/Doc/howto/sockets.rst
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1**************************** 2 Socket Programming HOWTO 3**************************** 4 5:Author: Gordon McMillan 6 7 8.. topic:: Abstract 9 10 Sockets are used nearly everywhere, but are one of the most severely 11 misunderstood technologies around. This is a 10,000 foot overview of sockets. 12 It's not really a tutorial - you'll still have work to do in getting things 13 operational. It doesn't cover the fine points (and there are a lot of them), but 14 I hope it will give you enough background to begin using them decently. 15 16 17Sockets 18======= 19 20Sockets are used nearly everywhere, but are one of the most severely 21misunderstood technologies around. This is a 10,000 foot overview of sockets. 22It's not really a tutorial - you'll still have work to do in getting things 23working. It doesn't cover the fine points (and there are a lot of them), but I 24hope it will give you enough background to begin using them decently. 25 26I'm only going to talk about INET sockets, but they account for at least 99% of 27the sockets in use. And I'll only talk about STREAM sockets - unless you really 28know what you're doing (in which case this HOWTO isn't for you!), you'll get 29better behavior and performance from a STREAM socket than anything else. I will 30try to clear up the mystery of what a socket is, as well as some hints on how to 31work with blocking and non-blocking sockets. But I'll start by talking about 32blocking sockets. You'll need to know how they work before dealing with 33non-blocking sockets. 34 35Part of the trouble with understanding these things is that "socket" can mean a 36number of subtly different things, depending on context. So first, let's make a 37distinction between a "client" socket - an endpoint of a conversation, and a 38"server" socket, which is more like a switchboard operator. The client 39application (your browser, for example) uses "client" sockets exclusively; the 40web server it's talking to uses both "server" sockets and "client" sockets. 41 42 43History 44------- 45 46Of the various forms of IPC (*Inter Process Communication*), sockets are by far 47the most popular. On any given platform, there are likely to be other forms of 48IPC that are faster, but for cross-platform communication, sockets are about the 49only game in town. 50 51They were invented in Berkeley as part of the BSD flavor of Unix. They spread 52like wildfire with the Internet. With good reason --- the combination of sockets 53with INET makes talking to arbitrary machines around the world unbelievably easy 54(at least compared to other schemes). 55 56 57Creating a Socket 58================= 59 60Roughly speaking, when you clicked on the link that brought you to this page, 61your browser did something like the following:: 62 63 #create an INET, STREAMing socket 64 s = socket.socket( 65 socket.AF_INET, socket.SOCK_STREAM) 66 #now connect to the web server on port 80 67 # - the normal http port 68 s.connect(("www.mcmillan-inc.com", 80)) 69 70When the ``connect`` completes, the socket ``s`` can now be used to send in a 71request for the text of this page. The same socket will read the reply, and then 72be destroyed. That's right - destroyed. Client sockets are normally only used 73for one exchange (or a small set of sequential exchanges). 74 75What happens in the web server is a bit more complex. First, the web server 76creates a "server socket". :: 77 78 #create an INET, STREAMing socket 79 serversocket = socket.socket( 80 socket.AF_INET, socket.SOCK_STREAM) 81 #bind the socket to a public host, 82 # and a well-known port 83 serversocket.bind((socket.gethostname(), 80)) 84 #become a server socket 85 serversocket.listen(5) 86 87A couple things to notice: we used ``socket.gethostname()`` so that the socket 88would be visible to the outside world. If we had used ``s.bind(('', 80))`` or 89``s.bind(('localhost', 80))`` or ``s.bind(('127.0.0.1', 80))`` we would still 90have a "server" socket, but one that was only visible within the same machine. 91 92A second thing to note: low number ports are usually reserved for "well known" 93services (HTTP, SNMP etc). If you're playing around, use a nice high number (4 94digits). 95 96Finally, the argument to ``listen`` tells the socket library that we want it to 97queue up as many as 5 connect requests (the normal max) before refusing outside 98connections. If the rest of the code is written properly, that should be plenty. 99 100OK, now we have a "server" socket, listening on port 80. Now we enter the 101mainloop of the web server:: 102 103 while 1: 104 #accept connections from outside 105 (clientsocket, address) = serversocket.accept() 106 #now do something with the clientsocket 107 #in this case, we'll pretend this is a threaded server 108 ct = client_thread(clientsocket) 109 ct.run() 110 111There's actually 3 general ways in which this loop could work - dispatching a 112thread to handle ``clientsocket``, create a new process to handle 113``clientsocket``, or restructure this app to use non-blocking sockets, and 114mulitplex between our "server" socket and any active ``clientsocket``\ s using 115``select``. More about that later. The important thing to understand now is 116this: this is *all* a "server" socket does. It doesn't send any data. It doesn't 117receive any data. It just produces "client" sockets. Each ``clientsocket`` is 118created in response to some *other* "client" socket doing a ``connect()`` to the 119host and port we're bound to. As soon as we've created that ``clientsocket``, we 120go back to listening for more connections. The two "clients" are free to chat it 121up - they are using some dynamically allocated port which will be recycled when 122the conversation ends. 123 124 125IPC 126--- 127 128If you need fast IPC between two processes on one machine, you should look into 129whatever form of shared memory the platform offers. A simple protocol based 130around shared memory and locks or semaphores is by far the fastest technique. 