The usual AJAX request/response we’re all used to doesn’t keep the connection open for this sort of use case. Instead we need a push-based method like WebSockets, Long Polling, Server-Sent Events (SSE) and more recently HTTP2 push.
Long Polling
Overview
Long polling is essentially a more efficient form of the original polling technique (with JavaScript - XMLHttpRequest). Making repeated requests to a server wastes resources, as each new incoming connection must be established, the HTTP headers must be parsed, a query for new data must be performed, and a response (usually with no new data to offer) must be generated and delivered.
The connection must then be closed and any resources cleaned up. Rather than having to repeat this process multiple times for every client until new data for a given client becomes available, long polling is a technique where the server elects to hold a client’s connection open for as long as possible, delivering a response only after data becomes available or a timeout threshold is reached.
A client initiates an XHR/AJAX request, requesting some data from a server.
The server does not immediately respond with request information but waits until there is new information available.
When there is new information available, the server responds with new information.
The client receives the new information and immediately sends another request to the server restarting the process.
Challenges
Message ordering and delivery guarantees : Cannot be guaranteed if the same client opens multiple connections to the server or if client was not able to receive the message then there will be possible message loss.
Performance and scaling.
Device support and fallbacks.
WebSockets
Overview
WebSocket is a computer communication protocol which provides full-duplex communication channels over a single TCP connection.
It is different from HTTP but compatible with HTTP.
Located at layer 7 in the OSI model and depends on TCP at layer 4.
Works over port 80 and 443 ( in case of TLS encrypted) and supports HTTP proxies and intermediaries.
To achieve compatibility, the WebSocket handshake uses Upgrade header to update the protocol to the WebSocket protocol.
Challenges
Hard to scale : connections to WebSocket server need to be persistent. Even nodes are scaled (horizontal and vertical), we need a solution for sharing data between nodes.
Intermediary/Edge caching is not possible with WebSockets unlike HTTP.
Security issues : https://blog.securelayer7.net/websocket-common-vulnerabilities-plaguing-it-and-managing-them/
Server-Sent Events
On this page.