Doc/howto/sockets.tex

\documentclass{howto}

\title{Socket Programming HOWTO}

\release{0.00}

\author{Gordon McMillan}
\authoraddress{\email{gmcm@hypernet.com}}

\begin{document}
\maketitle

\begin{abstract}
\noindent
Sockets are used nearly everywhere, but are one of the most severely
misunderstood technologies around. This is a 10,000 foot overview of
sockets. It's not really a tutorial - you'll still have work to do in
getting things operational. It doesn't cover the fine points (and there
are a lot of them), but I hope it will give you enough background to
begin using them decently.

This document is available from the Python HOWTO page at
\url{http://www.python.org/doc/howto}.

\end{abstract}

\tableofcontents

\section{Sockets}

Sockets are used nearly everywhere, but are one of the most severely
misunderstood technologies around. This is a 10,000 foot overview of
sockets. It's not really a tutorial - you'll still have work to do in
getting things working. It doesn't cover the fine points (and there
are a lot of them), but I hope it will give you enough background to
begin using them decently.

I'm only going to talk about INET sockets, but they account for at
least 99\% of the sockets in use. And I'll only talk about STREAM
sockets - unless you really know what you're doing (in which case this
HOWTO isn't for you!), you'll get better behavior and performance from
a STREAM socket than anything else. I will try to clear up the mystery
of what a socket is, as well as some hints on how to work with
blocking and non-blocking sockets. But I'll start by talking about
blocking sockets. You'll need to know how they work before dealing
with non-blocking sockets.

Part of the trouble with understanding these things is that "socket"
can mean a number of subtly different things, depending on context. So
first, let's make a distinction between a "client" socket - an
endpoint of a conversation, and a "server" socket, which is more like
a switchboard operator. The client application (your browser, for
example) uses "client" sockets exclusively; the web server it's
talking to uses both "server" sockets and "client" sockets.


\subsection{History}

Of the various forms of IPC (\emph{Inter Process Communication}),
sockets are by far the most popular.  On any given platform, there are
likely to be other forms of IPC that are faster, but for
cross-platform communication, sockets are about the only game in town.

They were invented in Berkeley as part of the BSD flavor of Unix. They
spread like wildfire with the Internet. With good reason --- the
combination of sockets with INET makes talking to arbitrary machines
around the world unbelievably easy (at least compared to other
schemes).  

\section{Creating a Socket}

Roughly speaking, when you clicked on the link that brought you to
this page, your browser did something like the following:

\begin{verbatim}
    #create an INET, STREAMing socket
    s = socket.socket(
        socket.AF_INET, socket.SOCK_STREAM)
    #now connect to the web server on port 80 
    # - the normal http port
    s.connect(("www.mcmillan-inc.com", 80))
\end{verbatim}

When the \code{connect} completes, the socket \code{s} can
now be used to send in a request for the text of this page. The same
socket will read the reply, and then be destroyed. That's right -
destroyed. Client sockets are normally only used for one exchange (or
a small set of sequential exchanges).

What happens in the web server is a bit more complex. First, the web
server creates a "server socket".

\begin{verbatim}
    #create an INET, STREAMing socket
    serversocket = socket.socket(
        socket.AF_INET, socket.SOCK_STREAM)
    #bind the socket to a public host, 
    # and a well-known port
    serversocket.bind((socket.gethostname(), 80))
    #become a server socket
    serversocket.listen(5)
\end{verbatim}

A couple things to notice: we used \code{socket.gethostname()}
so that the socket would be visible to the outside world. If we had
used \code{s.bind(('', 80))} or \code{s.bind(('localhost',
80))} or \code{s.bind(('127.0.0.1', 80))} we would still
have a "server" socket, but one that was only visible within the same
machine.

A second thing to note: low number ports are usually reserved for
"well known" services (HTTP, SNMP etc). If you're playing around, use
a nice high number (4 digits).

Finally, the argument to \code{listen} tells the socket library that
we want it to queue up as many as 5 connect requests (the normal max)
before refusing outside connections. If the rest of the code is
written properly, that should be plenty.