131 132If you do decide to use sockets, bind the "server" socket to ``'localhost'``. On 133most platforms, this will take a shortcut around a couple of layers of network 134code and be quite a bit faster. 135 136 137Using a Socket 138============== 139 140The first thing to note, is that the web browser's "client" socket and the web 141server's "client" socket are identical beasts. That is, this is a "peer to peer" 142conversation. Or to put it another way, *as the designer, you will have to 143decide what the rules of etiquette are for a conversation*. Normally, the 144``connect``\ ing socket starts the conversation, by sending in a request, or 145perhaps a signon. But that's a design decision - it's not a rule of sockets. 146 147Now there are two sets of verbs to use for communication. You can use ``send`` 148and ``recv``, or you can transform your client socket into a file-like beast and 149use ``read`` and ``write``. The latter is the way Java presents their sockets. 150I'm not going to talk about it here, except to warn you that you need to use 151``flush`` on sockets. These are buffered "files", and a common mistake is to 152``write`` something, and then ``read`` for a reply. Without a ``flush`` in 153there, you may wait forever for the reply, because the request may still be in 154your output buffer. 155 156Now we come the major stumbling block of sockets - ``send`` and ``recv`` operate 157on the network buffers. They do not necessarily handle all the bytes you hand 158them (or expect from them), because their major focus is handling the network 159buffers. In general, they return when the associated network buffers have been 160filled (``send``) or emptied (``recv``). They then tell you how many bytes they 161handled. It is *your* responsibility to call them again until your message has 162been completely dealt with. 163 164When a ``recv`` returns 0 bytes, it means the other side has closed (or is in 165the process of closing) the connection. You will not receive any more data on 166this connection. Ever. You may be able to send data successfully; I'll talk 167about that some on the next page. 168 169A protocol like HTTP uses a socket for only one transfer. The client sends a 170request, the reads a reply. That's it. The socket is discarded. This means that 171a client can detect the end of the reply by receiving 0 bytes. 172 173But if you plan to reuse your socket for further transfers, you need to realize 174that *there is no "EOT" (End of Transfer) on a socket.* I repeat: if a socket 175``send`` or ``recv`` returns after handling 0 bytes, the connection has been 176broken. If the connection has *not* been broken, you may wait on a ``recv`` 177forever, because the socket will *not* tell you that there's nothing more to 178read (for now). Now if you think about that a bit, you'll come to realize a 179fundamental truth of sockets: *messages must either be fixed length* (yuck), *or 180be delimited* (shrug), *or indicate how long they are* (much better), *or end by 181shutting down the connection*. The choice is entirely yours, (but some ways are 182righter than others). 183 184Assuming you don't want to end the connection, the simplest solution is a fixed 185length message:: 186 187 class mysocket: 188 '''demonstration class only 189 - coded for clarity, not efficiency 190 ''' 191 192 def __init__(self, sock=None): 193 if sock is None: 194 self.sock = socket.socket( 195 socket.AF_INET, socket.SOCK_STREAM) 196 else: 197 self.sock = sock 198 199 def connect(self, host, port): 200 self.sock.connect((host, port)) 201 202 def mysend(self, msg): 203 totalsent = 0 204 while totalsent < MSGLEN: 205 sent = self.sock.send(msg[totalsent:]) 206 if sent == 0: 207 raise RuntimeError, \ 208 "socket connection broken" 209 totalsent = totalsent + sent 210 211 def myreceive(self): 212 msg = '' 213 while len(msg) < MSGLEN: 214 chunk = self.sock.recv(MSGLEN-len(msg)) 215 if chunk == '': 216 raise RuntimeError, \ 217 "socket connection broken" 218 msg = msg + chunk 219 return msg 220 221The sending code here is usable for almost any messaging scheme - in Python you 222send strings, and you can use ``len()`` to determine its length (even if it has 223embedded ``\0`` characters). It's mostly the receiving code that gets more 224complex. (And in C, it's not much worse, except you can't use ``strlen`` if the 225message has embedded ``\0``\ s.) 226 227The easiest enhancement is to make the first character of the message an 228indicator of message type, and have the type determine the length. Now you have 229two ``recv``\ s - the first to get (at least) that first character so you can 230look up the length, and the second in a loop to get the rest. If you decide to 231go the delimited route, you'll be receiving in some arbitrary chunk size, (4096 232or 8192 is frequently a good match for network buffer sizes), and scanning what 233you've received for a delimiter. 