Client server computing

1
Client server computing

• Programming Language Design and Implementation
  (4th Edition)
• by T. Pratt and M. Zelkowitz
• Prentice Hall, 2001
• Section 11.4

2
Persistent systems

• Traditional software Data stored outside of
  program
• Persistent systems Data part of program
• Execution never stops
• Transaction systems

Program
Program
3
Development of persistent languages

• 1. Data
• Transient data For life of one execution
• Persistent data Exists across executions
• 2. Need a mechanism to indicate an object is
  persistent
• approach is to assume all data are transient and
  indicate which objects are persistent
• 3. Need a mechanism to address a persistent
  object
• The language can develop its own model of
  persistent storage
• 4. Need to synchronize simultaneous access to an
  individual persistent object
• Use of semaphores
• 5. Need to check type compatibility of persistent
  objects
• Because of the multiple programs that access
  persistent data, name equivalence is difficult to
  use, structural equivalence is the preferred
  method in persistent languages.

4
Client-server computing

• Evolution of computer technology
• 1960s Standalone mainframes. As machine speed
  increased, use of multiprogramming and time
  sharing
• 1970s Development of less expensive personal
  computers
• 1980s As personal computers evolved, need for
  centralized resources to collect and store data
  (e.g., to corporate database)
• Need for local area networks (LANs)
• Development of the ARPANET and Internet
• Need for communication protocols between
  computers
• Need for computing models to adapt to this
• 1990s Interconnection and globalization of
  computing
• The World wide web and global access for all

5
Loosely coupled systems

• Each processor has its own memory and disk
  storage
• Communication lines to other processors to
  transfer information
• Leads to millisecond times to access data
• Synchronization harder - Machine communicating
  with may not be accessible on network because of
  a variety of reasons.
• Systems developed on such architectures are
  called distributed systems

6
Distributed systems

• Distributed systems can be
• centralized, where a single processor does the
  scheduling and informs the other machines as to
  the tasks to execute
• When completed, the assigned processor will
• indicate that it is ready to do another task
• distributed or peer-to-peer, where each machine
  is an equal and the process of scheduling is
  spread among all of the machines
• - one processor broadcasts a message to
• all the processors, and an inactive
• processor could respond with the message
• I'll do it.
• - Alternatively, each machine in the network
• will individually invoke a task on a
• specified machine elsewhere in the network.
• In this way, work gradually gets propagated
• around the network

7
Client-server mediator architecture
8
Client-server mediator architecture

• Client machine
• Interacts with user
• Has protocol to communicate with server
• Server
• Decides where data is located
• Accesses data
• Mediator is software that uses encoded knowledge
  about sets of data to create information for a
  higher class of application.
• Issues
• May be communicating with multiple clients
  simultaneously
• Need to keep each such transaction separate
• Multiple local address spaces in server

9
Impact on language design

• Lack of a global data store for the program. Both
  the client and server only have limited access to
  the total information content of the program.
• Programs need to be divided into separate pieces
  to effectively compute an answer (e.g., tasks).
• The Remote Procedure Call (RPC)is another
  communication method. An RPC syntactically looks
  like a subprogram call except that the operating
  system will enact a program possibly on another
  processor.
• RPCs are related to message communication and
  often are implemented as a send-receive message
  strategy
• client sends an RPC message to a server
• server waits on a receive for the message
• then responds back to the client in a similar
  manner.