OK, now we have a "server" socket, listening on port 80. Now we enter
the mainloop of the web server:

\begin{verbatim}
    while 1:
        #accept connections from outside
        (clientsocket, address) = serversocket.accept()
        #now do something with the clientsocket
        #in this case, we'll pretend this is a threaded server
        ct = client_thread(clientsocket)
        ct.run()
\end{verbatim}

There's actually 3 general ways in which this loop could work -
dispatching a thread to handle \code{clientsocket}, create a new
process to handle \code{clientsocket}, or restructure this app
to use non-blocking sockets, and mulitplex between our "server" socket
and any active \code{clientsocket}s using
\code{select}. More about that later. The important thing to
understand now is this: this is \emph{all} a "server" socket
does. It doesn't send any data. It doesn't receive any data. It just
produces "client" sockets. Each \code{clientsocket} is created
in response to some \emph{other} "client" socket doing a
\code{connect()} to the host and port we're bound to. As soon as
we've created that \code{clientsocket}, we go back to listening
for more connections. The two "clients" are free to chat it up - they
are using some dynamically allocated port which will be recycled when
the conversation ends.

\subsection{IPC} If you need fast IPC between two processes
on one machine, you should look into whatever form of shared memory
the platform offers. A simple protocol based around shared memory and
locks or semaphores is by far the fastest technique.

If you do decide to use sockets, bind the "server" socket to
\code{'localhost'}. On most platforms, this will take a shortcut
around a couple of layers of network code and be quite a bit faster.


\section{Using a Socket}

The first thing to note, is that the web browser's "client" socket and
the web server's "client" socket are identical beasts. That is, this
is a "peer to peer" conversation. Or to put it another way, \emph{as the
designer, you will have to decide what the rules of etiquette are for
a conversation}. Normally, the \code{connect}ing socket
starts the conversation, by sending in a request, or perhaps a
signon. But that's a design decision - it's not a rule of sockets.

Now there are two sets of verbs to use for communication. You can use
\code{send} and \code{recv}, or you can transform your
client socket into a file-like beast and use \code{read} and
\code{write}. The latter is the way Java presents their
sockets. I'm not going to talk about it here, except to warn you that
you need to use \code{flush} on sockets. These are buffered
"files", and a common mistake is to \code{write} something, and
then \code{read} for a reply. Without a \code{flush} in
there, you may wait forever for the reply, because the request may
still be in your output buffer.

Now we come the major stumbling block of sockets - \code{send}
and \code{recv} operate on the network buffers. They do not
necessarily handle all the bytes you hand them (or expect from them),
because their major focus is handling the network buffers. In general,
they return when the associated network buffers have been filled
(\code{send}) or emptied (\code{recv}). They then tell you
how many bytes they handled. It is \emph{your} responsibility to call
them again until your message has been completely dealt with.

When a \code{recv} returns 0 bytes, it means the other side has
closed (or is in the process of closing) the connection.  You will not
receive any more data on this connection. Ever.  You may be able to
send data successfully; I'll talk about that some on the next page.

A protocol like HTTP uses a socket for only one transfer. The client
sends a request, the reads a reply.  That's it. The socket is
discarded. This means that a client can detect the end of the reply by
receiving 0 bytes.

But if you plan to reuse your socket for further transfers, you need
to realize that \emph{there is no "EOT" (End of Transfer) on a
socket.} I repeat: if a socket \code{send} or
\code{recv} returns after handling 0 bytes, the connection has
been broken.  If the connection has \emph{not} been broken, you may
wait on a \code{recv} forever, because the socket will
\emph{not} tell you that there's nothing more to read (for now).  Now
if you think about that a bit, you'll come to realize a fundamental
truth of sockets: \emph{messages must either be fixed length} (yuck),
\emph{or be delimited} (shrug), \emph{or indicate how long they are}
(much better), \emph{or end by shutting down the connection}. The
choice is entirely yours, (but some ways are righter than others).

Assuming you don't want to end the connection, the simplest solution
is a fixed length message:

\begin{verbatim}
    class mysocket:
        '''demonstration class only 
          - coded for clarity, not efficiency'''
        def __init__(self, sock=None):
            if sock is None:
                self.sock = socket.socket(
                    socket.AF_INET, socket.SOCK_STREAM)
            else:
                self.sock = sock

        def connect(self, host, port):
            self.sock.connect((host, port))

        def mysend(self, msg):
            totalsent = 0
            while totalsent < MSGLEN:
                sent = self.sock.send(msg[totalsent:])
                if sent == 0:
                    raise RuntimeError, \\
                        "socket connection broken"
                totalsent = totalsent + sent

        def myreceive(self):
            msg = ''
            while len(msg) < MSGLEN:
                chunk = self.sock.recv(MSGLEN-len(msg))
                if chunk == '':
                    raise RuntimeError, \\
                        "socket connection broken"
                msg = msg + chunk
            return msg
\end{verbatim}

The sending code here is usable for almost any messaging scheme - in
Python you send strings, and you can use \code{len()} to
determine its length (even if it has embedded \code{\e 0}
characters). It's mostly the receiving code that gets more
complex. (And in C, it's not much worse, except you can't use
\code{strlen} if the message has embedded \code{\e 0}s.)

