What are Long-Polling, Websockets, Server-Sent Events (SSE) and Comet?

Long-polling, WebSockets, Server-Sent Events (SSE), and Comet are all techniques used to enable real-time communication between a client (usually a web browser) and a server.

Long-polling is a technique in which the client sends a request to the server, and the server holds the request open until it has new data to send back to the client. Once the server has new data, it sends a response to the client and the client sends a new request to the server, establishing a new long-poll connection. This process repeats, allowing the server to push new data to the client as it becomes available.

WebSockets is a more efficient real-time communication protocol that allows a client and a server to send messages back and forth in real time over a single, long-lived connection. WebSockets uses a different protocol than HTTP and can be used to transmit any type of data, not just text.

Server-Sent Events (SSE) is a server push technology that allows a server to send data to a client over an HTTP connection. With SSE, the server can send events in the form of text messages to the client, and the client can listen for these events and react to them.

Comet is a general term used to describe a group of techniques that allow a server to send data to a client over an HTTP connection. These techniques, which include long-polling, SSE, and others, are used to enable real-time communication between a client and a server.