System Design Interview

In long polling, a client holds the connection open until there are actually new messages available or a timeout threshold has been reached. Once the client receives new messages, it immediately sends another request to the server, restarting the process. Long polling has a few drawbacks: •Sender and receiver may not connect to the same chat server. HTTP based servers are usually stateless. If you use round robin for load balancing, the server that receives the message might not have a long-polling connection with the client who receives the message. •A server has no good way to tell if a client is disconnected. •It is inefficient. If a user does not chat much, long polling still makes periodic connections after timeouts.