Systems Architecture, Fifth Edition

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Chapter Goals

• Describe client/server and multi-tier application
  architecture and discuss their advantages
  compared to centralized applications
• Explain how operating systems and network
  protocol stacks cooperate so users and programs
  can access remote resources
• Describe low-level protocols for interprocess
  communication across networks, including sockets,
  names pipes, RPC, and DCE

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Chapter Goals (continued)

• List and describe standard Internet protocols
  used to access distributed resources
• Discuss component-based application development
  and describe the protocols and standards that
  support component-based applications
• Explain the role and function of directory
  services and the LSAP standard

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Distributed Computing

• Distributing parts of an information system
  across many computer systems and locations
• Architectural models
• Client/server
• Three-layer
• N-layer

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Client/Server Architecture

• Server
• Manages system resources provides access to them
  through a well-defined communication interface
• Client
• Uses communication interface to request
  resources server responds to those requests
• Current dominant model for distributed computing

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One of the many different ways that client/server
architecture can be implemented.
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N-Layer Client/Server Architecture

• Three-layer architecture
• Divides application software into three tiers
• Data layer
• Business logic layer
• View layer
• Simplifies distributing or replicating
  application software across a network
• N-layer architectures
• Employ more than three layers

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The view layer acts as a client of the business
logic layer, which in turn acts as a client of
the data layer.
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Multi-Layer Architectures

• Require standard models and services to
  communicate with one another
• Middleware
• System software that implements communication
  standards and gives clients and servers the
  ability to interact

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Network Resource Access

• OS components enable distributed access by
• Distinguishing between local and remote resources
• Interacting with distant operating systems
• How does an OS do these things?

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Protocol Stacks

• Software that implements the lowest five levels
  of the OSI model
• Provide several advantages for implementing
  network I/O and services (shown two slides from
  now)

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Advantages of Protocol Stacks

• Divide network interaction into well-defined
  pieces that can be separately implemented,
  installed, and updated
• Provide flexibility to keep up with rapid
  protocol standard evolution
• Insulate application programs and portions of OS
  from details of low-level network communication
  protocols and physical network implementation
  (ensures software portability)

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Accessing Remote Resources

• Static connection
• Initialized by user or system administrator prior
  to accessing a remote resource
• Difficult to initialize and maintain (somebody
  has to do it what if it changes?)
• Example Using Windows to map a network drive to
  a remote folder (in My Computer / Tools / Map
  Network Drive)

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Accessing Remote Resources

• Dynamic connection
• Established through interaction between a
  resource layer and a primary resource
  registration repository
• More flexible but requires a distributed registry
  of resource name and locations

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Premises ofRemote Resource Access

• Location transparency
• Operating systems, application programs, and user
  interfaces are simpler if there is no distinction
  between local and remote resource access
• Also called network transparency
• Example a word processor shouldnt care if the
  file it is operating on is local or on a remote
  server

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Premises ofRemote Resource Access

• All resources are potentially shared across a
  network any computer system is potentially both
  a server and a client
• To provide remote access, all operating systems
  need to incorporate server-like functions
• OSs that implement this design feature are said
  to implement service-oriented resource access

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Service-Oriented Resource Access

• Need 2 layers between service layer and device
  drivers
• Service provider
• Server interface to specific resource
• Resource locator
• Locates resources referred to in service requests
  from local or remote users and programs
• Forwards service requests to appropriate service
  provider
• Maintains a resource registry

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Software components that support service-oriented
resource access
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Blurring Between Server OS and Client OS

• Because client machines are called upon to
  perform server operations, the distinction
  between server OS and client OS is getting fuzzy
• Windows Server 2003 and Windows XP have many
  similar features
• But Windows Server supports up to 32 CPUs and a
  sophisticated directory-based security system
  Windows XP Pro supports 1 or 2 CPUs more modest
  security

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Interprocess Communication

• Distributed processes must communicate with one
  another to exchange data and synchronize
  activities
• Peer-to-peer interprocess communication protocols
• Sockets
• Named pipes
• Remote procedure calls
• Distributed Computing Environment (DCE)

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Sockets

• Unique combinations of an IP number and a port
  number, separated by a colon (e.g.,
  129.24.8.153)
• Implement direct process-to-process communication
  via protocol stacks
• Supported by all modern OSs, which enable
  programs to initialize sockets, receive messages
  sent to a socket, and send messages to sockets
  anywhere on the Internet

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Each socket uniquely identifies a client or
server process on the Internet.
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Named Pipes

• Region of shared memory that enables multiple
  processes executing on the same machine to
  exchange data
• Commonly used for communication among OS
  components, for queuing requests to an OS service
  such as a Web server, and for exchanging messages
  among components within a large application
• A named pipe is permanently placed within file
  system directory (is treated like a file)
• And is able to communicate among processes on
  different computers

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OS assigns a free socket to named pipe when it is
created, allocates I/O buffers and routes data
flowing in/out of the pipe through low-level
network protocol stack.
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Remote Procedure Calls

• Allow one process to execute another as a
  subroutine with parameter passing and format
  translation
• The calling process
• Passes parameters to the called process
• Waits for the called process to complete its task
• Accepts parameters back from the called process
• Resumes execution with the instruction following
  the call

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Problems with RPC

• You are calling a routine, so you are passing
  parameters
• Are the parameters the same type? Same number?
  Big endian or little endian? Floating point
  consistency? ASCII or EBCDIC or Unicode?

