Client Server Model and Software Design

1
Client Server Model and Software Design

• TCP/IP allows a programmer to establish
  communication between two application and to pass
  data back and forth.
• Application can execute on the same machine or on
  different machines.
• One organizational method dominates the use of
  TCP/IP is known as the client-server paradigm.

2
Terminology and concept

• Client an application that initiates
  peer-to-peer communication is called client
• Each time a client application executes, it
  contacts a server, sends a request, and awaits a
  response.
• Clients are often easier to build than servers,
  and usually require no special system privileges
  to operate

3

• Server is any program that waits for incoming
  communication requests from a client. The server
  receives a clients request, performs the
  necessary computation, and returns the result to
  the client.
• Privilege and complexity
• Servers must contain code that handles the issues
  of Authentication, Authorization, data security,
  privacy and protection

4
Parameterization of Clients

• Some client software provides more generality
  than others. In particular, some client software
  allows the user to specify both remote machine on
  which a server operates and the protocol port
  number at which the server is listening.

5
Connectionless vs. connection-oriented servers

• When programmers design client-server software,
  they must choose between two types of
  interaction a connectionless or a
  connection-oriented style.
• TCP connection-oriented. It provides all the
  reliability needed to communicate across an
  internet.
• Verifies data arrives, automatically retransmits
• Compute checksum over data.
• Automatically delete duplicated package
• UDP User datagram protocol.connectionless. No
  guarantees about reliable delivery.

6
Chapter 3 Concurrent Processing in Client-Server
Software

• The term concurrency refers to real or apparent
  simultaneous computing.
• Concurrent processing is fundamental to
  distributed computing and occurs in many forms
• Many pairs of application programs can
  communicate concurrently, sharing the network
  that interconnects them
• Concurrency can occur within a given computer
  system. Multiple users on a timesharing system
• Set of all clients on a set of machines can
  execute concurrently
• Concurrency in server. A single server program
  must handle incoming requests concurrently

7
Programs vs. processes

• Process defines the fundamental unit of
  computation( some system use the terms task, job,
  or thread instead of process)
• A process differs from a program because the
  process concept includes only the active
  execution of a computation, not the code.
• After the code has been loaded into a computer,
  the operating system allows multiple processes to
  execute the same piece of code at the same time

8

• For( I 1, Ilt10 I)
• printf(d\n, I)
• If two or more processes execute the code segment
  concurrently, one of them may be on the sixth
  iteration when other starts the first iteration
• When multiple processes execute a piece of code
  concurrently, each process has its own,
  independent copy of the variables associated with
  the code

9
Procedure call

• The run-time system for procedure-oriented
  programming languages uses a stack mechanism to
  handle procedure calls. The run-time system
  pushes a procedure activation record on the stack
  whenever it makes a procedure call.
• When multiple procedures execute a piece of code
  concurrently, each has its own run-time stack of
  procedure activation records

10
An example of concurrent process creation

• includeltstdlib.hgt
• includeltstdio.hgt
• Int sum
• Main()
• int I
• sum 0
• for ( I 1 I lt 5 I)
• printf(The value of I is d\n,I)
• fflush(stdout) / flush the buffer/
• sum I
• printf(The sum is d\n, sum)
• Exit(0)

11
A concurrent version

• To create a new process in UNIX, a program calls
  the system function fork.
• fork divides the running program into two
  identical processes, both executing at the same
  place in the same code.
• The two processed continue just as if two users
  had simultaneously started two copies of the
  application.

12

• includeltstdlib.hgt
• includeltstdio.hgt
• int sum
• main()
• int I
• sum 0
• fork() / create a new process/
• for( I 1 I lt5 I)
• printf(The value of I is d\n, I)
• fflush(stdout)
• sum I
• printf(The sume is d \n, sum)
• exit(0)

13

• To understand the fork function, imagine that
  fork causes the operating system to make a copy
  of the executing program and allows both copies
  to run at the same time

Output The value of I is 1 The value of I is
2 The value of I is 3 The value of I is 4 The
value of I is 5 The sum is 15 The value of I is
1 The value of I is 2 The value of I is 3 The
value of I is 4 The value of I is 5 The sum is 15
The first process executed so rapidly that it was
able to complete execution before the second
process ran at all
14
timeslicing

• The operating system allocates the available CPU
  power to each process for a short time before
  moving on to the next. We use term timeslicing to
  describe system that share the available CPU
  among several processes concurrently
• To see the effect of timeslicing, extend the
  concurrent program to iterate 10,000 times
  instead 5 times

15
Process identifier or process id

• In practice, the process created by fork is not
  absolutely identical to the original process it
  differs in one small detail.fork is a function
  that returns a value to its caller.
• in the original process, fork returns a small
  positive integer that identifies the newly
  created process
• In the newly created process, the fork returns
  zero

16

• includeltstdlib.hgt
• int sum
• main()
• int pid
• sum 0
• pid fork()
• if (pid ! 0) / original process /
• printf(The original process prints this.\n)
• else / newly created process/
• printf(The new process print this\n)
• exit(0)