The easiest enhancement is to make the first character of the message
an indicator of message type, and have the type determine the
length. Now you have two \code{recv}s - the first to get (at
least) that first character so you can look up the length, and the
second in a loop to get the rest. If you decide to go the delimited
route, you'll be receiving in some arbitrary chunk size, (4096 or 8192
is frequently a good match for network buffer sizes), and scanning
what you've received for a delimiter.

One complication to be aware of: if your conversational protocol
allows multiple messages to be sent back to back (without some kind of
reply), and you pass \code{recv} an arbitrary chunk size, you
may end up reading the start of a following message. You'll need to
put that aside and hold onto it, until it's needed.

Prefixing the message with it's length (say, as 5 numeric characters)
gets more complex, because (believe it or not), you may not get all 5
characters in one \code{recv}. In playing around, you'll get
away with it; but in high network loads, your code will very quickly
break unless you use two \code{recv} loops - the first to
determine the length, the second to get the data part of the
message. Nasty. This is also when you'll discover that
\code{send} does not always manage to get rid of everything in
one pass. And despite having read this, you will eventually get bit by
it!

In the interests of space, building your character, (and preserving my
competitive position), these enhancements are left as an exercise for
the reader. Lets move on to cleaning up.

\subsection{Binary Data}

It is perfectly possible to send binary data over a socket. The major
problem is that not all machines use the same formats for binary
data. For example, a Motorola chip will represent a 16 bit integer
with the value 1 as the two hex bytes 00 01. Intel and DEC, however,
are byte-reversed - that same 1 is 01 00. Socket libraries have calls
for converting 16 and 32 bit integers - \code{ntohl, htonl, ntohs,
htons} where "n" means \emph{network} and "h" means \emph{host},
"s" means \emph{short} and "l" means \emph{long}. Where network order
is host order, these do nothing, but where the machine is
byte-reversed, these swap the bytes around appropriately.

In these days of 32 bit machines, the ascii representation of binary
data is frequently smaller than the binary representation. That's
because a surprising amount of the time, all those longs have the
value 0, or maybe 1. The string "0" would be two bytes, while binary
is four. Of course, this doesn't fit well with fixed-length
messages. Decisions, decisions.

\section{Disconnecting}

Strictly speaking, you're supposed to use \code{shutdown} on a
socket before you \code{close} it.  The \code{shutdown} is
an advisory to the socket at the other end.  Depending on the argument
you pass it, it can mean "I'm not going to send anymore, but I'll
still listen", or "I'm not listening, good riddance!".  Most socket
libraries, however, are so used to programmers neglecting to use this
piece of etiquette that normally a \code{close} is the same as
\code{shutdown(); close()}.  So in most situations, an explicit
\code{shutdown} is not needed.

One way to use \code{shutdown} effectively is in an HTTP-like
exchange. The client sends a request and then does a
\code{shutdown(1)}. This tells the server "This client is done
sending, but can still receive."  The server can detect "EOF" by a
receive of 0 bytes. It can assume it has the complete request.  The
server sends a reply. If the \code{send} completes successfully
then, indeed, the client was still receiving.

Python takes the automatic shutdown a step further, and says that when a socket is garbage collected, it will automatically do a \code{close} if it's needed. But relying on this is a very bad habit. If your socket just disappears without doing a \code{close}, the socket at the other end may hang indefinitely, thinking you're just being slow. \emph{Please} \code{close} your sockets when you're done.


\subsection{When Sockets Die}

Probably the worst thing about using blocking sockets is what happens
when the other side comes down hard (without doing a
\code{close}). Your socket is likely to hang. SOCKSTREAM is a
reliable protocol, and it will wait a long, long time before giving up
on a connection. If you're using threads, the entire thread is
essentially dead. There's not much you can do about it. As long as you
aren't doing something dumb, like holding a lock while doing a
blocking read, the thread isn't really consuming much in the way of
resources. Do \emph{not} try to kill the thread - part of the reason
that threads are more efficient than processes is that they avoid the
overhead associated with the automatic recycling of resources. In
other words, if you do manage to kill the thread, your whole process
is likely to be screwed up.  

\section{Non-blocking Sockets}

If you've understood the preceeding, you already know most of what you
need to know about the mechanics of using sockets. You'll still use
the same calls, in much the same ways. It's just that, if you do it
right, your app will be almost inside-out.

