Before dwelling into the subject of API Testing, we should 
understand what is the meaning of API or, Application 
Programming Interface.  

An API (Application Programming Interface) is a collection 
of software functions and procedures, called API calls, 
that can be executed by other software applications. 

API testing is mostly used for the system which has 
collection of API that needs to be tested. The system could 
be system software, application software or libraries. 

API testing is different from other testing types as GUI is 
rarely involved in API Testing. Even if GUI is not involved 
in API testing, you still need to setup initial 
environment, invoke API with required set of parameters and 
then finally analyze the result. 

Setting initial environment become complex because GUI is 
not involved. It is very easy to setup initial condition in 
GUI, In most cases you can find out in a glance whether 
system is ready or not. In case of API this is not the 
case, you need to have some way to make sure that system is 
ready for testing.  

This can be divided further in test environment setup and 
application setup. Things like database should be 
configured, server should be started are related to test 
environment setup. On the other hand object should be 
created before calling non static member of the class falls 
under application specific setup. 

Initial condition in API testing also involves creating 
conditions under which API will be called. Probably, API 
can be called directly 

or it can be called because of some event or in response of 
some exception. 

Output of API could be some data or status or it can just 
wait for some other call to complete in a-synchronized 
environment. Most of the test cases of API will be based on 
the output, if API 

Return value based on input condition 

This are relatively simple to test as input can be defined 
and results can be validated against expected return value. 
For example, It is very easy to write test cases for int add
(int a, int b) kind of API. You can pass different 
combinations of int a and int b and can validate these 
against known results. 

Does not return anything 

For cases like these you will probably have some mechanism 
to check behavior of API on the system. For example, if you 
need to write test cases for delete(ListElement) function 
you will probably validate size of the list, absence of 
list element in the list. 

Trigger some other API/event/interrupt 

If API is triggering some event or raising some interrupt, 
then you need to listen for those events and interrupt 
listener. Your test suite should call appropriate API and 
asserts should be on the interrupts and listener. 

Update data structure 

This category is also similar to the API category which 
does not return anything. Updating data structure will have 
some effect on the system and that should be validated. If 
you have other means of accessing the data structure, it 
should be used to validate that data structure is updated. 

Modify certain resources 

If API call is modifying some resources, for example 
updating some database, changing registry, killing some 
process etc, then it should be validated by accessing those 
resources. 

You should not get confused with API Testing and Unit 
Testing. API testing is not Unit testing. Unit testing is 
owned by dev team and API by QE team. API is mostly black 
box testing where as unit testing is essentially white box 
testing. Unit test cases are typically designed by the 
developers and there scope is limited to the unit under 
test. In API testing, test cases are designed by the QE 
team and there scope is not limited to any specific unit, 
but it normally cover complete system. 

Main Challenges of API Testing can be divided into 
following categories. 

Parameter Selection 
Parameter combination 
Call sequencing

A formalized set of software calls and routines that can be
referenced by an application program in order to access
supporting system or network services.