4. SocketServer
— A framework for network servers¶
Contents
Note
The SocketServer
module has been renamed to socketserver
in
Python 3. The 2to3 tool will automatically adapt imports when
converting your sources to Python 3.
The SocketServer
module simplifies the task of writing network servers.
There are four basic concrete server classes:
-
class
SocketServer.
TCPServer
(server_address, RequestHandlerClass, bind_and_activate=True)[source]¶ This uses the Internet TCP protocol, which provides for continuous streams of data between the client and server. If bind_and_activate is true, the constructor automatically attempts to invoke
server_bind()
andserver_activate()
. The other parameters are passed to theBaseServer
base class.
-
class
SocketServer.
UDPServer
(server_address, RequestHandlerClass, bind_and_activate=True)[source]¶ This uses datagrams, which are discrete packets of information that may arrive out of order or be lost while in transit. The parameters are the same as for
TCPServer
.
-
class
SocketServer.
UnixStreamServer
(server_address, RequestHandlerClass, bind_and_activate=True)[source]¶ -
class
SocketServer.
UnixDatagramServer
(server_address, RequestHandlerClass, bind_and_activate=True)[source]¶ These more infrequently used classes are similar to the TCP and UDP classes, but use Unix domain sockets; they’re not available on non-Unix platforms. The parameters are the same as for
TCPServer
.
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. Then call the
handle_request()
or
serve_forever()
method of the server object to
process one or many requests. Finally, call server_close()
to close the socket.
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.
4.1. Server Creation Notes¶
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.
-
class
SocketServer.
ForkingMixIn
[source]¶ -
class
SocketServer.
ThreadingMixIn
[source]¶ Forking and threading versions of each type of server can be created using these mix-in classes. For instance,
ThreadingUDPServer
is created as follows:class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass
The mix-in class comes first, since it overrides a method defined in
UDPServer
. Setting the various attributes also changes the behavior of the underlying server mechanism.
-
class
SocketServer.
ForkingTCPServer
[source]¶ -
class
SocketServer.
ForkingUDPServer
[source]¶ -
class
SocketServer.
ThreadingTCPServer
[source]¶ -
class
SocketServer.
ThreadingUDPServer
[source]¶ These classes are pre-defined using the mix-in classes.
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.
4.2. Server Objects¶
-
class
SocketServer.
BaseServer
(server_address, RequestHandlerClass)[source]¶ This is the superclass of all Server objects in the module. It defines the interface, given below, but does not implement most of the methods, which is done in subclasses. The two parameters are stored in the respective
server_address
andRequestHandlerClass
attributes.-
fileno
()¶ Return an integer file descriptor for the socket on which the server is listening. This function is most commonly passed to
select.select()
, to allow monitoring multiple servers in the same process.
-
handle_request
()[source]¶ Process a single request. This function calls the following methods in order:
get_request()
,verify_request()
, andprocess_request()
. If the user-providedhandle()
method of the handler class raises an exception, the server’shandle_error()
method will be called. If no request is received withintimeout
seconds,handle_timeout()
will be called andhandle_request()
will return.
-
serve_forever
(poll_interval=0.5)[source]¶ Handle requests until an explicit
shutdown()
request. Poll for shutdown every poll_interval seconds. Ignores thetimeout
attribute. If you need to do periodic tasks, do them in another thread.
-
shutdown
()[source]¶ Tell the
serve_forever()
loop to stop and wait until it does.New in version 2.6.
-
address_family
¶ The family of protocols to which the server’s socket belongs. Common examples are
socket.AF_INET
andsocket.AF_UNIX
.
-
RequestHandlerClass
¶ The user-provided request handler class; an instance of this class is created for each request.
-
server_address
¶ The address on which the server is listening. The format of addresses varies depending on the protocol family; see the documentation for the
socket
module for details. For Internet protocols, this is a tuple containing a string giving the address, and an integer port number:('127.0.0.1', 80)
, for example.
-
socket
¶ The socket object on which the server will listen for incoming requests.
The server classes support the following class variables:
-
allow_reuse_address
¶ Whether the server will allow the reuse of an address. This defaults to
False
, and can be set in subclasses to change the policy.
-
request_queue_size
¶ The size of the request queue. If it takes a long time to process a single request, any requests that arrive while the server is busy are placed into a queue, up to
request_queue_size
requests. Once the queue is full, further requests from clients will get a “Connection denied” error. The default value is usually 5, but this can be overridden by subclasses.
-
socket_type
¶ The type of socket used by the server;
socket.SOCK_STREAM
andsocket.SOCK_DGRAM
are two common values.
