"""Generic socket server classes. This module tries to capture the various aspects of defining a server: - address family: - AF_INET: IP (Internet Protocol) sockets (default) - AF_UNIX: Unix domain sockets - others, e.g. AF_DECNET are conceivable (see - socket type: - SOCK_STREAM (reliable stream, e.g. TCP) - SOCK_DGRAM (datagrams, e.g. UDP) - client address verification before further looking at the request (This is actually a hook for any processing that needs to look at the request before anything else, e.g. logging) - how to handle multiple requests: - synchronous (one request is handled at a time) - forking (each request is handled by a new process) - threading (each request is handled by a new thread) The classes in this module favor the server type that is simplest to write: a synchronous TCP/IP server. This is bad class design, but save some typing. (There's also the issue that a deep class hierarchy slows down method lookups.) There are four classes in an inheritance diagram that represent synchronous servers of four types: +-----------+ +------------------+ | TCPServer |------->| UnixStreamServer | +-----------+ +------------------+ | v +-----------+ +--------------------+ | UDPServer |------->| UnixDatagramServer | +-----------+ +--------------------+ Note that UnixDatagramServer derives from UDPServer, not from UnixStreamServer -- the only difference between an IP and a Unix stream server is the address family, which is simply repeated in both unix server classes. Forking and threading versions of each type of server can be created using the ForkingServer and ThreadingServer mix-in classes. For instance, a threading UDP server class is created as follows: class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass The Mix-in class must come first, since it overrides a method defined in UDPServer! To implement a service, you must derive a class from BaseRequestHandler and redefine its handle() method. You can then run various versions of the service by combining one of the server classes with your request handler class. The request handler class must be different for datagram or stream services. This can be hidden by using the mix-in request handler classes StreamRequestHandler or DatagramRequestHandler. Of course, you still have to use your head! For instance, it makes no sense to use a forking server if the service contains state in memory that can be modified by requests (since the modifications in the child process would never reach the initial state kept in the parent process and passed to each child). In this case, you can use a threading server, but you will probably have to use locks to avoid two requests that come in nearly simultaneous to apply conflicting changes to the server state. On the other hand, if you are building e.g. an HTTP server, where all data is stored externally (e.g. in the file system), a synchronous class will essentially render the service "deaf" while one request is being handled -- which may be for a very long time if a client is slow to reqd all the data it has requested. Here a threading or forking server is appropriate. In some cases, it may be appropriate to process part of a request synchronously, but to finish processing in a forked child depending on the request data. This can be implemented by using a synchronous server and doing an explicit fork in the request handler class's handle() method. Another approach to handling multiple simultaneous requests in an environment that supports neither threads nor fork (or where these are too expensive or inappropriate for the service) is to maintain an explicit table of partially finished requests and to use select() to decide which request to work on next (or whether to handle a new incoming request). This is particularly important for stream services where each client can potentially be connected for a long time (if threads or subprocesses can't be used). Future work: - Standard classes for Sun RPC (which uses either UDP or TCP) - Standard mix-in classes to implement various authentication and encryption schemes - Standard framework for select-based multiplexing XXX Open problems: - What to do with out-of-band data? """ __version__ = "0.2" import socket import sys import os class TCPServer: """Base class for various socket-based server classes. Defaults to synchronous IP stream (i.e., TCP). Methods for the caller: - __init__(server_address, RequestHandlerClass) - serve_forever() - handle_request() # if you don't use serve_forever() - fileno() -> int # for select() Methods that may be overridden: - server_bind() - server_activate() - get_request() -> request, client_address - verify_request(request, client_address) - process_request(request, client_address) - handle_error() Methods for derived classes: - finish_request(request, client_address) Class variables that may be overridden by derived classes or instances: - address_family - socket_type - request_queue_size (only for stream sockets) - reuse_address Instance variables: - server_address - RequestHandlerClass - socket """ address_family = socket.AF_INET socket_type = socket.SOCK_STREAM request_queue_size = 5 allow_reuse_address = 1 def __init__(self, server_address, RequestHandlerClass): """Constructor. May be extended, do not override.""" self.server_address = server_address self.RequestHandlerClass = RequestHandlerClass self.socket = socket.socket(self.address_family, self.socket_type) self.server_bind() self.server_activate() def server_bind(self): """Called by constructor to bind the socket. May be overridden. """ if self.allow_reuse_address: self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) self.socket.bind(self.server_address) def server_activate(self): """Called by constructor to activate the server. May be overridden. """ self.socket.listen(self.request_queue_size) def fileno(self): """Return socket file number. Interface required by select(). """ return self.socket.fileno() def serve_forever(self): """Handle one request at a time until doomsday.""" while 1: self.handle_request() # The distinction between handling, getting, processing and # finishing a request is fairly arbitrary. Remember: # # - handle_request() is