An API, or application programming interface, facilitates communication between software systems through various functions. API testing differs from GUI testing by focusing on the business logic layer, using tools like Postman to send requests and analyze responses. The document provides a comprehensive guide on API architecture, testing methodologies using Postman, and automating API tests with detailed procedural steps.

2. API is Application Programming Interface. It is used to establish
communication and data exchange between two software systems.
A software system implementing an API consists of various
functions and sub-routines which can be executed by another
software system. API testing is totally different from GUI testing. It
basically concentrates on the business logic layer of the software
architecture. In API Testing, you use software to send calls to the
API, get output, and note down the system's response.

3. API Architecture: API calls collection includes
mainly two things
1. HTTP headers: HTTP headers are always dependent on your
application.
i) Authorization: A token included with requests to identify the
requester. This header has to be included in all requests other than
the login request.
ii) Content-Type: A standard MIME type describes the format of
object data. Content type in most of the requests and responses will
be application/json.

4. 2. HTTP Request: There are mainly four types of requests.
i) POST– Used to Create or Update data
ii) PUT- Used to Update data
iii) GET- Used to Retrieve data
iv) DELETE– Used to Delete data

5. What is Postman?
Postman is used to fire requests to an API. It is available as add-in
on Chrome and also available for Mac application. It is very fast in
response and also light weight which puts no impact on browser
performance. Tests can be created with verifications for certain
conditions on the response. With Postman, you can create different
kinds of HTTP requests such as GET, POST, PUT, PATCH and
DELETE. It also provides supports for adding headers in the
requests. It’s a very convenient API tool with very useful features.

6. How to perform Automation Testing of APIs
using Postman?
* First, install ‘POSTMAN’ add on in Google Chrome.
* Open Postman.
* Click on ‘Create New Collection’ button, fill details and Click
‘Create’ button.
* Now go to newly created Collection and Create New Folder by
selecting ‘Add Folder’ option of side menu bar.
Now Click ‘+’ Button for opening new Tab.
* Now Fill URL, headers, Select Method (Get/POST/PATCH etc.),
Body etc. and click ‘Send’. It will show the result in a response. It
should return desired results. If there is a problem Please recheck
every prerequisites and requirement.

7. * You can add desired assertions in a particular request in tests tab
like:
tests[“Check: Status code is 201”] = responseCode.code === 201;
* Now ‘Save as’ request by filling its Name, Folder Name, Description
etc. It will show the request under your selected Folder/Collection.
* Now Click on Setting Environment Button (Setting Icon on Top right
corner under Heart Icon) and select ‘Manage Environments’.
* Click on ‘Add’ Button, Enter Name for environments, Enter Keys and
Values. Keys(Name of the variable like URL), Value(Value of Variable
like: http://bugraptors.com/). Click ‘Add’. It will create environments.
* Now select newly created environments from DDL (Top right corner
with setting Icon).

8. * Now use the variable as a requirement. Syntax to use is {{Key of
variable}}.Like: {{URL}}. It will show variables in a list and you can
select from there too. If the variable is accessible it will automatically
convert it into orange color either it will reflect in Red color.
* Do the same process for all your APIs.
* Now you can run the whole collection or single folder by Clicking
on ‘Runner’ (Top Left corner Button) or Click run from the submenu
of the desired collection. It will open Runner Window. Select
Collection/Folder, Select required environments, Fill iteration count
(If you want to run it multiple times), Fill Delay, Choose Data File if
required, and click ‘Start Test’. It will Give you result of every request.

9. * You can fetch Data from the response of a particular request and
store that in variables. The syntax of it is like:
var jsonData = JSON.parse(responseBody);
postman.setEnvironmentVariable(“variable_key”,
“variable_value”);
* For ‘report’ you have to install ‘Newman’ on your PC. It is
command line tool that can generate the report by running
POSTMAN collections.
* Now export Postman Collection by selecting Export from the left
menu bar of the desired collection.
* Also, export the required environments on manage environments
window from download icon button.