# 11.18 `asyncore` -- Asynchronous socket handler

This module provides the basic infrastructure for writing asynchronous socket service clients and servers.

There are only two ways to have a program on a single processor do ``more than one thing at a time.'' Multi-threaded programming is the simplest and most popular way to do it, but there is another very different technique, that lets you have nearly all the advantages of multi-threading, without actually using multiple threads. It's really only practical if your program is largely I/O bound. If your program is CPU bound, then pre-emptive scheduled threads are probably what you really need. Network servers are rarely CPU-bound, however.

If your operating system supports the `select()` system call in its I/O library (and nearly all do), then you can use it to juggle multiple communication channels at once; doing other work while your I/O is taking place in the ``background.'' Although this strategy can seem strange and complex, especially at first, it is in many ways easier to understand and control than multi-threaded programming. The module documented here solves many of the difficult problems for you, making the task of building sophisticated high-performance network servers and clients a snap.

`dispatcher` ()
  The first class we will introduce is the `dispatcher` class. This is a thin wrapper around a low-level socket object. To make it more useful, it has a few methods for event-handling on it. Otherwise, it can be treated as a normal non-blocking socket object.

  The direct interface between the select loop and the socket object are the `handle_read_event()` and `handle_write_event()` methods. These are called whenever an object `fires' that event.

  The firing of these low-level events can tell us whether certain higher-level events have taken place, depending on the timing and the state of the connection. For example, if we have asked for a socket to connect to another host, we know that the connection has been made when the socket fires a write event (at this point you know that you may write to it with the expectation of success). The implied higher-level events are:

  Event | Description
  --- | ---
  `handle_connect()` | Implied by a write event
  `handle_close()` | Implied by a read event with no data available
  `handle_accept()` | Implied by a read event on a listening socket

This set of user-level events is larger than the basics. The full set of methods that can be overridden in your subclass are:

`handle_read` ()
  Called when there is new data to be read from a socket.

`handle_write` ()
  Called when there is an attempt to write data to the object. Often this method will implement the necessary buffering for performance. For example:

        def handle_write(self):
            sent = self.send(self.buffer)
            self.buffer = self.buffer[sent:]

`handle_expt` ()
  Called when there is out of band (OOB) data for a socket connection. This will almost never happen, as OOB is tenuously supported and rarely used.

`handle_connect` ()
  Called when the socket actually makes a connection. This might be used to send a ``welcome'' banner, or something similar.

`handle_close` ()
  Called when the socket is closed.

`handle_accept` ()
  Called on listening sockets when they actually accept a new connection.

`readable` ()
  Each time through the `select()` loop, the set of sockets is scanned, and this method is called to see if there is any interest in reading. The default method simply returns `1`, indicating that by default, all channels will be interested.

`writeable` ()
  Each time through the `select()` loop, the set of sockets is scanned, and this method is called to see if there is any interest in writing. The default method simply returns `1`, indicating that by default, all channels will be interested.

In addition, there are the basic methods needed to construct and manipulate ``channels,'' which are what we will call the socket connections in this context. Note that most of these are nearly identical to their socket partners.

`create_socket` (family, type)
  This is identical to the creation of a normal socket, and will use the same options for creation. Refer to the `socket` documentation for information on creating sockets.

`connect` (address)
  As with the normal socket object, address is a tuple with the first element the host to connect to, and the second the port.

`send` (data)
  Send data out the socket.

`recv` (buffer_size)
  Read at most buffer_size bytes from the socket.

`listen` ([backlog])
  Listen for connections made to the socket. The backlog argument specifies the maximum number of queued connections and should be at least 1; the maximum value is system-dependent (usually 5).

`bind` (address)
  Bind the socket to address. The socket must not already be bound. (The format of address depends on the address family -- see above.)

`accept` ()
  Accept a connection. The socket must be bound to an address and listening for connections. The return value is a pair `(conn, address)` where conn is a new socket object usable to send and receive data on the connection, and address is the address bound to the socket on the other end of the connection.

`close` ()
  Close the socket. All future operations on the socket object will fail. The remote end will receive no more data (after queued data is flushed). Sockets are automatically closed when they are garbage-collected.

Subsections

- 11.18.1 Example basic HTTP client