Resource Sharing and Challenges - Distributed Computing

Distributed Computing

1. Resource Sharing and Challenges Unit - I

2. 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 Remote Invocation

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

4. 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

5. 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.

6. 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

7. 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

8. 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.

9. 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.

10. 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

11. 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

12. 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

13. WWW – World Wide Web 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.

14. WWW – World Wide Web 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]

15. WWW – World Wide Web URL

16. WWW – World Wide Web 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