Resource Sharing and Challenges - Distributed Computing

Resource Sharing and
Challenges
Unit - I

Resource Sharing
• Hardware resources such as printers, data resources such as files, and resources with more
specific functionality such as search engines.
• Search engine on the Web
• provides a facility to users throughout the world
• users who need never come into contact with one another directly.
• In computer-supported cooperative working (CSCW)
• a group of users who cooperate directly share resources such as documents in a small, closed group.
• Service
• manages a collection of related resources and presents their functionality to users and applications.
• eg: File Services - read, write and delete operation on files
• Server
• refers to a running program (a process) on a networked computer that accepts requests from programs
running on other computers to perform a service and responds appropriately.
• Approaches
• Client server computing

Challenges
• Heterogeneity
• Openness
• Security
• Scalability
• Failure Handling
• Concurrency
• Transparency
• Quality of Service

Heterogeneity
• applies to
• networks
• computer hardware
• operating systems
• programming languages
• implementations by different developers
• Different programming languages use different representations for characters and data structures such as
arrays and records
• Middleware
• applies to a software layer that provides a programming abstraction as well as masking the heterogeneity
• provides a uniform computational model for use by the programmers of servers and distributed applications
• eg: SQL, COBRA
• Mobile code
• used to refer to program code that can be transferred from one computer to another and run at the destination
• eg: Java applets

Openness
• The characteristic that determines whether the system can be extended and
reimplemented in various ways.
• Determined primarily by the degree to which new resource-sharing services
can be added and be made available for use by a variety of client programs.
• Characterized by the fact that their key interfaces are published.
• Based on the provision of a uniform communication mechanism and
published interfaces for access to shared resources.
• Constructed from heterogeneous hardware and software, possibly from
different vendors.
• Open distributed systems to emphasize the fact that they are extensible.

Security
• Security for information resources has three components:
• confidentiality
• protection against disclosure to unauthorized individuals
• integrity
• protection against alteration or corruption
• availability
• protection against interference with the means to access the resources.
• Security Challenges
• Denial of service attacks
• Disrupt a service for some reason
• Bombarding the service with such a large number of pointless requests that the serious users are
unable to use it.
• Security of Mobile code

Scalability
• Ranging from a small intranet to the Internet.
• Design Challenges
• Controlling the cost of physical resources
• As the demand for a resource grows, it should be possible to extend the system, at reasonable cost, to
meet it.
• Controlling the performance loss
• Consider the management of a set of data whose size is proportional to the number of users or
resources in the system
• Preventing software resources running out
• larger Internet addresses will occupy extra space in messages and in computer storage.
• Avoiding performance bottlenecks
• algorithms should be decentralized to avoid having performance bottlenecks
• The Domain Name System removed this bottleneck
• partitioning the name table between servers located throughout the Internet and administered locally

Transparency
• the concealment from the user and the application programmer of the separation
of components in a distributed system
• Access transparency
• enables local and remote resources to be accessed using identical operations.
• Location transparency
• enables resources to be accessed without knowledge of their physical or network location.
• Concurrency transparency
• enables several processes to operate concurrently using shared resources without
interference between them.
• Replication transparency
• enables multiple instances of resources to be used to increase reliability and performance
without knowledge of the replicas by users or application programmers.

Transparency
• Failure transparency
• enables the concealment of faults, allowing users and application programs to
complete their tasks despite the failure of hardware or software components.
• Mobility transparency
• allows the movement of resources and clients within a system without affecting
the operation of users or programs.
• Performance transparency
• allows the system to be reconfigured to improve performance as loads vary.
• Scaling transparency
• allows the system and applications to expand in scale without change to the
system structure or the application algorithms.

QoS – Quality of Service
• Functionality
• require of a service, such as the file service in a distributed system
• Nonfunctional properties
• affect the quality of the service experienced by clients and users are reliability,
security and performance.
• Adaptability
• meet changing system configurations and resource availability has been
recognized as a further important aspect of service quality.
• Reliability and security issues are critical in the design of most computer
systems

WWW – World Wide Web
• Feature of the Web
• provides a hypertext structure among the documents that it stores, reflecting the users’ requirement to
organize their knowledge.
• Web is an open system
• First, its operation is based on communication standards and document or content standards that are
freely published and widely implemented.
• Second, the Web is open with respect to the types of resource that can be published and shared on it.
• Three main standard technological components:
• HyperText Markup Language (HTML)
• a language for specifying the contents and layout of pages as they are displayed by web browsers
• Uniform Resource Locators (URLs)
• identify documents and other resources stored as part of the Web;
• a client-server system architecture
• with standard rules for interaction

• HTML
• to specify the text and images that make up the contents of a web page
• to specify how they are laid out and formatted for presentation to the user.
• A web page contains such structured items as headings, paragraphs, tables and
images.
• HTML is also used to specify links and which resources are associated with them.

• URL
• To identify a variety of resource.
• Also known as Uniform Resource Identifier (URI).
• Browsers examine URLs in order to access the corresponding resources.
• Every URL, in its full, absolute form, has two top-level components:
scheme : scheme-specific-identifier
• The first component, the ‘scheme’, declares which type of URL this is.
• The second component are used to identify the resource.
• HTTP URL has two main jobs:
• to identify which web server maintains the resource
• to identify which of the resources at that server is required.
• HTTP URLs are of the following form:
http:// servername [:port] [/pathName] [?query] [ #fragment]

• URL

• Functions
• Publishing a resource
• HTTP
• Reply – Request Interactions
• Content Types
• One Resource per request
• Simple access control
• Dynamic Pages
• Downloaded code
• Common Interface Gateway (CGI) program runs at the server
• AJAX and applet
• Web Services

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