HOWTO sobre a Programação de Sockets

Autor

Gordon McMillan

Resumo

Os soquetes são usados em quase todos os lugares, mas são uma das tecnologias mais mal compreendidas. Esta é uma visão geral contendo 10.000 pontos a respeitos dos Sockets. Não é realmente um tutorial - ainda terás o trabalho para tornar tudo operacionais. Não abrange os pontos finos (e há muitos deles), mas espero que lhe dê uma base suficiente para começar a utilizar de maneira correta.

Sockets

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.

Parte do problema em compreender o funcionamento se dá pelo fato de que o “soquete” pode ter significados que são sutilmente diferentes, dependendo do contexto. Então, primeiro, vamos fazer uma distinção entre um soquete “cliente” - o ponto final de uma conversa e um soquete “servidor”, que mais parece como um operador de painel. O aplicativo Cliente (seu navegador, por exemplo) usa soquetes “cliente” exclusivos; O servidor Web com o qual conversamos faz uso de soquetes “servidor” e soquetes de “cliente”.

História

Das várias formas de IPC, os sockets são, de longe, os mais populares. Em qualquer plataforma, muito provável existirão outras formas de IPC que sejam mais rápidas, porém, para a comunicação entre plataformas, os soquetes são sobre o único jogo na cidade.

Os mesmos foram criados em Berkeley como parte do saboroso BSD do Unix. Eles se espalharam como um “incêndios” na Internet. Por uma boa razão — a combinação do uso de soquetes com o INET permite conversas com as mais variadas máquinas existentes ao redor do mundo de uma forma incrivelmente fácil (pelo menos, em comparação com outras tecnologias semelhantes).

Criando um Scoker

De forma geral, quando clicastes no link que te trouxe até esta página (documentação do Python), o que o teu navegador fez, foi o seguinte:

#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))

Quando a connect (conexão) foi estabelecida, o socket s pode ser utilizado para enviar uma solicitação de texto para a página. O mesmo soquete é que irá ler a resposta e, em seguida, o mesmo será destruído. Isso mesmo, será destruído. Os soquetes de Clientes normalmente são usados apenas numa única transação (troca) (ou um pequeno conjunto sequencial de transações).

O que acontece no lado do servidor Web é um pouco mais complexo. Primeiro, o Servidor Web cria um “soquete tipo servidor”:

#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)

Algumas coisas que deves observar: usamos socket.gethostname() para que o soquete esteja visível ao mundo exterior. Se tivéssemos usado s.bind(('localhost', 80)) ou s.bind(('127.0.0.1', 80)) ainda teríamos um soquete do tipo “servidor”, mas o mesmo só estaria visível dentro do computador em que está sendo executado. s.bind(('', 80)) determina que o soquete estará acessível por qualquer computador que possuas o endereço IP do computador.

Uma segunda coisa que precisas observar é: as portas baixas, normalmente estão reservadas para serviços “bem conhecidos”, tais como (HTTP, SNMP etc). Caso estejas brincando, utilize um número alto (números contendo 4 dígitos).

Por fim, o argumento “listen” diz à biblioteca Socket que queremos enfileirar no máximo 5 requisições de conexão (normalmente o máximo) antes de recusar começar a recusar conexões externas. Caso o resto do código esteja escrito corretamente, isso deverá ser o suficiente.

Agora que temos um Socket tipo “servidor”, que está ouvindo a porta 80, podemos entrar no mainloop do servidor web:

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()

Na verdade, existem 3 formas gerais nas quais este loop pode ser implementado - despachando um handle que lide com o clientsocket, criando um novo processo que funcione com o clientsocket, ou reestruture o aplicativo para trabalhar com Sockets não bloqueáveis e multiplexar entre os nossos Socket tipo “servidor” e qualquer cliente ativo que faça uso do select. Trataremos mais sobre isso posteriormente. O importante que temos que saber neste momento é isso: isso é tudo o que um Socket tipo “Servidor” irá fazer. O mesmo não enviar quaisquer dado. Não recebe dados. Apenas produz Sockets “cliente”. Cada clientsocket será criado em resposta a algum outro Socket “cliente” que fará uma connect() ao um determinado host numa determinada porta que estamos vinculados. Será dessa forma que criamos aquele “cliente”, voltaremos a ouvir mais conexões. Os dois “clientes” são livres para conversar - eles estão usando uma porta alocada dinamicamente que será descartada quando a conversa terminar.

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 'localhost'. On most platforms, this will take a shortcut around a couple of layers of network code and be quite a bit faster.

Usando um Socket

A primeira coisa que precisamos observar, é que o soquete “cliente” do navegador e o soquete “cliente” do servidor Web são “animais” idênticos. Ou seja, esta é uma conversa “peer to peer”. Ou, em outras palavras, como designer, terás que decidir quais são as regras da etiqueta para uma conversa. Normalmente, o soquete ``connect``ando inicia a conversa, enviando uma solicitação ou talvez um sinal. Mas essa é uma decisão de design - não é uma regra de soquetes.

