The socketserver module simplifies the task of writing network servers.
There are four basic server classes: TCPServer uses the Internet TCP protocol, which provides for continuous streams of data between the client and server. UDPServer uses datagrams, which are discrete packets of information that may arrive out of order or be lost while in transit. The more infrequently used UnixStreamServer and UnixDatagramServer classes are similar, but use Unix domain sockets; they’re not available on non-Unix platforms. For more details on network programming, consult a book such as W. Richard Steven’s UNIX Network Programming or Ralph Davis’s Win32 Network Programming.
These four classes process requests synchronously; each request must be completed before the next request can be started. This isn’t suitable if each request takes a long time to complete, because it requires a lot of computation, or because it returns a lot of data which the client is slow to process. The solution is to create a separate process or thread to handle each request; the ForkingMixIn and ThreadingMixIn mix-in classes can be used to support asynchronous behaviour.
Creating a server requires several steps. First, you must create a request handler class by subclassing the BaseRequestHandler class and overriding its handle() method; this method will process incoming requests. Second, you must instantiate one of the server classes, passing it the server’s address and the request handler class. Finally, call the handle_request() or serve_forever() method of the server object to process one or many requests.
When inheriting from ThreadingMixIn for threaded connection behavior, you should explicitly declare how you want your threads to behave on an abrupt shutdown. The ThreadingMixIn class defines an attribute daemon_threads, which indicates whether or not the server should wait for thread termination. You should set the flag explicitly if you would like threads to behave autonomously; the default is False, meaning that Python will not exit until all threads created by ThreadingMixIn have exited.
Server classes have the same external methods and attributes, no matter what network protocol they use.
There are five classes in an inheritance diagram, four of which represent synchronous servers of four types:
+------------+ | BaseServer | +------------+ | v +-----------+ +------------------+ | 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 ForkingMixIn and ThreadingMixIn 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. Setting the various member variables also changes the behavior of the underlying server mechanism.
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 handler subclasses 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 different 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 protect the integrity of the shared data.
On the other hand, if you are building an HTTP server where all data is stored externally (for instance, 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 receive 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 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 cannot be used). See asyncore for another way to manage this.
The server classes support the following class variables:
There are various server methods that can be overridden by subclasses of base server classes like TCPServer; these methods aren’t useful to external users of the server object.
The request handler class must define a new handle() method, and can override any of the following methods. A new instance is created for each request.
This function must do all the work required to service a request. The default implementation does nothing. Several instance attributes are available to it; the request is available as self.request; the client address as self.client_address; and the server instance as self.server, in case it needs access to per-server information.
The type of self.request is different for datagram or stream services. For stream services, self.request is a socket object; for datagram services, self.request is a pair of string and socket. However, this can be hidden by using the request handler subclasses StreamRequestHandler or DatagramRequestHandler, which override the setup() and finish() methods, and provide self.rfile and self.wfile attributes. self.rfile and self.wfile can be read or written, respectively, to get the request data or return data to the client.
This is the server side:
import socketserver class MyTCPHandler(socketserver.BaseRequestHandler): """ The RequestHandler class for our server. It is instantiated once per connection to the server, and must override the handle() method to implement communication to the client. """ def handle(self): # self.request is the TCP socket connected to the client self.data = self.request.recv(1024).strip() print("%s wrote:" % self.client_address) print(self.data) # just send back the same data, but upper-cased self.request.send(self.data.upper()) if __name__ == "__main__": HOST, PORT = "localhost", 9999 # Create the server, binding to localhost on port 9999 server = socketserver.TCPServer((HOST, PORT), MyTCPHandler) # Activate the server; this will keep running until you # interrupt the program with Ctrl-C server.serve_forever()
An alternative request handler class that makes use of streams (file-like objects that simplify communication by providing the standard file interface):
class MyTCPHandler(socketserver.StreamRequestHandler): def handle(self): # self.rfile is a file-like object created by the handler; # we can now use e.g. readline() instead of raw recv() calls self.data = self.rfile.readline().strip() print("%s wrote:" % self.client_address) print(self.data) # Likewise, self.wfile is a file-like object used to write back # to the client self.wfile.write(self.data.upper())
The difference is that the readline() call in the second handler will call recv() multiple times until it encounters a newline character, while the single recv() call in the first handler will just return what has been sent from the client in one send() call.
