All About APIs

API Application Programming Interfaces. It is a software intermediary that allows 2 applications to talk to each other. API is like a messenger. There is a message that you need to send to a server such as a website or mobile application. Similarly if this server, i.e. website or application is trying to send you a message, it will use an API.

• The requests can go online as well as offline.
• Eg. of Offline mode of API Request – Camera sending pictures to Photo Editor within your mobile phones.

What is a Web Service ?

• An API that goes Online is termed as a webservice.
• Web Services are a subset of APIs. i.e. All Web Services are APIs but not all APIs are Web Services.

Data Types for APIs

• While sending an API request online, you can send it with data in 2 types:

• Each type of data has a different protocol for transit.

Protocols for Data Transit

• HTTP of Hyper Text Transfer Protocol, is used to transfer data through internet.
• APIs are all about transferring data b/w 2 applications or 2 servors.
• For online APIs/Webservices the protocol used to transfer data is called HTTP
• Hence communication between client computers and web servers is done by sending HTTP requests and receiving HTTP responses.
• Clients are mostly browsers, programs or devices and servers are mostly computers in the cloud.

Steps in Communication

1. Client ( e.g. a browser) sends an HTTP request to the web.
2. A web server receives the request.
3. The server runs an application to process the request.
4. The server returns an HTTP response (O/P) to the browser.
5. The client (the browser) receives the response.

• When the client communicates to the server, there are many methods for this communication called HTTP methods.
• There are mainly over 10 methods, out of which the main methods include – GET, POST, PUT, DELETE which are in line with the CRUD operations

Methods

• C: Create: POST: Submit data to Server
• R: Read: GET: Retrieve data from Server
• U: Update: PUT: Update data already on Server
• D: Delete: DELETE: Deletes data from Server

• The method is specified while you are sending the request, not as a response.

HTTP Status Codes (Responses)

• 2XX: The request was successfully received, understood and accepted.
• 4XX: Problem with client. e.g. accessing a page that doesn’t exist.
• 5XX: Problem with server. e.g. Too many people accessing a website, causing the site to crash and unable to fulfill requests.

While sending an API request, say a ‘POST’ request, you are sending some data with request. Similarly, while receiving a response from the server, the response will also include data in it. The data that is sent in request or response is of 2 types: XML and JSON.

XML – Extensible Markup Language

• It is a markup language like HTML.
• It is created by WEC, the same organization that created HTML.
• To send XML data, we use SOAP API. These are older APIs, and SOAP stands for – Simple Object Access Protocol. Here is an example ->

<?xml version="1.0" encoding="UTF-8"?>
<library>
    <book>
        <title>The Great Gatsby</title>
        <author>F. Scott Fitzgerald</author>
        <year>1925</year>
        <genre>Fiction</genre>
    </book>
    <book>
        <title>To Kill a Mockingbird</title>
        <author>Harper Lee</author>
        <year>1960</year>
        <genre>Fiction</genre>
    </book>
    <book>
        <title>A Brief History of Time</title>
        <author>Stephen Hawking</author>
        <year>1988</year>
        <genre>Non-Fiction</genre>
    </book>
</library>

JSON: Java Script Object Notation

• It is built using JavaScript
• JSON code is smaller than XML
• JSON code data is send using REST APIs.
• REST – Representation State Transfer.
• In JSON there are 3 main components:
  • Strings: Described in double quotes – “ABCD”
  • Lists: Described with square brackets – [1,2,3]
  • Objects: Collection of key value pairs, Described with Curly Brackets – {“Key”:”value”}

• Sample of JSON code

{
  "library": [
    {
      "title": "The Great Gatsby",
      "author": "F. Scott Fitzgerald",
      "year": 1925,
      "genre": "Fiction"
    },
    {
      "title": "To Kill a Mockingbird",
      "author": "Harper Lee",
      "year": 1960,
      "genre": "Fiction"
    },
    {
      "title": "A Brief History of Time",
      "author": "Stephen Hawking",
      "year": 1988,
      "genre": "Non-Fiction"
    }
  ]
}

Testing APIs

• Software testing validates APIs functioning.
• API testing is done on the business logic layer and not on UX part.
• Purpose of API testing:
  • Functionality
  • Reliability/ Repeatability
  • Performance
  • Security

API Documentation

• Technical content deliverable with instructions on how to use and integrate the API
• Since everything has APIs, documentation is needed to keep track of them as well as to increase adoption.

Other Important Points

• Difference between Patch and PUT
  • Put is used to modify resources where the client sends data that updates the entire record.
  • e.g. you cannot send only the User ID or Password. you have to send them together.
  • Patch is used for partial update of records.
• .