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Distributed Computing Environment

• A standard for distributed OS services defined by
  the Open Group
• Partially supported by most major OSs
• Combines all peer-to-peer approaches and adds
  security (Kerberos) and minimal directory
  services
• Promotes interoperability of distributed software
  across operating systems and middleware products

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DCE functions are incorporated directly into an
operating system or supplied as an optional
component.
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The Internet
Internet Global collection of networks interconnected using TCP/IP Infrastructure upon which the Web is based
WWW Collection of resources (programs, files, services), accessible over the Internet by standard protocols (FTP, HTTP) Organized using client/server architecture
Intranet Private network that uses Internet protocols accessible only to a limited set of internal users Set of privately accessible resources, organized and delivered via Web protocols over a TCP/IP network
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Standard Web Protocols and Services

• Web protocols
• Define valid resource formats and a standard
  means of requesting resources
• Identified by a unique Uniform Resource Locator
  (URL)

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Resource
Host
Port
Protocol
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The Internet as an Application Platform

• Application program executes on a Web server that
  can be accessed from any computer with an
  Internet connection

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Database is back-end server and is accessed
via pipes/sockets/RPC https or Kerberos used
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The Internet as an Application Platform
Advantages Disadvantages
Expands accessibility Eliminates need to install custom client software Cheaper to develop and deploy built around existing Web standards and browser software already installed on clients Security Performance Reliability
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Components and Distributed Objects

• Component
• Standardized, interchangeable software module
  that is executable, has a unique identifier, and
  has a well-known interface
• Distributed object
• Cooperating process that implements a public
  interface to the services it provides

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Component-Based Software

• Enables construction of complex programs and
  applications from smaller previously developed
  parts
• Requires protocols and infrastructure for
  component registration, discovery, and
  communication

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Components and Objects

• Send and respond to messages
• Encapsulate internal data
• Interact with other components through
  awell-defined interface

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Connection Standards and Infrastructure

• Interoperability requires well-defined and widely
  adopted standards
• Standard network protocols do not address
• Format and content of valid messages and
  responses
• Way to uniquely identify each component on the
  Internet and route messages to/from that computer
• Additional standards provided by CORBA, COM,
  SOAP, J2EE

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Common Object Request Broker Architecture (CORBA)

• Specifies middleware used by objects to interact
  across networks
• Key components
• Object Request Broker (ORB)
• Internet Inter-ORB Protocol (IIOP)
• Robust, scalable, independent of programming
  language, OS, and CPU architecture
• Disadvantage Complexity

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Component Object Model Plus (COM)

• Like CORBA, defines component registration,
  message routing services, and component
  communication protocol
• Unlike CORBA, components are not assigned
  permanent identifier, and are registered in
  Windows Registry of client machine where
  installed
• Disadvantage Dependence on propriety technology
  and limited support outside of Microsoft products

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Simple Object Access Protocol (SOAP)

• Attempts to address shortcomings of CORBA and
  COM
• Has few infrastructure requirements and
  relatively simple programming interface
• Relies on existing Internet protocols
• Disadvantage Security and message delivery
  guarantees

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SOAP Messages are encoded in XML and transmitted
using HTTP enables objects to be located
anywhere on the Internet.
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Java 2 Enterprise Edition (J2EE)

• Standards for developing/deploying
  component-based distributed applications written
  in Java
• Key elements follow three-layer architecture
• Component interactions based on many standards
• Remote Method Invocation (RMI)
• Java Naming and Directory Interface (JNDI)
• Java Authentication and Authorization Service
  (JAAS)
• Java Database Connectivity (JDBC)

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Directory Services

• Middleware that
• Stores name and network address of distributed
  resources
• Responds to directory queries
• Accepts directory updates
• Synchronizes replicated or distributed directory
  copies
• Integral components of network operating systems

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Information Stored in Network OS Directories

• Registered users and their permissions to access
  directory objects
• Shared hardware resources
• Shared files, databases, and programs
• Computer systems and specialized hardware devices

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Lightweight Directory Access Protocol (LDAP)

• Widely deployed directory service standard that
  can track users, distributed resources, and
  objects
• Limited interoperability among different LDAP
  directories does not define standard content
  templates
• Defines several standard container types and an
  attribute called distinguished name (DN) which
  uniquely identifies the object within an
  objectclass

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• Container object types
• Country (C)
• Organization (O)
• Organizational Unit (OU)

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Microsoft Active Directory

• Directory service and security system built into
  Windows server
• Stores information about network resources
• Every resource or container object has an access
  control list that describes access rights
• Based on LDAP and the Internet Domain Naming
  Service (DNS)
• Does not support distributed or component-based
  software directly

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Summary

• Distributed computing
• Network resource access
• Interprocess communication
• The Internet
• Components and distributed objects
• Distributed computing