2. 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
Systems Architecture, Fifth Edition

3. 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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4. Systems Architecture, Fifth Edition

5. 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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6. Client/Server Architecture
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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7. One of the many different ways that client/server architecture can be implemented.
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8. 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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9. 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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10. 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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11. 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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12. 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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14. 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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15. 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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16. 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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17. Premises of Remote 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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18. Premises of Remote 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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19. 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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20. Software components that support service-oriented resource access
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21. 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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22. 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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24. Sockets
Unique combinations of an IP number and a port number, separated by a colon (e.g., :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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25. Each socket uniquely identifies a client or server process on the Internet.
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26. 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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27. 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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28. 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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29. 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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30. 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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31. DCE functions are incorporated directly into an operating system or supplied as an optional component.
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32. 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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33. Standard Web Protocols and Services
Define valid resource formats and a standard means of requesting resources
Identified by a unique Uniform Resource Locator (URL)
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34. Resource
Host
Protocol
Port
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37. 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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38. Database is back-end server and is accessed via
pipes/sockets/RPC; https or Kerberos used
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39. 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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40. Components and Distributed Objects
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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41. 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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42. Components and Objects
Send and respond to messages
Encapsulate internal data
Interact with other components through a well-defined interface
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43. 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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44. 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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45. 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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46. 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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47. SOAP: Messages are encoded in XML and transmitted using HTTP; enables objects to be located anywhere on the Internet.
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48. 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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50. 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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51. 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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52. 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
Systems Architecture, Fifth Edition

53. Container object types: Country (C) Organization (O)
Organizational Unit (OU)
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54. 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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57. Summary Distributed computing Network resource access
Interprocess communication
The Internet
Components and distributed objects
Systems Architecture, Fifth Edition