What is the principle and mechanism of WebSockets?

WebSocket is a protocol for full-duplex communication on a single TCP connection, allowing real-time two-way communication by establishing a long-lasting connection after an HTTP handshake.

The mechanism of the WebSocket protocol is as follows:

1. The client initiates a WebSocket handshake request, which is similar to a regular HTTP request but includes some special header information such as Upgrade, Connection, and Sec-WebSocket-Key.
2. Upon receiving a WebSocket handshake request, the server verifies its validity. If the request is valid, the server responds with an HTTP response containing status code 101, indicating a successful handshake.
3. After receiving the handshake response from the server, the client also needs to undergo verification. Once verified, real-time bidirectional communication between the client and server begins through a TCP connection.
4. Both parties can send and receive messages using the WebSocket protocol, which can be in the form of text or binary data.
5. Either party can close the WebSocket connection at any time by sending a special close frame.

Here are the main features of the WebSocket protocol:

1. Real-time communication: After establishing a WebSocket connection, it allows for real-time, bidirectional communication without the need for continuously initiating new HTTP requests.
2. Low latency: The delay in message transmission is reduced due to the establishment of long connections.
3. Less communication overhead: The WebSocket protocol uses binary frames for message transmission, which results in lower communication overhead compared to the text transmission of the HTTP protocol.
4. Cross-origin support: The WebSocket protocol allows for cross-origin communication, enabling real-time bidirectional communication between browsers and servers.