Lecture 9: Socket Programming Interface

Multi-Threaded Servers

The server in the previous slide was single threaded, meaning that only one client could be connected to it at any instant. This is a serious restriction in a "Real World(tm)" environment.

Whilst the Java code for a multi-threaded server is too complex to include into the lecture, the basic concept is that the accept() method returns a new, connected, socket. A program can start up a new thread of execution which handles communications using the connected socket, whilst allowing the "main program" to again go back to "accepting connections".

Conceptual Conundrum -- how can the newly-created program thread perform communications on (eg) port 7277 at the same time as the server is "waiting for connections" at the same port number?

The answer to this is very subtle. In fact, an active TCP connection is characterised by a 4-tuple, consisting of the IP addresses of each of the computers involved, plus both of the port numbers (both client and server) in use. Because this 4-tuple is unique for every TCP connection (Exercise: why?), it's possible to have many server threads serving client requests simultaneously (or, more correctly, concurrently)

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