-
timeout
¶ Timeout duration, measured in seconds, or
None
if no timeout is desired. Ifhandle_request()
receives no incoming requests within the timeout period, thehandle_timeout()
method is called.
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.-
finish_request
()[source]¶ Actually processes the request by instantiating
RequestHandlerClass
and calling itshandle()
method.
-
get_request
()¶ Must accept a request from the socket, and return a 2-tuple containing the new socket object to be used to communicate with the client, and the client’s address.
-
handle_error
(request, client_address)[source]¶ This function is called if the
handle()
method of aRequestHandlerClass
instance raises an exception. The default action is to print the traceback to standard output and continue handling further requests.
-
handle_timeout
()[source]¶ This function is called when the
timeout
attribute has been set to a value other thanNone
and the timeout period has passed with no requests being received. The default action for forking servers is to collect the status of any child processes that have exited, while in threading servers this method does nothing.
-
process_request
(request, client_address)[source]¶ Calls
finish_request()
to create an instance of theRequestHandlerClass
. If desired, this function can create a new process or thread to handle the request; theForkingMixIn
andThreadingMixIn
classes do this.
-
server_activate
()[source]¶ Called by the server’s constructor to activate the server. The default behavior for a TCP server just invokes
listen()
on the server’s socket. May be overridden.
-
server_bind
()¶ Called by the server’s constructor to bind the socket to the desired address. May be overridden.
-
verify_request
(request, client_address)[source]¶ Must return a Boolean value; if the value is
True
, the request will be processed, and if it’sFalse
, the request will be denied. This function can be overridden to implement access controls for a server. The default implementation always returnsTrue
.
-
4.3. Request Handler Objects¶
-
class
SocketServer.
BaseRequestHandler
[source]¶ This is the superclass of all request handler objects. It defines the interface, given below. A concrete request handler subclass must define a new
handle()
method, and can override any of the other methods. A new instance of the subclass is created for each request.-
setup
()[source]¶ Called before the
handle()
method to perform any initialization actions required. The default implementation does nothing.
-
handle
()[source]¶ 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 asself.client_address
; and the server instance asself.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.
-
-
class
SocketServer.
StreamRequestHandler
[source]¶ -
class
SocketServer.
DatagramRequestHandler
[source]¶ These
BaseRequestHandler
subclasses override thesetup()
andfinish()
methods, and provideself.rfile
andself.wfile
attributes. Theself.rfile
andself.wfile
attributes can be read or written, respectively, to get the request data or return data to the client.
4.4. Examples¶
4.4.1. SocketServer.TCPServer
Example¶
This is the server side:
import SocketServer
class MyTCPHandler(SocketServer.BaseRequestHandler):
"""
The request handler 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 "{} wrote:".format(self.client_address[0])
print self.data
# just send back the same data, but upper-cased
self.request.sendall(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 "{} wrote:".format(self.client_address[0])
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 sendall()
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)
try:
# Connect to server and send data
sock.connect((HOST, PORT))
sock.sendall(data + "\n")
# Receive data from the server and shut down
received = sock.recv(1024)
finally:
sock.close()
print "Sent: {}".format(data)
print "Received: {}".format(received)
The output of the example should look something like this:
Server:
$ python TCPServer.py
127.0.0.1 wrote:
hello world with TCP
127.0.0.1 wrote:
python is nice
Client:
$ python TCPClient.py hello world with TCP
Sent: hello world with TCP
Received: HELLO WORLD WITH TCP
$ python TCPClient.py python is nice
Sent: python is nice
Received: PYTHON IS NICE
4.4.2. SocketServer.UDPServer
Example¶
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[0].strip()
socket = self.request[1]
print "{} wrote:".format(self.client_address[0])
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(data + "\n", (HOST, PORT))
received = sock.recv(1024)
print "Sent: {}".format(data)
print "Received: {}".format(received)
The output of the example should look exactly like for the TCP server example.
4.4.3. Asynchronous Mixins¶
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 = "{}: {}".format(cur_thread.name, data)
self.request.sendall(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))
try:
sock.sendall(message)
response = sock.recv(1024)
print "Received: {}".format(response)
finally:
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.daemon = True
server_thread.start()
print "Server loop running in thread:", server_thread.name
client(ip, port, "Hello World 1")
client(ip, port, "Hello World 2")
client(ip, port, "Hello World 3")
server.shutdown()
server.server_close()
The output of the example should look something like this:
$ python ThreadedTCPServer.py
Server loop running in thread: Thread-1
Received: Thread-2: Hello World 1
Received: Thread-3: Hello World 2
Received: Thread-4: Hello World 3
The ForkingMixIn
class is used in the same way, except that the server
will spawn a new process for each request.