Web-sockets(Socket.io) and HTTP both are communication protocols. HTTP uses HTTP:// or HTTPS:// and web-socket uses ws:// or wss:// to communicate with client.
Web-sockets are designed to maintain real time connection with client. But HTTP not maintaining real time connection with client. It response to the request of the client then terminate connection with client.
So if server only provide something that doesn’t change in real time or the client doesn’t expect to see changes in real time then having real time connection to client is a waste of server resources(server load, traffic, etc.).
Example: Think you are searching something in google. Search results appeared in google is same as after 1 hour(or more) for the same keyword. search results are not change in real time. So think if google server use socket connection instead of HTTP connection with clients. Google servers have to maintain billions of simultaneous connections with their clients for nothing.
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