234 235One complication to be aware of: if your conversational protocol allows multiple 236messages to be sent back to back (without some kind of reply), and you pass 237``recv`` an arbitrary chunk size, you may end up reading the start of a 238following message. You'll need to put that aside and hold onto it, until it's 239needed. 240 241Prefixing the message with it's length (say, as 5 numeric characters) gets more 242complex, because (believe it or not), you may not get all 5 characters in one 243``recv``. In playing around, you'll get away with it; but in high network loads, 244your code will very quickly break unless you use two ``recv`` loops - the first 245to determine the length, the second to get the data part of the message. Nasty. 246This is also when you'll discover that ``send`` does not always manage to get 247rid of everything in one pass. And despite having read this, you will eventually 248get bit by it! 249 250In the interests of space, building your character, (and preserving my 251competitive position), these enhancements are left as an exercise for the 252reader. Lets move on to cleaning up. 253 254 255Binary Data 256----------- 257 258It is perfectly possible to send binary data over a socket. The major problem is 259that not all machines use the same formats for binary data. For example, a 260Motorola chip will represent a 16 bit integer with the value 1 as the two hex 261bytes 00 01. Intel and DEC, however, are byte-reversed - that same 1 is 01 00. 262Socket libraries have calls for converting 16 and 32 bit integers - ``ntohl, 263htonl, ntohs, htons`` where "n" means *network* and "h" means *host*, "s" means 264*short* and "l" means *long*. Where network order is host order, these do 265nothing, but where the machine is byte-reversed, these swap the bytes around 266appropriately. 267 268In these days of 32 bit machines, the ascii representation of binary data is 269frequently smaller than the binary representation. That's because a surprising 270amount of the time, all those longs have the value 0, or maybe 1. The string "0" 271would be two bytes, while binary is four. Of course, this doesn't fit well with 272fixed-length messages. Decisions, decisions. 273 274 275Disconnecting 276============= 277 278Strictly speaking, you're supposed to use ``shutdown`` on a socket before you 279``close`` it. The ``shutdown`` is an advisory to the socket at the other end. 280Depending on the argument you pass it, it can mean "I'm not going to send 281anymore, but I'll still listen", or "I'm not listening, good riddance!". Most 282socket libraries, however, are so used to programmers neglecting to use this 283piece of etiquette that normally a ``close`` is the same as ``shutdown(); 284close()``. So in most situations, an explicit ``shutdown`` is not needed. 285 286One way to use ``shutdown`` effectively is in an HTTP-like exchange. The client 287sends a request and then does a ``shutdown(1)``. This tells the server "This 288client is done sending, but can still receive." The server can detect "EOF" by 289a receive of 0 bytes. It can assume it has the complete request. The server 290sends a reply. If the ``send`` completes successfully then, indeed, the client 291was still receiving. 292 293Python takes the automatic shutdown a step further, and says that when a socket 294is garbage collected, it will automatically do a ``close`` if it's needed. But 295relying on this is a very bad habit. If your socket just disappears without 296doing a ``close``, the socket at the other end may hang indefinitely, thinking 297you're just being slow. *Please* ``close`` your sockets when you're done. 298 299 300When Sockets Die 301---------------- 302 303Probably the worst thing about using blocking sockets is what happens when the 304other side comes down hard (without doing a ``close``). Your socket is likely to 305hang. SOCKSTREAM is a reliable protocol, and it will wait a long, long time 306before giving up on a connection. If you're using threads, the entire thread is 307essentially dead. There's not much you can do about it. As long as you aren't 308doing something dumb, like holding a lock while doing a blocking read, the 309thread isn't really consuming much in the way of resources. Do *not* try to kill 310the thread - part of the reason that threads are more efficient than processes 311is that they avoid the overhead associated with the automatic recycling of 312resources. In other words, if you do manage to kill the thread, your whole 313process is likely to be screwed up. 314 315 316Non-blocking Sockets 317==================== 318 319If you've understood the preceeding, you already know most of what you need to 320know about the mechanics of using sockets. You'll still use the same calls, in 321much the same ways. It's just that, if you do it right, your app will be almost 322inside-out. 323 324In Python, you use ``socket.setblocking(0)`` to make it non-blocking. In C, it's 325more complex, (for one thing, you'll need to choose between the BSD flavor 326``O_NONBLOCK`` and the almost indistinguishable Posix flavor ``O_NDELAY``, which 327is completely different from ``TCP_NODELAY``), but it's the exact same idea. You 328do this after creating the socket, but before using it. (Actually, if you're 329nuts, you can switch back and forth.) 