion during the test.	Unit testing is performed when the project is created.
3.	In API testing, there is a wide scope of testing.	In Unit testing, there is a limited scope of testing; we can test only the basic functionality.
4.	API testing is done by the testers. The whole purpose of API testing is end to end testing of the functionality.	Unit testing is done by the developer. In unit testing, every functionality is separately tested.

24) What is a RESTFUL web service?

There are two kinds of web services

SOAP Web Services
RESTFUL Web Services

1. SOAP (Simple Object Access Protocol) – SOAP is a XML based method which is used in Web Services.

2. RESTFUL Web Services – To implement the concept of REST architecture HTTP method is used. RESTFUL Web Services defines URI (Uniform Resource Identifier), and also provides resource representation like JSON and a set of HTTP methods.

25) What isa Resource in REST?

REST architecture treats any content as resource, which can be text files, HTML pages, images, videos or dynamic business information. REST server gives the functionality to access the resources and modifies them. We can identify the each resources by URIs/ global IDs.

26) What is the way to represent the resource in REST?

REST uses different representation to define the resources like text, JSON and XML. The most popular representation of resources is JSON and XML.

27) What protocol is used by the RESTFUL Web Services?

RESTFUL Web Services uses the HTTP protocol. They use the HTTP protocol as a medium of communication between the client and the server.

28) What are the characteristics of REST?

Here are the two characteristics of REST.

REST is stateless. With the use of the REST API, the server has no status; we can restart the server between two calls, despite of whether all the data is transferred to the server.
Web Services uses the POST method to perform operations, while REST uses the GET method to access the resources.

29) What is messaging in RESTFUL Web Services?

RESTFUL Web Services use the HTTP protocol as a communication tool between the client and the server. This is the technique when the client sends a message in the form of HTTP request the server send back the HTTP reply which is called Messaging. This message consists message data and Meta data i.e. information on the message itself.

30) What are the components of an HTTP request?

An HTTP request has five components. These are:

Action showing HTTP method like GET, PUT, POST, DELETE.
Uniform Resource Identifier (URI): URI is the identifier for the resource on the server.
HTTP version: Indicate the HTTP version like- HTTP V1.1.
Request Header: Request Header carries metadata for the HTTP request message. Metadata could be a client type, format supported by the client, format of a message body, cache setting etc.
Request Body: Resource body indicates message content or resource representation.

31) What is the HTTP protocol supported by REST?

GET: GET is used to request data from the specified resource.

GET request can be cached and bookmark. It remains in the browser history and has length restriction. When dealing with sensitive data GET requests should not be used.

POST: POST is used to send data to the server for creating or updating the resources. POST requests are never cached or bookmarked.

PUT: PUT replaces the current representation of the target resource with the request payload.

DELETE: DELETE removes the specified resource.

OPTIONS: OPTION is used to describe the communication option for the target resources.

HEAD: HEAD asks for the response, which is identical to GET requests, but without the response body.

32) Can we use a GET request instead of a PUT to create a resource?

PUT or POST method is used to create a resource. GET is only used to request the resources.

33) What is URI? What is the purpose of a web-based service, and what is its format?

URI stands for Uniform Resource Identifier. It is a string of characters designed for unambiguous identification of resources and extensibility by the URI scheme. The purpose of URI is to locate the resource on the server hosting the web service. A URIs format is <protocol>://<service-name>/<Resource Type>/<ResourceID>

34) What are SOAP Web Services?

SOAP (Simple Object Access Protocol) is defined as an XML-based protocol. SOAP is also known for developing and designing web services and enabling the communication between the applications developed on different platforms by using different programming languages on the internet. SOAP is platform and language-independent.

35) When can we use the OAP API?

We can use the SOAP API to operate on records like create, retrieve, update, or delete. We can use the API to manage passwords, perform searches, etc.