Your Poetry is Served

Part 11: Your Poetry is Served

This continues the introduction started here. You can find an index to the entire series here.

A Twisted Poetry Server

Now that we’ve learned so much about writing clients with Twisted, let’s turn around and re-implement our poetry server with Twisted too. And thanks to the generality of Twisted’s abstractions, it turns out we’ve already learned almost everything we need to know. Take a look at our Twisted poetry server located in twisted-server-1/fastpoetry.py. It’s called fastpoetry because this server sends the poetry as fast as possible, without any delays at all. Note there’s significantly less code than in the client!

Let’s take the pieces of the server one at a time. First, the PoetryProtocol:

class PoetryProtocol(Protocol):

    def connectionMade(self):
        self.transport.write(self.factory.poem)
        self.transport.loseConnection()

Like the client, the server uses a separate Protocol instance to manage each different connection (in this case, connections that clients make to the server). Here the Protocol is implementing the server-side portion of our poetry protocol. Since our wire protocol is strictly one-way, the server’s Protocol instance only needs to be concerned with sending data. If you recall, our wire protocol requires the server to start sending the poem immediately after the connection is made, so we implement the connectionMade method, a callback that is invoked after a Protocol instance is connected to a Transport.

Our method tells the Transport to do two things: send the entire text of the poem (self.transport.write) and close the connection (self.transport.loseConnection). Of course, both of those operations are asynchronous. So the call to write() really means “eventually send all this data to the client” and the call to loseConnection() really means “close this connection once all the data I’ve asked you to write has been written”.

As you can see, the Protocol retrieves the text of the poem from the Factory, so let’s look at that next:

class PoetryFactory(ServerFactory):

    protocol = PoetryProtocol

    def __init__(self, poem):
        self.poem = poem

Now that’s pretty darn simple. Our factory’s only real job, besides making PoetryProtocol instances on demand, is storing the poem that each PoetryProtocol sends to a client.

Notice that we are sub-classing ServerFactory instead of ClientFactory. Since our server is passively listening for connections instead of actively making them, we don’t need the extra methods ClientFactory provides. How can we be sure of that? Because we are using the listenTCP reactor method and the documentation for that method explains that the factory argument should be an instance of ServerFactory.

Here’s the main function where we call listenTCP:

def main():
    options, poetry_file = parse_args()

    poem = open(poetry_file).read()

    factory = PoetryFactory(poem)

    from twisted.internet import reactor

    port = reactor.listenTCP(options.port or 0, factory,
                             interface=options.iface)

    print 'Serving %s on %s.' % (poetry_file, port.getHost())

    reactor.run()

It basically does three things:

1. Read the text of the poem we are going to serve.
2. Create a PoetryFactory with that poem.
3. Use listenTCP to tell Twisted to listen for connections on a port, and use our factory to make the protocol instances for each new connection.

After that, the only thing left to do is tell the reactor to start running the loop. You can use any of our previous poetry clients (or just netcat) to test out the server.

Discussion

Recall Figure 8 and Figure 9 from Part 5. Those figures illustrated how a new Protocol instance is created and initialized after Twisted makes a new connection on our behalf. It turns out the same mechanism is used when Twisted accepts a new incoming connection on a port we are listening on. That’s why both connectTCP and listenTCP require factory arguments.

One thing we didn’t show in Figure 9 is that the connectionMade callback is also called as part of Protocol initialization. This happens no matter what, but we didn’t need to use it in the client code. And the Protocol methods that we did use in the client aren’t used in the server’s implementation. So if we wanted to, we could make a shared library with a single PoetryProtocol that works for both clients and servers. That’s actually the way things are typically done in Twisted itself. For example, the NetstringReceiver Protocol can both read and write netstrings from and to a Transport.

We skipped writing a low-level version of our server, but let’s think about what sort of things are going on under the hood. First, calling listenTCP tells Twisted to create a listening socket and add it to the event loop. An “event” on a listening socket doesn’t mean there is data to read; instead it means there is a client waiting to connect to us.

Twisted will automatically accept incoming connection requests, thus creating a new client socket that links the server directly to an individual client. That client socket is also added to the event loop, and Twisted creates a new Transport and (via the PoetryFactory) a new PoetryProtocol instance to service that specific client. So the Protocol instances are always connected to client sockets, never to the listening socket.

We can visualize all of this in Figure 26:

In the figure there are three clients currently connected to the poetry server. Each Transport represents a single client socket, and the listening socket makes a total of four file descriptors for the select loop to monitor. When a client is disconnected the associated Transport and PoetryProtocol will be dereferenced and garbage-collected (assuming we haven’t stashed a reference to one of them somewhere, a practice we should avoid to prevent memory leaks). The PoetryFactory, meanwhile, will stick around as long as we keep listening for new connections which, in our poetry server, is forever. Like the beauty of poetry. Or something. At any rate, Figure 26 certainly cuts a fine figure of a Figure, doesn’t it?

The client sockets and their associated Python objects won’t live very long if the poem we are serving is relatively short. But with a large poem and a really busy poetry server we could end up with hundreds or thousands of simultaneous clients. And that’s OK — Twisted has no built-in limits on the number of connections it can handle. Of course, as you increase the load on any server, at some point you will find it cannot keep up or some internal OS limit is reached. For highly-loaded servers, careful measurement and testing is the order of the day.

Twisted also imposes no limit on the number of ports we can listen on. In fact, a single Twisted process could listen on dozens of ports and provide a different service on each one (by using a different factory class for each listenTCP call). And with careful design, whether you provide multiple services with a single Twisted process or several is a decision you could potentially even postpone to the deployment phase.

There’s a couple things our server is missing. First of all, it doesn’t generate any logs that might help us debug problems or analyze our network traffic. Furthermore, the server doesn’t run as a daemon, making it vulnerable to death by accidental Ctrl-C (or just logging out). We’ll fix both those problems in a future Part but first, in Part 12, we’ll write another server to perform poetry transformation.

Suggested Exercises

1. Write an asynchronous poetry server without using Twisted, like we did for the client in Part 2. Note that listening sockets need to be monitored for reading and a “readable” listening socket means we can accept a new client socket.
2. Write a low-level asynchronous poetry server using Twisted, but without using listenTCP or protocols, transports, and factories, like we did for the client in Part 4. So you’ll still be making your own sockets, but you can use the Twisted reactor instead of your own select loop.
3. Make the high-level version of the Twisted poetry server a “slow server” by using callLater or LoopingCall to make multiple calls to transport.write(). Add the --num-bytes and --delay command line options supported by the blocking server. Don’t forget to handle the case where the client disconnects before receiving the whole poem.
4. Extend the high-level Twisted server so it can serve multiple poems (on different ports).
5. What are some reasons to serve multiple services from the same Twisted process? What are some reasons not to?