This is the client side:
import socket import sys HOST, PORT = "localhost", 9999 data = " ".join(sys.argv[1:]) # Create a socket (SOCK_STREAM means a TCP socket) sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM) # Connect to server and send data sock.connect((HOST, PORT)) sock.send(bytes(data + "\n","utf8")) # Receive data from the server and shut down received = sock.recv(1024) sock.close() print("Sent: %s" % data) print("Received: %s" % received)
The output of the example should look something like this:
$ python TCPServer.py 127.0.0.1 wrote: b'hello world with TCP' 127.0.0.1 wrote: b'python is nice'
$ python TCPClient.py hello world with TCP Sent: hello world with TCP Received: b'HELLO WORLD WITH TCP' $ python TCPClient.py python is nice Sent: python is nice Received: b'PYTHON IS NICE'
This is the server side:
import socketserver class MyUDPHandler(socketserver.BaseRequestHandler): """ This class works similar to the TCP handler class, except that self.request consists of a pair of data and client socket, and since there is no connection the client address must be given explicitly when sending data back via sendto(). """ def handle(self): data = self.request.strip() socket = self.request print("%s wrote:" % self.client_address) print(data) socket.sendto(data.upper(), self.client_address) if __name__ == "__main__": HOST, PORT = "localhost", 9999 server = socketserver.UDPServer((HOST, PORT), MyUDPHandler) server.serve_forever()
This is the client side:
import socket import sys HOST, PORT = "localhost", 9999 data = " ".join(sys.argv[1:]) # SOCK_DGRAM is the socket type to use for UDP sockets sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) # As you can see, there is no connect() call; UDP has no connections. # Instead, data is directly sent to the recipient via sendto(). sock.sendto(bytes(data + "\n","utf8"), (HOST, PORT)) received = sock.recv(1024) print("Sent: %s" % data) print("Received: %s" % received)
The output of the example should look exactly like for the TCP server example.
To build asynchronous handlers, use the ThreadingMixIn and ForkingMixIn classes.
An example for the ThreadingMixIn class:
import socket import threading import socketserver class ThreadedTCPRequestHandler(socketserver.BaseRequestHandler): def handle(self): data = self.request.recv(1024) cur_thread = threading.current_thread() response = bytes("%s: %s" % (cur_thread.getName(), data),'ascii') self.request.send(response) class ThreadedTCPServer(socketserver.ThreadingMixIn, socketserver.TCPServer): pass def client(ip, port, message): sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM) sock.connect((ip, port)) sock.send(message) response = sock.recv(1024) print("Received: %s" % response) sock.close() if __name__ == "__main__": # Port 0 means to select an arbitrary unused port HOST, PORT = "localhost", 0 server = ThreadedTCPServer((HOST, PORT), ThreadedTCPRequestHandler) ip, port = server.server_address # Start a thread with the server -- that thread will then start one # more thread for each request server_thread = threading.Thread(target=server.serve_forever) # Exit the server thread when the main thread terminates server_thread.setDaemon(True) server_thread.start() print("Server loop running in thread:", server_thread.name) client(ip, port, b"Hello World 1") client(ip, port, b"Hello World 2") client(ip, port, b"Hello World 3") server.shutdown()
The output of the example should look something like this:
$ python ThreadedTCPServer.py Server loop running in thread: Thread-1 Received: b"Thread-2: b'Hello World 1'" Received: b"Thread-3: b'Hello World 2'" Received: b"Thread-4: b'Hello World 3'"
The ForkingMixIn class is used in the same way, except that the server will spawn a new process for each request.