Agora, há dois conjuntos de verbos que podemos usar na comunicação. Podemos usar send e recv, ou podemos transformar o seu soquete cliente numa “besta” semelhante a um arquivo e usar read e write. O último é o modo como Java apresenta seus soquetes. Não tratarei disso aqui, exceto para avisar que precisas usar o flush com soquetes. Estes são “arquivos” em buffer e um erro comum é “escrever” algo e, em seguida, “ler” uma resposta. Sem que haja um flush lá dentro, poderás esperar eternamente pela resposta, porque o pedido ainda poderá estar no buffer de saída.

Agora, chegamos ao principal obstáculo no uso dos sockets - send e recv operam nos buffers de rede. Eles não lidam necessariamente com todos os bytes que você os entrega (ou esperam deles), porque o foco principal deles é lidar com os buffers de rede. Em geral, os mesmos retornam quando os buffers de rede associados foram preenchidos (send) ou esvaziados (recv). Eles então lhe dizem quantos bytes eles trataram. É sua responsabilidade chamá-los novamente até que a sua mensagem tenha sido completamente tratada.

Quando um recv retornar 0 bytes, significa que o outro lado finalizou a conexão (ou está no processo de fechamento) da conexão. Não receberás mais dados nesta conexão. Pra sempre. Poderás enviar dados com sucesso; Falaremos mais sobre isso posteriormente.

Um protocolo como o HTTP utiliza um soquete para apenas uma transferência. O cliente envia uma solicitação e depois lê uma resposta. É isso aí. O soquete é descartado. Isso significa que um cliente pode detectar o final da resposta ao receber 0 bytes.

But if you plan to reuse your socket for further transfers, you need to realize that there is no EOT on a socket. I repeat: if a socket send or recv returns after handling 0 bytes, the connection has been broken. If the connection has not been broken, you may wait on a recv forever, because the socket will 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: messages must either be fixed length (yuck), or be delimited (shrug), or indicate how long they are (much better), 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:

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):
        chunks = []
        bytes_recd = 0
        while bytes_recd < MSGLEN:
            chunk = self.sock.recv(min(MSGLEN - bytes_recd, 2048))
            if chunk == '':
                raise RuntimeError("socket connection broken")
            chunks.append(chunk)
            bytes_recd = bytes_recd + len(chunk)
        return ''.join(chunks)

The sending code here is usable for almost any messaging scheme - in Python you send strings, and you can use len() to determine its length (even if it has embedded \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 strlen if the message has embedded \0s.)

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 recvs - 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.

Uma complicação a ter em conta: se o seu protocolo de conversa permitir que várias mensagens sejam enviadas de volta para trás (sem algum tipo de resposta) e você passar recv um tamanho arbitrário de bloco, você pode acabar lendo o início de uma mensagem seguinte. Você precisará deixar isso de lado e segurá-lo até que seja necessário.

Prefixing the message with its length (say, as 5 numeric characters) gets more complex, because (believe it or not), you may not get all 5 characters in one 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 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 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.

Dados Binários

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 - ntohl, htonl, ntohs, htons where “n” means network and “h” means host, “s” means short and “l” means 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.

Desconectando

Strictly speaking, you’re supposed to use shutdown on a socket before you close it. The 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 close is the same as shutdown(); close(). So in most situations, an explicit shutdown is not needed.

One way to use shutdown effectively is in an HTTP-like exchange. The client sends a request and then does a 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 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 close if it’s needed. But relying on this is a very bad habit. If your socket just disappears without doing a close, the socket at the other end may hang indefinitely, thinking you’re just being slow. Please close your sockets when you’re done.

Quando o Socket Morre

Probably the worst thing about using blocking sockets is what happens when the other side comes down hard (without doing a 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 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.

Sockets não-bloqueantes

If you’ve understood the preceding, 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 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 O_NONBLOCK and the almost indistinguishable Posix flavor O_NDELAY, which is completely different from 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.)

A principal diferença mecânica é que send, recv, connect e accept podem retornar sem terem feito nada. Terás (claro) uma série de escolhas. Poderás verificar o código de retorno e os códigos de erro que geralmente nos deixam loucos. Se não acreditas em mim, tente alguma vez. Seu aplicativo vai crescendo, buggy e sugam CPU. Então, vamos ignorar as soluções mortas no cérebro e fazê-lo direito.

Utilize select.

In C, coding 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 select in Python, you’ll have little trouble with it in C:

ready_to_read, ready_to_write, in_error = \
               select.select(
                  potential_readers,
                  potential_writers,
                  potential_errs,
                  timeout)

You pass 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 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 contain the sockets that are actually readable, writable and in error. Each of these lists is a subset (possibly empty) of the corresponding list you passed in.

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 recv on that socket will return something. Same idea for the writable list. You’ll be able to send something. Maybe not all you want to, but 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 accept will (almost certainly) work. If you have created a new socket to connect to someone else, put it in the potential_writers list. If it shows up in the writable list, you have a decent chance that it has connected.

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

Actually, 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.

Portability alert: On Unix, select works both with the sockets and files. Don’t try this on Windows. On Windows, 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.

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 select instead of recv.