API Testing

API testing helps examine application business logic. It’s a part of integration testing, and no quality assurance process is complete without it. In simple words, API stands for Application Programming Interface. API acts as an interface between two software applications and allows the two software applications to communicate with each other.

What is an API?

Application Programming Interface (or API) is a set of routines, rules, methods, tools, and protocols used when building mobile applications. In other words, it’s a way for different software components to interact with each other. API reduces development time and provides developers with access to a number of practical features and services.

Routine: a program that performs a particular task. Routine is also known as procedure, function, or subroutine.

Protocols: A format for transmitting data between two systems.

API architecture

The majority of apps have three layers to them:

1. Database
2. Business Logic
3. Presentation Layer

Database Layer: where data is extracted from and stored in databases (DB). The client system handles both Presentation and Application layers and the Server system handles the Database layer. It is also known as a client-server application. The communication takes place between the Client and the Server. The client system sends the request to the server system and the server system processes the request and sends back the data to the Client System.

Business Logic Layer: which consists of APIs; its main purposes are data processing between different layers, logical decision making, and application management. Business logic is the programming that manages communication between an end-user interface and a database. The main components of business logic are business rules and workflows.

A Business Logic Layer (BLL) that serves as an intermediary for data exchange between the presentation layer and the DAL. In a real-world application, the BLL should be implemented as a separate Class Library project in the App_Code folder in order to simplify the project structure. below illustrates the architectural relationships among the presentation layer, BLL, and DAL.

Presentation Layer: This represents the user interface (UI), it is also known as the Client layer. The topmost layer of an application. This is the layer we see when we use the software. By using this layer we can access the webpages. The main function of this layer is to communicate with the Application layer. This layer passes the information which is given by the user in terms of keyboard actions, mouse clicks to the Application Layer.
For example, the login page of Gmail where an end-user could see text boxes and buttons to enter user id, password, and to click on sign-in.

Businesses that implement an API-centric architecture, where apps use APIs on the backend to connect with data and services, have increased flexibility and agility to meet increasing customer demands and changing market needs.

API architecture spans the bigger picture of APIs and can be seen from several perspectives: API architecture may refer to the architecture of the complete solution, consisting not only of the API itself but also of an API client such as a mobile app and several other components.

API testing is a type of software testing that involves testing APIs directly and also as a part of integration testing to check whether the API meets expectations in terms of functionality, reliability, performance, and security of an application. In API Testing our main focus will be on a Business logic layer of the software architecture. API testing can be performed on any software system which contains multiple APIs. API testing won’t concentrate on the look and feel of the application. API testing is entirely different from GUI Testing.

Let’s see, how is UI testing is not similar to API testing

UI (User Interface) testing is to test the graphical interface part of the application. Its main focus is to test the look and feel of an application. On the other hand, API testing enables the communication between two different software systems. Its main focus is on the business layer of the application.

API Testing Types

API testing typically involves the following practices:

• Unit testing: To test the functionality of individual operation
• Functional testing: To test the functionality of broader scenarios by using a block of unit test results tested together
• Load testing: To test the functionality and performance under load
• Runtime/Error Detection: To monitor an application to identify problems such as exceptions and resource leaks
• Security testing: To ensure that the implementation of the API is secure from external threats
• UI testing: It is performed as part of end-to-end integration tests to make sure every aspect of the user interface functions as expected
• Interoperability and WS Compliance testing: Interoperability and WS Compliance Testing is a type of testing that applies to SOAP APIs. Interoperability between SOAP APIs is checked by ensuring conformance to the Web Services Interoperability Profiles. WS-* compliance is tested to ensure standards such as WS-Addressing, WS-Discovery, WS-Federation, WS-Policy, WS-Security, and WS-Trust are properly implemented and utilized
• Penetration testing: To find vulnerabilities of an application from attackers
• Fuzz testing: To test the API by forcibly input into the system in order to attempt a forced crash

Common tests on APIs:

Some of the common tests we perform on APIs are as follows.

• To verify whether the return value is based on the input condition. The response of the APIs should be verified based on the request.
• To verify whether the system is authenticating the outcome when the API is updating any data structure
• To verify whether the API triggers some other event or request another API
• To verify the behavior of the API when there is no return value

Advantages of API Testing:

• API Testing is time effective when compared to GUI Testing. API test automation requires less code so it can provide faster and better test coverage.
• API Testing helps us to reduce the testing cost. With API Testing we can find minor bugs before the GUI Testing. These minor bugs will become bigger during GUI Testing. So finding those bugs in the API Testing will be cost-effective to the Company.
• API Testing is language independent.
• API Testing is quite helpful in testing Core Functionality. We can test the APIs without a user interface. In GUI Testing, we need to wait until the application is available to test the core functionalities.
• API Testing helps us to reduce the risks.

Why is it necessary to test APIs?

We test APIs to improve application test coverage on the Business Logic layer. Testing APIs is important, because whenever they don’t work properly, major issues may occur in application logic, performance, and security.

What exactly needs to be verified in API Testing?

Basically, on API Testing, we send a request to the API with the known data and we analyze the response.

• Data accuracy
• HTTP status codes
• Response time
• Error codes in case API return any errors
• Authorization checks
• Non-functional testing such as performance testing, security testing

Challenges in API testing:

Some of the challenges we face while doing API testing are as follows

• Selecting proper parameters and its combinations
• Categorizing the parameters properly
• Proper call sequencing is required as this may lead to inadequate coverage in testing
• Verifying and validating the output
• Due to the absence of GUI, it is quite difficult to provide input values

Types of bugs we face when performing API testing:

Issues observed when performing API testing are

• Stress, performance, and security issues
• Duplicate or missing functionality
• Reliability issues
• Improper messaging
• Incompatible error handling mechanism
• Multi-threaded issues
• Improper errors

API Testing Best Practices:

• Test for the expected results
• Add stress to the system by sending a series of API load tests
• Group API test cases by test category
• Create test cases with all possible inputs combinations for complete test coverage
• Prioritize API function calls to make it easy to test
• Create tests to handle unforeseen problems
• Automate API testing wherever it is possible

Top API testing tools covering both open-source and commercial solutions

1. SoapUI
2. Katalon Studio
3. Postman
4. Tricentis Tosca
5. Apigee
6. JMeter
7. Rest-Assured

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