What is API testing in software testing?

Application Programming Interface (API) is a bunch of code that acts as an interface between two computers. APIs offer computing services from external software that we can deploy into our own programs.

API Testing is the stage in software testing where we test the functionality, working, reliability, performance, and other aspects of an API.

Why is API testing necessary?

As defined above, we implement API in a program to perform communication between two components of a software application.

We conduct API testing to confirm whether the output received from the first application or database is accurate, well-structured, and useful to another application.

API testing also determines the response of an API. There is a set of defined rules that check how the data should behave, based on the following:

• The type of input
• The required authentication parameters
• Whether sensitive data is transmitted securely over the network
• How long an API takes to return the value

Layers in a software / application

Usually, applications have several layers. Most often, they have these three layers:

1. Data layer
2. Service layer (API)
3. Presentation layer

The service layer where the APIs resides is also known as a business layer. API testing is involved at this stage of an application, and is the most crucial part in software testing.

Consider the following structure of an application.

How to perform API testing

We can conduct API testing on a software application manually or through online tools available in both free and paid versions. API testing involves some types that are helpful in determining various aspects of API.

1. Functionality testing
2. Load testing
3. Reliability testing
4. Security testing
5. Validation testing, and several more

These types vary from software to software. Large scale softwares involve in-depth testing of an API, while small-scale ones do not require much testing.

Postman, SoapUI, RapidAPI, and Katalon Studio are a few widely used API testing tools available for developers and testers.

Advantages of API testing

API testing is very useful in a software application and it includes various advantages, such as the following:

• API testing improves our test coverage.
• It eases our load by executing faster releases of products.
• It benefits us with easier test maintenance of work.
• Automated API testing reduces testing cost as compared to manual testing of an API.
• Earlier testing helps the tester to determine those errors that could occur during GUI testing.