In the previous chapter, we looked at exchanging data between devices by using the sockets interface. In this chapter, we're going to use sockets to build network applications. Sockets follow one of the main models of computer networking, that is, the client/server model. We'll look at this with a focus on structuring server applications. We'll cover the following topics:
- Designing a simple protocol
- Building an echo server and client
- Building a chat server and client
- Multithreaded and event-driven server architectures
- The
eventletandasynciolibraries
The examples in this chapter are best run on Linux or a Unix operating system. The Windows sockets implementation has some idiosyncrasies, and these can create some error conditions, which we will not be covering here. Note that Windows does not support the poll interface that we'll use in one example. If you do use Windows, then you'll probably need to use ctrl + break to kill these processes in the console, rather than using ctrl - c because Python in a Windows command prompt doesn't respond to ctrl – c when it's blocking on a socket send or receive, which will be quite often in this chapter! (and if, like me, you're unfortunate enough to try testing these on a Windows laptop without a break key, then be prepared to get very familiar with the Windows Task Manager's End task button).
The basic setup in the client/server model is one device, the server that runs a service and patiently waits for clients to connect and make requests to the service. A 24-hour grocery shop may be a real world analogy. The shop waits for customers to come in and when they do, they request certain products, purchase them and leave. The shop might advertise itself so people know where to find it, but the actual transactions happen while the customers are visiting the shop.
A typical computing example is a web server. The server listens on a TCP port for clients that need its web pages. When a client, for example a web browser, requires a web page that the server hosts, it connects to the server and then makes a request for that page. The server replies with the content of the page and then the client disconnects. The server advertises itself by having a hostname, which the clients can use to discover the IP address so that they can connect to it.
In both of these situations, it is the client that initiates any interaction – the server is purely responsive to that interaction. So, the needs of the programs that run on the client and server are quite different.
Client programs are typically oriented towards the interface between the user and the service. They retrieve and display the service, and allow the user to interact with it. Server programs are written to stay running for indefinite periods of time, to be stable, to efficiently deliver the service to the clients that are requesting it, and to potentially handle a large number of simultaneous connections with a minimal impact on the experience of any one client.
In this chapter, we will look at this model by writing a simple echo server and client, and then upgrading it to a chat server, which can handle a session with multiple clients. The socket module in Python perfectly suits this task.