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.

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

Server: is any program that waits for incoming communication requests from a client. The server receives a client’s 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

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.

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.

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

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”

For( I = 1, I<=10; 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

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

An example of concurrent process creation #include Int sum; Main() { int I; sum = 0; for ( I = 1; I <= 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); }

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.

#include int sum; main() {int I; sum =0; fork(); /* create a new process*/ for( I =1; I <=5; I++) { printf(“The value of I is %d\n”, I); fflush(stdout); sum+= I; } printf(“The sume is %d \n”, sum); exit(0); }

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

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

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

#include 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); }