197 lines
7.4 KiB
TeX
197 lines
7.4 KiB
TeX
\section{\module{asyncore} ---
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Asynchronous socket handler}
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\declaremodule{builtin}{asyncore}
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\modulesynopsis{A base class for developing asynchronous socket
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handling services.}
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\moduleauthor{Sam Rushing}{rushing@nightmare.com}
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\sectionauthor{Christopher Petrilli}{petrilli@amber.org}
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% Heavily adapted from original documentation by Sam Rushing.
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This module provides the basic infrastructure for writing asynchronous
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socket service clients and servers.
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There are only two ways to have a program on a single processor do
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``more than one thing at a time.'' Multi-threaded programming is the
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simplest and most popular way to do it, but there is another very
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different technique, that lets you have nearly all the advantages of
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multi-threading, without actually using multiple threads. It's really
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only practical if your program is largely I/O bound. If your program
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is processor bound, then pre-emptive scheduled threads are probably what
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you really need. Network servers are rarely processor bound, however.
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If your operating system supports the \cfunction{select()} system call
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in its I/O library (and nearly all do), then you can use it to juggle
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multiple communication channels at once; doing other work while your
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I/O is taking place in the ``background.'' Although this strategy can
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seem strange and complex, especially at first, it is in many ways
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easier to understand and control than multi-threaded programming.
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The module documented here solves many of the difficult problems for
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you, making the task of building sophisticated high-performance
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network servers and clients a snap.
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\begin{classdesc}{dispatcher}{}
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The first class we will introduce is the \class{dispatcher} class.
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This is a thin wrapper around a low-level socket object. To make
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it more useful, it has a few methods for event-handling on it.
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Otherwise, it can be treated as a normal non-blocking socket object.
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The direct interface between the select loop and the socket object
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are the \method{handle_read_event()} and
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\method{handle_write_event()} methods. These are called whenever an
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object `fires' that event.
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The firing of these low-level events can tell us whether certain
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higher-level events have taken place, depending on the timing and
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the state of the connection. For example, if we have asked for a
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socket to connect to another host, we know that the connection has
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been made when the socket fires a write event (at this point you
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know that you may write to it with the expectation of success).
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The implied higher-level events are:
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\begin{tableii}{l|l}{code}{Event}{Description}
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\lineii{handle_connect()}{Implied by a write event}
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\lineii{handle_close()}{Implied by a read event with no data available}
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\lineii{handle_accept()}{Implied by a read event on a listening socket}
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\end{tableii}
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\end{classdesc}
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This set of user-level events is larger than the basics. The
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full set of methods that can be overridden in your subclass are:
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\begin{methoddesc}{handle_read}{}
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Called when there is new data to be read from a socket.
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\end{methoddesc}
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\begin{methoddesc}{handle_write}{}
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Called when there is an attempt to write data to the object.
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Often this method will implement the necessary buffering for
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performance. For example:
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\begin{verbatim}
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def handle_write(self):
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sent = self.send(self.buffer)
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self.buffer = self.buffer[sent:]
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\end{verbatim}
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\end{methoddesc}
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\begin{methoddesc}{handle_expt}{}
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Called when there is out of band (OOB) data for a socket
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connection. This will almost never happen, as OOB is
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tenuously supported and rarely used.
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\end{methoddesc}
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\begin{methoddesc}{handle_connect}{}
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Called when the socket actually makes a connection. This
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might be used to send a ``welcome'' banner, or something
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similar.
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\end{methoddesc}
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\begin{methoddesc}{handle_close}{}
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Called when the socket is closed.
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\end{methoddesc}
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\begin{methoddesc}{handle_accept}{}
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Called on listening sockets when they actually accept a new
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connection.
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\end{methoddesc}
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\begin{methoddesc}{readable}{}
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Each time through the \method{select()} loop, the set of sockets
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is scanned, and this method is called to see if there is any
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interest in reading. The default method simply returns \code{1},
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indicating that by default, all channels will be interested.
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\end{methoddesc}
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\begin{methoddesc}{writable}{}
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Each time through the \method{select()} loop, the set of sockets
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is scanned, and this method is called to see if there is any
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interest in writing. The default method simply returns \code{1},
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indicating that by default, all channels will be interested.
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\end{methoddesc}
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In addition, there are the basic methods needed to construct and
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manipulate ``channels,'' which are what we will call the socket
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connections in this context. Note that most of these are nearly
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identical to their socket partners.
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\begin{methoddesc}{create_socket}{family, type}
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This is identical to the creation of a normal socket, and
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will use the same options for creation. Refer to the
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\refmodule{socket} documentation for information on creating
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sockets.
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\end{methoddesc}
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\begin{methoddesc}{connect}{address}
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As with the normal socket object, \var{address} is a
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tuple with the first element the host to connect to, and the
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second the port.
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\end{methoddesc}
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\begin{methoddesc}{send}{data}
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Send \var{data} out the socket.
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\end{methoddesc}
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\begin{methoddesc}{recv}{buffer_size}
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Read at most \var{buffer_size} bytes from the socket.
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\end{methoddesc}
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\begin{methoddesc}{listen}{backlog}
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Listen for connections made to the socket. The \var{backlog}
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argument specifies the maximum number of queued connections
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and should be at least 1; the maximum value is
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system-dependent (usually 5).
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\end{methoddesc}
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\begin{methoddesc}{bind}{address}
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Bind the socket to \var{address}. The socket must not already
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be bound. (The format of \var{address} depends on the address
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family --- see above.)
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\end{methoddesc}
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\begin{methoddesc}{accept}{}
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Accept a connection. The socket must be bound to an address
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and listening for connections. The return value is a pair
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\code{(\var{conn}, \var{address})} where \var{conn} is a
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\emph{new} socket object usable to send and receive data on
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the connection, and \var{address} is the address bound to the
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socket on the other end of the connection.
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\end{methoddesc}
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\begin{methoddesc}{close}{}
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Close the socket. All future operations on the socket object
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will fail. The remote end will receive no more data (after
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queued data is flushed). Sockets are automatically closed
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when they are garbage-collected.
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\end{methoddesc}
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\subsection{Example basic HTTP client \label{asyncore-example}}
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As a basic example, below is a very basic HTTP client that uses the
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\class{dispatcher} class to implement its socket handling:
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\begin{verbatim}
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class http_client(asyncore.dispatcher):
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def __init__(self, host,path):
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asyncore.dispatcher.__init__(self)
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self.path = path
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self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
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self.connect( (host, 80) )
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self.buffer = 'GET %s HTTP/1.0\r\n\r\n' % self.path
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def handle_connect(self):
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pass
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def handle_read(self):
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data = self.recv(8192)
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print data
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def writable(self):
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return (len(self.buffer) > 0)
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def handle_write(self):
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sent = self.send(self.buffer)
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self.buffer = self.buffer[sent:]
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\end{verbatim}
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