How to test an API.

Before going to API testing, let's first understand

What is an API?

API is an acronym for Application Programming Interface.

It enables communication and data exchange between two separate software systems. A software system implementing an API contains functions/sub-routines which can be executed by another software system.

What is API testing?

API testing is entirely different from GUI testing and mainly concentrates on the business logic layer of the software architecture. This testing won't concentrate on the look and feel of an application.

Instead of using standard user inputs(keyboard) and outputs, in API Testing, you use software to send calls to the API, get output, and note down the system's response.

API Testing requires an application to interact with API. In order to test an API, you will need to

• Use Testing Tool to drive the API
• Write your own code to test the API

Set-up of API Test environment

• API testing is different than other testing as GUI is not available, and yet you are required to setup initial environment that invoke API with required set of parameters and then finally examine the test result.
• Hence, Setting up testing environment for API testing seems little complex.
• Database and server should be configured as per the application requirements.
• Once installation is done, API Function should be called to check whether that API is working.

Types of Output of an API

Output of API could be

1. Any type of data
2. Status (say Pass or Fail)
3. Call to another API function.

Let's look at an example of each of above Types

Any Type of Data

Example: There is an API function which should add two integer numbers.

?

1	Long add(int a, int b)

The numbers have to be given as input parameters. Output should be summation of two integer numbers. This output needs to be verified with expected outcome.

Calling needs to be done such as

?

1	add (1234, 5656)

Exceptions have to be handled if number is exceeding the integer limit.

Status (say Pass or Fail)

Consider the below API function -

1. Lock()
2. Unlock()
3. Delete()

They return any value such as True (in case of success) or false (In case of error) as an output.

A more accurate test case would be, can call the functions in any of the script and later check for changes either in the database or the Application GUI.

Calling of another API / Event

In this case, we call one of the API function which in turn will call another function.

For example - First API function can be used for deleting a specified record in the table and this function in turn call another function to REFRESH the database.

Test Cases for API Testing:

Test cases of API testing are based on

• Return value based on input condition: it is relatively easy to test, as input can be defined and results can be authenticated
• Does not return anything: When there is no return value, behavior of API on the system to be checked
• Trigger some other API/event/interrupt: If output of an API triggers some event or interrupt, then those events and interrupt listeners should be tracked
• Update data structure: Updating data structure will have some outcome or effect on the system, and that should be authenticated
• Modify certain resources: If API call modifies some resources then it should be validated by accessing respective resources

Approach of API Testing:

Following points helps the user to do API Testing approach:

1. Understanding the functionality of the API program and clearly define the scope of the program
2. Apply testing techniques such as equivalence classes, boundary value analysis and error guessing and write test cases for the API
3. Input Parameters for the API need to be planned and defined appropriately
4. Execute the test cases and compare expected and actual results.