2 systems architecture, fifth edition chapter goals describe client/server and multi-tier...

2Systems Architecture, Fifth Edition

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

3Systems Architecture, Fifth Edition

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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5Systems Architecture, Fifth Edition

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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14Systems Architecture, Fifth Edition

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)

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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17Systems Architecture, Fifth Edition

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 shouldn’t 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

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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22Systems Architecture, Fifth Edition

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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24Systems Architecture, Fifth Edition

Sockets

• Unique combinations of an IP number and a port number, separated by a colon (e.g., 129.24.8.1:53)

• 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

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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30Systems Architecture, Fifth Edition

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 InternetInternet • 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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PortProtocol

Host

Resource

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36Systems Architecture, Fifth Edition

37Systems Architecture, Fifth Edition

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 viapipes/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 a

well-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.

48Systems Architecture, Fifth Edition

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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50Systems Architecture, Fifth Edition

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)

54Systems Architecture, Fifth Edition

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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56Systems Architecture, Fifth Edition

57Systems Architecture, Fifth Edition

Summary

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