Client/Server
Programming
COMP476
Networked Computer Systems
• Applications exchange messages
through resulting connection
• Connecting application uses internet
protocol to contact listener
• Listening application informs local
protocol software that it is ready to
accept incoming messages
Establishing Contact through IP
• Waits for return message
• Sends message to server
• Client application establishes
connection
• Returns results
• Performs service
• Waits for incoming message
• Server application is “listener”
Client-Server Paradigm
Can access multiple services
• Must know (or find) the address of
server
• Initiates contact with server
• Runs locally on user's computer
• Invoked directly by user
• Also performs other computations
• Becomes client when network service is
needed
• Arbitrary application program
Characteristics of Client
• Will accept requests from arbitrary
clients; provides one service to each
client
• Waits for service requests from clients;
loops to wait for next request
• Runs on a remote computer (usually
centralized, shared computer)
• Starts at system initialization time
• Special purpose application dedicated
to providing network service
Characteristics of Server
consider streaming audio
• Server may send multiple response -
• Client may send multiple requests; server
sends multiple responses
• Server send response, perhaps with data
• Client sends request, perhaps with data
• Typically, client and server exchange
messages:
Message Exchanges
• Both client and server must have same
protocol stack and both interact with transport
• Clients and servers exchange messages
through transport protocols; e.g., TCP or UDP
Transport Protocols and
Client-Server Paradigm
• Servers run as independent processes
and can manage clients simultaneously
Multiple Services on One Computer
• Failure of single computer can bring
down multiple servers
• One server can affect others by
exhausting computer resources
• Reduces management overhead - only
one computer to maintain
• Can reduce costs by sharing resources
among multiple services
Multiple Services on One Computer
• Thus, client endpoints are unique, and server
computer protocol software can deliver
messages to correct server process
• No two clients on one computer can use
same source port
• (IP address, port number) on client
• (IP address, port number) on server
• Each transport session has two unique
identifiers
• How do incoming messages get delivered to
the correct server?
Selecting from Multiple Servers
– portability - allow the application to be used
on different systems
– load balancing - select server with lowest
utilization
– failure recovery - select a different server if
original fails
– efficiency – avoid many messages or
broadcasts
• How and when does a client application
learn the location of a service?
• Goals:
Binding of Server Location
Write server name in code
Read a file of server addresses
Broadcast request for a server
Ask a human
Name server
UDDI
• Much more on this topic later on
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Finding a Service
• Servers may become clients of other
servers
• Clients may access different servers for
one service
• Clients can access multiple services
Client-Server Interactions
• Often, but not always, there is an
application protocol
• Clients and servers use transport
protocols to communicate
• Server waits for client requests
• Client requests service when needed
• Client-server paradigm used in almost
every distributed computation
Summary