In Python, you use \code{socket.setblocking(0)} to make it
non-blocking. In C, it's more complex, (for one thing, you'll need to
choose between the BSD flavor \code{O_NONBLOCK} and the almost
indistinguishable Posix flavor \code{O_NDELAY}, which is
completely different from \code{TCP_NODELAY}), but it's the
exact same idea. You do this after creating the socket, but before
using it. (Actually, if you're nuts, you can switch back and forth.)

The major mechanical difference is that \code{send},
\code{recv}, \code{connect} and \code{accept} can
return without having done anything. You have (of course) a number of
choices. You can check return code and error codes and generally drive
yourself crazy. If you don't believe me, try it sometime. Your app
will grow large, buggy and suck CPU. So let's skip the brain-dead
solutions and do it right.

Use \code{select}.

In C, coding \code{select} is fairly complex. In Python, it's a
piece of cake, but it's close enough to the C version that if you
understand \code{select} in Python, you'll have little trouble
with it in C.

\begin{verbatim}    ready_to_read, ready_to_write, in_error = \\
                   select.select(
                      potential_readers, 
                      potential_writers, 
                      potential_errs, 
                      timeout)
\end{verbatim}

You pass \code{select} three lists: the first contains all
sockets that you might want to try reading; the second all the sockets
you might want to try writing to, and the last (normally left empty)
those that you want to check for errors.  You should note that a
socket can go into more than one list. The \code{select} call is
blocking, but you can give it a timeout. This is generally a sensible
thing to do - give it a nice long timeout (say a minute) unless you
have good reason to do otherwise.

In return, you will get three lists. They have the sockets that are
actually readable, writable and in error. Each of these lists is a
subset (possbily empty) of the corresponding list you passed in. And
if you put a socket in more than one input list, it will only be (at
most) in one output list.

If a socket is in the output readable list, you can be
as-close-to-certain-as-we-ever-get-in-this-business that a
\code{recv} on that socket will return \emph{something}. Same
idea for the writable list. You'll be able to send
\emph{something}. Maybe not all you want to, but \emph{something} is
better than nothing. (Actually, any reasonably healthy socket will
return as writable - it just means outbound network buffer space is
available.)

If you have a "server" socket, put it in the potential_readers
list. If it comes out in the readable list, your \code{accept}
will (almost certainly) work. If you have created a new socket to
\code{connect} to someone else, put it in the ptoential_writers
list. If it shows up in the writable list, you have a decent chance
that it has connected.

One very nasty problem with \code{select}: if somewhere in those
input lists of sockets is one which has died a nasty death, the
\code{select} will fail. You then need to loop through every
single damn socket in all those lists and do a
\code{select([sock],[],[],0)} until you find the bad one. That
timeout of 0 means it won't take long, but it's ugly.

Actually, \code{select} can be handy even with blocking sockets.
It's one way of determining whether you will block - the socket
returns as readable when there's something in the buffers.  However,
this still doesn't help with the problem of determining whether the
other end is done, or just busy with something else.

\textbf{Portability alert}: On Unix, \code{select} works both with
the sockets and files. Don't try this on Windows. On Windows,
\code{select} works with sockets only. Also note that in C, many
of the more advanced socket options are done differently on
Windows. In fact, on Windows I usually use threads (which work very,
very well) with my sockets. Face it, if you want any kind of
performance, your code will look very different on Windows than on
Unix. (I haven't the foggiest how you do this stuff on a Mac.)

\subsection{Performance}

There's no question that the fastest sockets code uses non-blocking
sockets and select to multiplex them. You can put together something
that will saturate a LAN connection without putting any strain on the
CPU. The trouble is that an app written this way can't do much of
anything else - it needs to be ready to shuffle bytes around at all
times.

Assuming that your app is actually supposed to do something more than
that, threading is the optimal solution, (and using non-blocking
sockets will be faster than using blocking sockets). Unfortunately,
threading support in Unixes varies both in API and quality. So the
normal Unix solution is to fork a subprocess to deal with each
connection. The overhead for this is significant (and don't do this on
Windows - the overhead of process creation is enormous there). It also
means that unless each subprocess is completely independent, you'll
need to use another form of IPC, say a pipe, or shared memory and
semaphores, to communicate between the parent and child processes.

Finally, remember that even though blocking sockets are somewhat
slower than non-blocking, in many cases they are the "right"
solution. After all, if your app is driven by the data it receives
over a socket, there's not much sense in complicating the logic just
so your app can wait on \code{select} instead of
\code{recv}.

\end{document}