the top-level call. It calls # get_request(), verify_request() and process_request() # - get_request() is different for stream or datagram sockets # - process_request() is the place that may fork a new process # or create a new thread to finish the request # - finish_request() instantiates the request handler class; # this constructor will handle the request all by itself def handle_request(self): """Handle one request, possibly blocking.""" try: request, client_address = self.get_request() except socket.error: return if self.verify_request(request, client_address): try: self.process_request(request, client_address) except: self.handle_error(request, client_address) def get_request(self): """Get the request and client address from the socket. May be overridden. """ return self.socket.accept() def verify_request(self, request, client_address): """Verify the request. May be overridden. Return true if we should proceed with this request. """ return 1 def process_request(self, request, client_address): """Call finish_request. Overridden by ForkingMixIn and ThreadingMixIn. """ self.finish_request(request, client_address) def finish_request(self, request, client_address): """Finish one request by instantiating RequestHandlerClass.""" self.RequestHandlerClass(request, client_address, self) def handle_error(self, request, client_address): """Handle an error gracefully. May be overridden. The default is to print a traceback and continue. """ print '-'*40 print 'Exception happened during processing of request from', print client_address import traceback traceback.print_exc() print '-'*40 class UDPServer(TCPServer): """UDP server class.""" socket_type = socket.SOCK_DGRAM max_packet_size = 8192 def get_request(self): data, client_addr = self.socket.recvfrom(self.max_packet_size) return (data, self.socket), client_addr def server_activate(self): # No need to call listen() for UDP. pass class ForkingMixIn: """Mix-in class to handle each request in a new process.""" active_children = None max_children = 40 def collect_children(self): """Internal routine to wait for died children.""" while self.active_children: if len(self.active_children) < self.max_children: options = os.WNOHANG else: # If the maximum number of children are already # running, block while waiting for a child to exit options = 0 try: pid, status = os.waitpid(0, options) except os.error: pid = None if not pid: break self.active_children.remove(pid) def process_request(self, request, client_address): """Fork a new subprocess to process the request.""" self.collect_children() pid = os.fork() if pid: # Parent process if self.active_children is None: self.active_children = [] self.active_children.append(pid) return else: # Child process. # This must never return, hence os._exit()! try: self.socket.close() self.finish_request(request, client_address) os._exit(0) except: try: self.handle_error(request, client_address) finally: os._exit(1) class ThreadingMixIn: """Mix-in class to handle each request in a new thread.""" def process_request(self, request, client_address): """Start a new thread to process the request.""" import threading t = threading.Thread(target = self.finish_request, args = (request, client_address)) t.start() class ForkingUDPServer(ForkingMixIn, UDPServer): pass class ForkingTCPServer(ForkingMixIn, TCPServer): pass class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass class ThreadingTCPServer(ThreadingMixIn, TCPServer): pass if hasattr(socket, 'AF_UNIX'): class UnixStreamServer(TCPServer): address_family = socket.AF_UNIX class UnixDatagramServer(UDPServer): address_family = socket.AF_UNIX class ThreadingUnixStreamServer(ThreadingMixIn, UnixStreamServer): pass class ThreadingUnixDatagramServer(ThreadingMixIn, UnixDatagramServer): pass class BaseRequestHandler: """Base class for request handler classes. This class is instantiated for each request to be handled. The constructor sets the instance variables request, client_address and server, and then calls the handle() method. To implement a specific service, all you need to do is to derive a class which defines a handle() method. The handle() method can find the request as self.request, the client address as self.client_address, and the server (in case it needs access to per-server information) as self.server. Since a separate instance is created for each request, the handle() method can define arbitrary other instance variariables. """ def __init__(self, request, client_address, server): self.request = request self.client_address = client_address self.server = server try: self.setup() self.handle() self.finish() finally: sys.exc_traceback = None # Help garbage collection def setup(self): pass def __del__(self): pass def handle(self): pass def finish(self): pass # The following two classes make it possible to use the same service # class for stream or datagram servers. # Each class sets up these instance variables: # - rfile: a file object from which receives the request is read # - wfile: a file object to which the reply is written # When the handle() method returns, wfile is flushed properly class StreamRequestHandler(BaseRequestHandler): """Define self.rfile and self.wfile for stream sockets.""" # Default buffer sizes for rfile, wfile. # We default rfile to buffered because otherwise it could be # really slow for large data (a getc() call per byte); we make # wfile unbuffered because (a) often after a write() we want to # read and we need to flush the line; (b) big writes to unbuffered # files are typically optimized by stdio even when big reads # aren't. rbufsize = -1 wbufsize = 0 def setup(self): self.connection = self.request self.rfile = self.connection.makefile('rb', self.rbufsize) self.wfile = self.connection.makefile('wb', self.wbufsize) def finish(self): self.wfile.flush() self.wfile.close() self.rfile.close() class DatagramRequestHandler(BaseRequestHandler): """Define self.rfile and self.wfile for datagram sockets.""" def setup(self): import StringIO self.packet, self.socket = self.request self.rfile = StringIO.StringIO(self.packet) self.wfile = StringIO.StringIO(self.packet) def finish(self): self.socket.sendto(self.wfile.getvalue(), self.client_address)