Picture this:
A moviegoer is trying to purchase movie tickets online. He or she has selected the seats, entered the payment details, and submitted. He or she gets an error message saying that the tickets they tried to book have sold out.
Consider an application, which gives detailed information about the stock market and allows purchasing/selling stocks. When someone enters payment details and submits these details, they get an error saying that the purchase has been rejected as the price of the stock has now changed.
Initially, in applications where real-time data was required over HTTP, developers realized that they needed bidirectional communication between the client side and server side. It was generally implemented using one of the following approaches:
