Lab 3 Design of Server Software

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Lab 3 Design of Server Software

• Dr. Tony K. C. Chan

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• Acknowledgements
• Thanks for Prof. Y. W. Leung providing valuable
  materials by which this teaching materials is
  prepared.

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• References
• 1 D. E. Comer and D. L. Stevens,
  Internetworking with TCP/IP Volume III
  Client-Server Programming and Applications,
  Prentice-Hall, Inc, 1997.
• 2 Beejs Guide to Network Programming Using
  Internet Sockets Online Available
  http//beej.us/guide/bgnet/
• 3 Winsock Functions Online Available
  http//msdn2.microsoft.com/en-us/library/ms741394.
  aspx.

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1. A Short Review

• Four Basic Types of Servers

Iterative Connectionless
Iterative Connection-Oriented
Concurrent Connectionless
Concurrent Connection-Oriented
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2. Concurrent Processing

• 2.1 Process and Thread
• We can use processes and threads to realize
  concurrency on a processor via time sharing.
• Process
• Process is a fundamental unit of computation.
  The OS has to store various information about
  each process.

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2. Concurrent Processing

• 2.1 Process and Thread, cont.
• Thread
• Windows provides a second form of concurrent
  execution known as threads of execution of
  threads.
• Each thread must be associated with a single
  process.
• A thread is executed independently of other
  threads.
• All threads in a process share
• Global variables and
• Resources that the OS allocates to the process.
• Each thread in a process has its own local
  variables.

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2. Concurrent Processing

• 2.2 Creating Threads in Windows
• In Windows, a program calls _beginthread() to
  create a new thread to execute a specified
  function.
• ? The program and the newly created thread are
  executed concurrently.
• _beginthread() requires three parameters
• 1st parameter start address of the function to
  be executed.
• 2nd parameter stack size for the new thread (we
  can use 0 for our purpose).
• 3rd parameter pointer to a list of parameters to
  be passed to the function if no parameter is
  passed, specify NULL.
• For more details, read the help pages in MS
  Visual C

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2. Concurrent Processing

• 2.2 Creating Threads in Windows, cont.
• A program can call _endthread() (without any
  parameter) to terminate a thread explicitly.
  However, when the function has completed
  execution, _endthread() is called automatically.
• Remarks In MS visual C, remember to include
  the header file ltprocess.hgt and link to the
  library libcmt.lib.

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2. Concurrent Processing

• 2.2 Creating Threads in Windows, cont.
• Example
• We consider an add process as follows.
• void add(int count)
• int i, sum
• sum0
• for (i1 iltcount i)
• printf("i d\n", i)
• sum i
• printf("The sum of i is d\n", sum)

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2. Concurrent Processing

• 2.2 Creating Threads in Windows, cont.
• Example, cont.
• What will be printed in the following codes?
• include ltstdio.hgt
• void add(int)
• int main(int argc, char argv)
• add(3)
• add(5)
• return 0

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2. Concurrent Processing

• 2.2 Creating Threads in Windows, cont.
• Example, cont.
• What will be printed in the following codes?
• pragma comment( linker, "/defaultliblibcmt.lib"
  )
• include ltstdio.hgt
• include ltprocess.hgt
• void add(int)
• int main(int argc, char argv)
• _beginthread(add, 0, (void )3) / Execute in
  a thread /
• add(5) / Execute in the main process /
• return 0

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Exercises

• 1. Write a multithread program using
  _beginthread()
• 2. Write a concurrent program that starts five
  threads. Arrange for each thread to print a few
  lines of output and then halt. Is output from the
  threads intermixed?

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3. Implementing Concurrent Server

• A concurrent server executes a main thread and
  multiple service threads.
• Main Thread (or Master Thread)
• When a server program starts execution, the main
  thread is executed.
• The main thread creates a socket and waits for
  clients requests.
• When a clients request arrives, the main thread
  creates a new service thread to serve this
  client. Meanwhile, the main thread waits for
  another request.

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3. Implementing Concurrent Server

• Service Thread (or Slave Thread)
• Each service thread is created to serve one
  client.
• When the service is completed, the service thread
  is terminated.
• If the server computer in serving N clients,
  there are one main thread and N service threads.

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Exercises

• 1. Write a concurrent connection-oriented server
  that provide ECHO service.
• 2. Modify the server in 1 so that the server
  keeps a log of the time at which it creates each
  slave thread and the time at which the slave
  terminates.
• 3. Build an iterative implementation of an ECHO
  server. Conduct an experiment to determine if a
  human can sense the difference in response time
  between the concurrent and the iterative versions.

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4. Asynchronous I/O

• An application calls the select(), asking the OS
  which socket becomes ready for I/O.
• The call returns when at least one socket becomes
  ready or timer expires.
• Then the application can perform I/O through the
  ready sockets.

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4. Asynchronous I/O

• select() uses the following structure to
  represent a set of socket descriptors to be
  handled
• Typedef struct fd_set
• u_int fd_count
• SOCKET fd_arrayFD_SETSIZE
• fd_set
• where
• fd_count is the number of socket descriptors in
  the set
• fd_array is an array of socket descriptors and
  FD_SETSIZE is the maximum number of descriptors
  in a set

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4. Asynchronous I/O

• Some useful macros for asynchronous I/O(Let set
  be a set of socket descriptors fd_set set)
• To initialize set to empty, execute
• FD_ZERO(set)
• To add the socket descriptor s to set, execute
• FD_SET(s, set)
• To remove s from set, execute
• FD_CLR(s, set)
• To test if s belongs to set, execute
• FD_ISSET(s, set)
• If s belongs to set, the return value is
  non-zero otherwise, zero.

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4. AsynchronousI/O

• The select function determinesthe status of one
  or moresockets, waiting if necessary,to perform
  synchronous I/O
• Example
• fd_set rfds/ Call select() to select
  socketsthat are ready for reading /select(0,
  rfds, (fd_set )0, (fd_set )0, (struct
  timeval )0)

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5. Implementing Single-Thread Concurrent Server

• Use asynchronous I/O and a single thread to
  provide apparent concurrency.
• Example
• SOCKET msock, s
• fd_set afds, / set of all sockets created /
• fd_set rfds / set of sockets having input I/O
  /
• int i
• / create master socket here /
• FD_ZERO(afds)
• FD_SET(msock, afds)
• for () / an infinitive loop for handling
  the incoming requests /

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5. Implementing Single-Thread Concurrent Server

• Example, cont.
• / codes inside the an infinitive loop /
• memcpy(rfds, afds, sizeof(rfds))
• select(0, rfds, (fd_set )0, (fd_set )0,
  (struct timeval )0)
• if (FD_ISSET(msock, rfds))
• / handle the request form the master socket /
• for (i 0 ilt rfds.fd_count i)
• s rfds.fd_arrayi
• if (s ! msock FD_ISSET(s, rfds))
• / handle the incoming message from the i-th
  socket in the set /

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Exercises

• 1. Write a ECHO service using asynchronous I/O.
• 2. Conduct an experiment that proves the ECHO
  server in 1 can handle connections concurrently.

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THE END