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Socket Python Module Python Select Module Threads CS 460 Lecture 4 Application Layer Python Network Programming Daniel Zappala Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads 1 Python Socket Module Introduction Creating a Server Creating a Client Catching Exceptions Python Select Module Introduction A Server with Select A Client For Select Threads Introduction Threaded Server 2 3 Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Introduction provides direct access to the standard BSD socket interface why use Python? similar to what you will do with C sockets, but simpler to learn socket addressing easier, bu er allocation done for you several higher-level abstractions available Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Overview 1 2 3 4 5 6 create a socket bind the socket to an address and port listen for incoming connections wait for clients accept a client send and receive data Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Address Families AF UNIX communication between two processes on the same machine represented as a string AF INET communication over the Internet, with IP version 4 represented as a tuple of (host, port), host is a string host name, port is an integer port number host can be a Internet host name (www.cnn.com) or an Ip address (64.236.24.20) AF INET6 communication over the Internet, with IP version 6 represented using a tuple of (host, port, ow info, scope id) ow info is a ow identi er used for Quality of Service (e.g. low delay or guaranteed bandwidth) scope id is a scope identi er, which can limit packet delivery to various administrative boundaries Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Create a Socket 1 socket ( family , type [ , protocol ] ) returns a socket identi er family is AF UNIX, AF INET, or AF INET6 type is usually SOCK STREAM for TCP, or SOCK DGRAM for UDP protocol is ignored in most cases 1 2 from s o c k e t i m p o r t s = s o c k e t ( AF INET , SOCK STREAM) Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Bind the Socket 1 bind ( address ) address is a tuple de ned by the address family 1 2 3 host = p o r t = 50000 s . bind ( ( host , port ) ) AF INET is a (host,port) tuple setting host to the empty string tells the OS to use any address associated with the host port number must not be currently used, or else an exception is raised Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Listen 1 l i s t e n ( backlog ) tells the server to listen for incoming connections backlog is an integer specifying the maximum number of connections the server will hold in a queue use a minimum of one, OS maximum is usually 5 use threads to service the queue of connections quickly if service time for a connection is large 1 2 backlog = 5 s . l i s t e n ( backlog ) Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Accept a Client 1 accept () returns a tuple (socket,address) socket is a new socket identi er for the client address is the client address, a tuple de ned by the address family (host, port for AF INET) 1 c l i e n t , address = s . accept () Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Receive Data 1 recv ( b u f f e r s i z e [ , flags ]) returns a string representing the data received bu ersize is the maximum size of the data to be received see Linux recv man page for ags possible that less data is received than the maximum 1 2 s i z e = 1024 data = c l i e n t . recv ( s i z e ) Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Send Data 1 send ( s t r i n g [ , f l a g s ] ) returns the number of bytes sent string is the data to be sent see Linux send man page for ags possible that some of the data is not sent must check return value and resend if necessary 1 2 d a t a = H e l l o World c l i e n t . send ( data ) Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions A Simple Echo Server 1 2 3 4 5 6 7 8 9 10 11 12 13 14 import socket host = p o r t = 50000 backlog = 5 s i z e = 1024 s = s o c k e t . s o c k e t ( s o c k e t . AF INET , s o c k e t . SOCK STREAM) s . bind ( ( host , port ) ) s . l i s t e n ( backlog ) while 1: c l i e n t , address = s . accept () data = c l i e n t . recv ( s i z e ) i f data : c l i e n t . send ( data ) c l i e n t . close () doesn t check for exceptions doesn t check if entire data is echoed back Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Overview 1 2 3 create a socket connect to the server send and receive data Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Connect to the Server 1 connect ( address ) address is a tuple de ned by the address family 1 2 3 host = l o c a l h o s t p o r t = 50000 s . connect ( ( host , port ) ) use a (host,port) tuple just like bind must use the address and port of the server, not the client using localhost means the server is running on the local machine use an Internet host name or an IP address for a remote machine server must be listening for clients, or else an exception is raised Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions A Simple Echo Client 1 2 3 4 5 6 7 8 9 10 11 import socket host = l o c a l h o s t p o r t = 50000 s i z e = 1024 s = s o c k e t . s o c k e t ( s o c k e t . AF INET , s o c k e t . SOCK STREAM) s . connect ( ( host , port ) ) s . send ( Hello , world ) data = s . recv ( s i z e ) s . close () p r i n t Received : , data doesn t check for exceptions doesn t check if entire data is sent or echoed back Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Server Exceptions Demonstration: What happens if the server s port is being used? Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Catching Exceptions 1 2 3 4 5 6 7 8 9 10 s = None try : s = s o c k e t . s o c k e t ( s o c k e t . AF INET , o s c k e t . SOCK STREAM) s . bind ( ( host , port ) ) s . l i s t e n ( backlog ) e x c e p t s o c k e t . e r r o r , ( code , message ) : if s: s . close () p r i n t C o u l d n o t open s o c k e t : + message sys . e x i t (1) socket module de nes exceptions socket.error returns a tuple initialize socket to None and then close it if it exists Demonstration: exceptions handled correctly Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Client Exceptions Demonstration: what happens if the server is not listening? Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Creating a Server Creating a Client Catching Exceptions Catching Exceptions 1 2 3 4 5 6 7 8 9 s = None try : s = s o c k e t . s o c k e t ( s o c k e t . AF INET , s o c k e t . SOCK STREAM) s . connect ( ( host , port ) ) e x c e p t s o c k e t . e r r o r , ( code , message ) : if s: s . close () p r i n t C o u l d n o t open s o c k e t : + message sys . e x i t (1) Demonstration: exceptions handled correctly Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction A Server with Select A Client For Select Introduction allows an application to wait for input from multiple sockets at a time does not use threads process data from one client a time can interleave client requests closely follows C interface for select Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction A Server with Select A Client For Select Using Select 1 s e l e c t ( input , output , exception [ , timeout ] ) returns a tuple of three lists: the subset of input sockets and les that have input, output, or exception events if a timeout has occurred without any input, output, or exception events, then all three lists will be empty input, output, exceptions are lists of socket or le identi ers that are waiting for events timeout is a time given as a oating point number of seconds: the socket call will return after this period of time if no sockets are ready 1 2 input = [ server , sys . stdin ] i r e a d y , oready , eready = s e l e c t . s e l e c t ( input , [ ] , [ ] ) Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction A Server with Select A Client For Select Echo Server with Select 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 input = [ server , sys . stdin ] running = 1 while running : i r e a d y , oready , eready = s e l e c t . s e l e c t ( input , [ ] , [ ] ) for s in inputready : i f s == s e r v e r : # server socket c l i e n t , address = s e r v e r . accept () i n p u t . append ( c l i e n t ) e l i f s == s y s . s t d i n : # standard input junk = sys . s t d i n . r e a d l i n e () running = 0 else : # a l l other sockets data = s . recv ( s i z e ) i f data : s . send ( data ) else : s . close () i n p u t . remove ( s ) Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction A Server with Select A Client For Select A Long-Lived Echo Client 1 2 3 4 5 6 7 8 9 10 11 12 s y s . s t d o u t . w r i t e ( % ) while 1: # r e a d from k e y b o a r d l i n e = sys . stdin . readline () i f l i n e == : break s . send ( l i n e ) data = s . recv ( s i z e ) sys . stdout . w r i t e ( data ) s y s . s t d o u t . w r i t e ( % ) s . close () Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction A Server with Select A Client For Select Demonstrations Demonstration: clients handled one at a time Demonstration: clients handled with select Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Threaded Server Introduction thread module: low-level interface to threads threading module: high-level interface built on thread module locks semaphores event objects thread and timer classes Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Threaded Server Threaded Server: Initialize Client Threads 1 2 3 4 5 6 c l a s s C l i e n t ( t h r e a d i n g . Thread ) : def init ( self ,( client , address ) ) : t h r e a d i n g . Thread . init ( self ) self . client = client s e l f . address = address s e l f . s i z e = 1024 subclasses the Thread class from threading module must call Thread initialization method initalize method variables Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Threaded Server Threaded Server: Run Client Threads 1 2 3 4 5 6 7 8 9 def run ( s e l f ) : running = 1 while running : data = s e l f . c l i e n t . recv ( s e l f . s i z e ) i f data : s e l f . c l i e n t . send ( data ) else : s e l f . c l i e n t . close () running = 0 overrides run method for Thread class loop to read data from socket until client closes no need for select on client sockets! Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Threaded Server Threaded Server: Initialize Server Thread 1 2 3 4 5 6 7 8 class Server : def init ( self ): s e l f . host = s e l f . p o r t = 50000 s e l f . backlog = 5 s e l f . s i z e = 1024 s e l f . s e r v e r = None s e l f . threads = [ ] basic initialization keep a list of client threads Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Threaded Server Threaded Server: Open Socket 1 2 3 4 5 6 7 8 9 10 def open socket ( s e l f ) : try : s e l f . s e r v e r = s o c k e t . s o c k e t ( s o c k e t . AF INET , s o c k e s e l f . s e r v e r . bind ( ( s e l f . host , s e l f . port ) ) s e l f . server . l i s t e n (5) e x c e p t s o c k e t . e r r o r , ( code , message ) : if s: s . close () p r i n t C o u l d n o t open s o c k e t : + message sys . e x i t (1) open socket Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Threaded Server Threaded Server: Run Server Thread 1 2 3 4 5 6 7 8 9 10 11 12 13 14 def run ( s e l f ) : s e l f . open socket () input = [ s e l f . server , sys . stdin ] running = 1 while running : i r e a d y , oready , eready = s e l e c t . s e l e c t ( input , [ ] , [ ] ) for s in iready : i f s == s e l f . s e r v e r : c = Client ( s e l f . s e r v e r . accept ()) c . start () s e l f . t h r e a d s . append ( c ) e l i f s == s y s . s t d i n : junk = sys . s t d i n . r e a d l i n e () running = 0 handle server socket and stdin with select spawn new thread for each client Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Threaded Server Threaded Server: Close Client Threads 1 2 3 4 # close a l l threads s e l f . server . close () for c in s e l f . threads : c . join () when server is exiting, wait for all threads to nish Daniel Zappala CS 460 Lecture 4 Python Socket Module Python Select Module Threads Introduction Threaded Server Threaded Server: Start the Server 1 2 3 if name == s = Server () s . run ( ) main : using main from global namespace allows us to either run the server directly or load it as a module Daniel Zappala CS 460 Lecture 4

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