Top 30+ Web API Testing Interview Questions

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Definition & Functions of an API (Common Web API Testing interview questions)

What is an API? An API (Application Programming Interface) is a software intermediary that enables two applications to communicate with each other. It comprises a number of subroutine definitions, logs, and tools for creating application software.

In an API testing interview, you could be asked to give some API examples, here are the well-known ones: Google Maps API, Amazon Advertising API, Twitter API, YouTube API, etc.

What are the main differences between API and Web Service? All Web services are APIs but not all APIs are Web services. Web services might not contain all the specifications and cannot perform all the tasks that APIs would perform. A Web service uses only three styles of use: SOAP, REST, and XML-RPC for communication whereas API may be exposed in multiple ways. A Web service always needs a network to operate while APIs don’t need a network for operation.
What are the Limits of API Usage? Many APIs have a certain limit set up by the provider. Thus, try to estimate your usage and understand how that will impact the overall cost of the offering. Whether this will be a problem depends in large part on how data is leveraged. Getting caught by a quota and effectively cut off because of budget limitations will render the service (and any system or process depending on it) virtually useless.

Creating an API (Common Web API Testing interview questions)

What are some architectural styles for creating a Web API? This is one of the fundamental Web API interview questions. Bellows are four common Web API architectural styles:

HTTP for client-server communication
XML/JSON as formatting language
Simple URI as the address for the services
Stateless communication

Who can use a Web API? Web API can be consumed by any clients which support HTTP verbs such as GET, PUT, DELETE, and POST. Since Web API services do not require configuration, they can be easily used by any client. In fact, even portable devices such as mobile devices can easily use Web API, which is undoubtedly the biggest advantage of this technology.

Testing an API – Top Web API Testing interview questions & answers

What is API Testing?
API testing is a kind of software testing that determines if the developed APIs meet expectations regarding the functionality, reliability, performance, and security of the application.
What are the advantages of API Testing?

In an API interview, they are likely to ask about the advantages of API testing. So be prepared with the significant ones such as:

Test for Core Functionality: API testing provides access to the application without a user interface. The core and code-level of functionalities of the application will be tested and evaluated early before the GUI tests. This will help detect minor issues which can become bigger during the GUI testing.
Time Effective: API testing usually is less time-consuming than functional GUI testing. The web elements in GUI testing must be polled, which makes the testing process slower. Particularly, API test automation requires less code so it can provide better and faster test coverage compared to GUI test automation. These will result in cost saving for the testing project.
Language-Independent: In API testing, data is exchanged using XML or JSON. These transfer modes are completely language-independent, allowing users to select any coding language when adopting automation testing services for the project.
Easy Integration with GUI: API tests enable highly integrable tests, which is particularly useful if you want to perform functional GUI tests after API testing. For instance, simple integration would allow new user accounts to be created within the application before a GUI test started.

Some common protocols used in API testing?
Many protocols are now available to be used in API testing, such as JMS, REST, HTTP, UDDI and SOAP.
What is the test environment of API?
Setting up the API’s test environment is not an easy task, so you should have a ready answer if your API testing interview is coming. The test environment of API is a bit complete and requires the configuration of the database and server, depending on the software requirements. No GUI (Graphical User Interface) is available in this test form.

When the installation process is complete, API is verified for proper operation. Throughout the process, the API called from the original environment is set up with different parameters to study the test results.

What are the principles of an API test design? The five most important principles of an API test design are:

Setup: Create objects, start services, initialize data, etc
Execution: Steps to apply API or the scenario, including logging
Verification: Oracles to evaluate the result of the execution
Reporting: Pass, failed, or blocked
Clean up: Pre-test state

What are the common API testing types?
While there are certainly specialty tests, and no list can be asked to be comprehensive in this realm, most tests fit broadly into the following nine categories that you should remember before attending an API testing interview.
Validation Testing
Functional Testing
UI testing
Load testing
Runtime/ Error Detection
Security testing

Also read :Top 30+ Web API Testing Interview Questions

Penetration testing
Fuzz testing
Interoperability and WS Compliance testing
What is the procedure to perform API testing?
Choose the suite to add the API test case
Choose the test development mode
Demand the development of test cases for the required API methods
Configure the control parameters of the application and then test conditions
Configure method validation
Execute the API test
Check test reports and filter API test cases
Arrange all API test cases
What must be checked when performing API testing?
During the API testing process, a request is raised to the API with the known data. This way you can analyze the validation response. While testing an API, you should consider:

Accuracy of data
Schema validation
HTTP status codes
Data type, validations, order, and completeness
Authorization checks
Implementation of response timeout
Error codes in case API returns, and
Non-functional testing like performance and security testing

What is the best approach method to perform API testing? The following factors should be considered when performing API testing:

Defining the correct input parameters
Verifying the calls of the mixture of two or more added value parameters
Defining the basic functionality and scope of the API program
Writing appropriate API test cases and making use of testing techniques such as equivalence class, boundary value, etc. to check the operability
Testing case execution
Comparing the test result with the expected result
Verifying the API behavior under conditions such as connection to files and so on.

What tools could be used for API testing? There is myriad different API testing tools available. A few common tools are Katalon Studio, Postman, SoapUi Pro, Apigee, etc. While doing Unit and API testing, both target source code. If an API method uses code based in .NET then another supporting tool must have .NET.

Learn more: SoapUI vs Postman, Katalon Studio: A Review of Top 3 API Tools