1. Networks and Client/Server Applications
Handling Multiple Clients Concurrently

2. Problem One server handles all clients
Other clients will have to wait if one client has a big request
Server
Network
Client 1
Client 2
Client 3
Client N

3. A One-on-One Therapy Server
while True:
print('Waiting for connection ')
client, address = server.accept()
print('... connected from:', address)
dr = Doctor()
client.send(bytes(dr.greeting()), 'ascii'))
message = decode(client.recv(BUFSIZE), 'ascii')
if not message:
print('Client disconnected')
client.close()
break
else:
client.send(bytes(dr.reply(message)), 'ascii'))
Other patients must wait for the current patient to finish

4. Multithreading
A modern computer can run several processes or threads concurrently
Each thread executes it own algorithm
Examples:
Edit text in a word processor while a spell checker runs in the background
Load an image file in the background while the Web browser lays out and displays a page

5. Multithreading
Threads can share a single hardware processor by swapping (timesharing)
Or they can run on separate processors in a multicore system

6. States in the Life of a Thread
new
start
The CPU
ready
The ready queue
Schedules threads for the CPU

7. States in the Life of a Thread
new
start
yield or timed out
running
The CPU
run
ready
The ready queue

8. States in the Life of a Thread
dead
new
complete
start
yield or timed out
running
The CPU
run
ready
The ready queue

9. The Thread Class Imported from the threading module Interface
Thread() # Returns a new instance
start() # Places the thread on the ready queue
run() # Contains the code for the thread to execute;
# pass by default

10. Using a Thread
Define a subclass of Thread with a new run method that contains the code to execute
Create an instance and run the start method to activate it

11. A One-on-One Therapy Server
while True:
print('Waiting for connection ')
client, address = server.accept()
print('... connected from:', address)
dr = Doctor()
client.send(bytes(dr.greeting()), 'ascii'))
message = decode(client.recv(BUFSIZE), 'ascii')
if not message:
print('Client disconnected')
client.close()
break
else:
client.send(bytes(dr.reply(message)), 'ascii'))
Other patients must wait for the current patient to finish

12. A One-on-One Therapy Server
while True:
print('Waiting for connection ')
client, addr = server.accept()
print('... connected from:', addr)
dr = Doctor()
handler = ClientHandler(client, dr)
handler.start()
Each client handler manages a conversation between the doctor and a client
Because the client handlers run as separate threads, many of them can run concurrently
The server can cycle back immediately to listen for another client

13. The ClientHandler Class
from threading import Thread
class ClientHandler(Thread):
The ClientHandler class is a subclass of Thread
As such, objects of type ClientHandler can be used wherever threads are used

14. The ClientHandler Class
from threading import Thread
class ClientHandler(Thread):
def __init__(self, client, dr):
Thread.__init__(self)
self.client = client
self.dr = dr
The __init__ method
runs the __init__ method in the Thread class
receives the client socket and the doctor object from the server and transfers these to instance variables

15. The ClientHandler Class
from threading import Thread
class ClientHandler(Thread):
def __init__(self, client, dr):
Thread.__init__(self)
self.client = client
self.dr = dr
def run(self):
self.client.send(bytes(self.dr.greeting(), 'ascii'))
while True:
message = decode(self.client.recv(BUFSIZE), 'ascii')
if not message:
print('Client disconnected')
client.close()
break
else:
self.client.send(bytes(self.dr.reply(message),
'ascii'))

16. Extensions Create a separate Doctor object for each client
Save the client’s history list to a file
If a file for a client exists, load the history list from it when she logs in
Add separate GUIs for the client and the server

17. Extensibility
The client/server pattern can be applied to many situations, such as the ATM and bank manager applications of this week’s lab
Many ATM clients (on many different computers)
One server/manager running on a single host

18. For Friday
Review and wrapup