330 331The major mechanical difference is that ``send``, ``recv``, ``connect`` and 332``accept`` can return without having done anything. You have (of course) a 333number of choices. You can check return code and error codes and generally drive 334yourself crazy. If you don't believe me, try it sometime. Your app will grow 335large, buggy and suck CPU. So let's skip the brain-dead solutions and do it 336right. 337 338Use ``select``. 339 340In C, coding ``select`` is fairly complex. In Python, it's a piece of cake, but 341it's close enough to the C version that if you understand ``select`` in Python, 342you'll have little trouble with it in C. :: 343 344 ready_to_read, ready_to_write, in_error = \ 345 select.select( 346 potential_readers, 347 potential_writers, 348 potential_errs, 349 timeout) 350 351You pass ``select`` three lists: the first contains all sockets that you might 352want to try reading; the second all the sockets you might want to try writing 353to, and the last (normally left empty) those that you want to check for errors. 354You should note that a socket can go into more than one list. The ``select`` 355call is blocking, but you can give it a timeout. This is generally a sensible 356thing to do - give it a nice long timeout (say a minute) unless you have good 357reason to do otherwise. 358 359In return, you will get three lists. They have the sockets that are actually 360readable, writable and in error. Each of these lists is a subset (possibly 361empty) of the corresponding list you passed in. And if you put a socket in more 362than one input list, it will only be (at most) in one output list. 363 364If a socket is in the output readable list, you can be 365as-close-to-certain-as-we-ever-get-in-this-business that a ``recv`` on that 366socket will return *something*. Same idea for the writable list. You'll be able 367to send *something*. Maybe not all you want to, but *something* is better than 368nothing. (Actually, any reasonably healthy socket will return as writable - it 369just means outbound network buffer space is available.) 370 371If you have a "server" socket, put it in the potential_readers list. If it comes 372out in the readable list, your ``accept`` will (almost certainly) work. If you 373have created a new socket to ``connect`` to someone else, put it in the 374potential_writers list. If it shows up in the writable list, you have a decent 375chance that it has connected. 376 377One very nasty problem with ``select``: if somewhere in those input lists of 378sockets is one which has died a nasty death, the ``select`` will fail. You then 379need to loop through every single damn socket in all those lists and do a 380``select([sock],[],[],0)`` until you find the bad one. That timeout of 0 means 381it won't take long, but it's ugly. 382 383Actually, ``select`` can be handy even with blocking sockets. It's one way of 384determining whether you will block - the socket returns as readable when there's 385something in the buffers. However, this still doesn't help with the problem of 386determining whether the other end is done, or just busy with something else. 387 388**Portability alert**: On Unix, ``select`` works both with the sockets and 389files. Don't try this on Windows. On Windows, ``select`` works with sockets 390only. Also note that in C, many of the more advanced socket options are done 391differently on Windows. In fact, on Windows I usually use threads (which work 392very, very well) with my sockets. Face it, if you want any kind of performance, 393your code will look very different on Windows than on Unix. 394 395 396Performance 397----------- 398 399There's no question that the fastest sockets code uses non-blocking sockets and 400select to multiplex them. You can put together something that will saturate a 401LAN connection without putting any strain on the CPU. The trouble is that an app 402written this way can't do much of anything else - it needs to be ready to 403shuffle bytes around at all times. 404 405Assuming that your app is actually supposed to do something more than that, 406threading is the optimal solution, (and using non-blocking sockets will be 407faster than using blocking sockets). Unfortunately, threading support in Unixes 408varies both in API and quality. So the normal Unix solution is to fork a 409subprocess to deal with each connection. The overhead for this is significant 410(and don't do this on Windows - the overhead of process creation is enormous 411there). It also means that unless each subprocess is completely independent, 412you'll need to use another form of IPC, say a pipe, or shared memory and 413semaphores, to communicate between the parent and child processes. 414 415Finally, remember that even though blocking sockets are somewhat slower than 416non-blocking, in many cases they are the "right" solution. After all, if your 417app is driven by the data it receives over a socket, there's not much sense in 418complicating the logic just so your app can wait on ``select`` instead of 419``recv``. 420