Introduction of Distributed Systems CSC 361 Qassim University Computer Science Department By Reham Althunyan George Coulouris, Jean Dooimore, Tim Kindberg, Distributed Systems: Concepts and Design (Fifth edition), Addison-Wesley publishers Limited, 2012 Chapter 1: Characterization of Distributed Systems ◼ ◼ ◼ ◼ ◼ Introduction Examples of distributed systems Resource sharing and the web Challenges Summary Distributed System: Definition General definition: ◼ A distributed system is one in which hardware or software components located at networked computers communicate and coordinate their actions only by passing messages. امللفات تكون مخزنه على االنترنت.. الدروب بوكس، اكبر مثال عليه االنترنت تعريف واسع وعام جدا تدخل فيه اشياء كثيره جدا Distributed System: Definition ◼ A distributed system is a piece of software that ensures that: ◼ ◼ a collection of independent computers appears to its users as a single coherent system Two aspects ◼ Text (1) independent computers and (2) single system => middleware. Characters of Distributed System مهم ويجي باالختبارات ◼ ◼ ◼ Concurrency of components • Concurrent programs execution • Sharing resources No global clock • programs cooperate by coordinating their actions and exchanging messages. • No single global notion of the correct time. Independent failures of components. • The failure of a computer or program somewhere is not known to others. Motivation of DS : Share resources ◼ ◼ ◼ It characterizes the range of the things that can usefully be shared in a networked computer system. It extends from hardware components such as disks and printers to software-defined entities such as files, databases and data objects. It includes the stream of video frames that emerge from a digital video camera and the audio connection from a mobile phone. Examples of distributed systems ◼ ◼ ◼ ◼ The Internet intranet Mobile and ubiquitous computing Distributed Computing as a Utility Internet ◼ ◼ ◼ It is a vast interconnected collection of computer networks of many different types. It is a very large distributed system that allows users, whenever they are, to make use of services such as web, email and file transfer. Internet protocols is a major technical achievement, enabling a program running anywhere to address messages to programs anywhere else. Internet intranet ◼ ◼ ◼ An intranet is a portion of the Internet that is separately administered and has a boundary that can be configured to enforce local security policies. Composed of several Local Area Networks (LANs) linked by backbone connections. It is connected to the Internet via a router to make use of services such as web, email or FTP. intranet intranet ◼ ◼ A firewall is to protect an intranet by preventing unauthorized messages leaving or entering the network. Issues to consider when designing an intranet : • • • File services : to enable users to share data. Firewall : tend to impede legitimate access. The cost of software installation and support. Mobile and ubiquitous computing ◼ ◼ Laptop computers Handheld devices • ◼ Wearable devices • ◼ Personal digital Assistants (PDAs), mobile and smart phones, pagers, video cameras and digital cameras. Smart watches. Embedded devices • Washing machines, hi-fi systems, cars and refrigerators. Portable and handheld devices in a distributed systems Mobile and ubiquitous computing ◼ Mobile computing • • ◼ Is the performance of computing tasks while the user is on the move. Location-aware computing: utilize resources such as printers that are conveniently nearby. Ubiquitous computing • The harnessing of many small, cheap computational devices that are present in users’ physical environment (home, office or elsewhere). Issues to Consider on Mobile Computing ◼ ◼ ◼ ◼ Support for discovery of resources in a host environment. Eliminating the need for users to reconfigure their mobile devices as they move around. Helping users to cope with limited connectivity as they travel. Providing privacy and other security guarantees to users and the environment they visit. Distributed Computing as a Utility :Cloud Computing ◼ Physical Resources • ◼ Storage and processing can be made available to networked computers , removing the needs to own such resources. Software Service • Can be made available across the global Internet. Many companies now offer a comprehensive range of services for effective rental. Distributed Computing as a Utility :Cloud Computing ◼ Cloud Computing • • • ◼ The vision of computing as utility. A set of Internet-based applications , storage and computing services sufficient to support most users’ need. Implemented on cluster computers to provide necessary scale and performance required by such services. Cluster Computers • • A set of interconnected computers that cooperate closely to provide a single , integrated high-performance capability . The overall goal of cluster computers to provide a range of cloud services, including high-performance computing capabilities, mass storage, rich application services. Distributed Computing as a Utility :Cloud Computing Resource sharing and the web بكل اختبار يجيييي ◼ ◼ The primary motivation of distributed systems is resource sharing. Types of Resources • Hardware (printers, scanners or cameras) Data (files, databases or web pages) • More specific functionality (search engines) • Resource sharing and the web Definitions : ◼ Service A distinct part of a computer system that manages a collection of related resources and presents their functionality to users and applications. • ◼ Server A running program (process) on a networked computer that accepts requests from programs running on other computers to perform a service and responds appropriately. • ◼ Client • Is the requesting process from the server. Resource sharing and the web Definitions : ◼ Remote invocation • ◼ ◼ A complete interaction between a client and a server, from the point when the client sends its request to when it receives the server’s response. Clients are active and servers are passive; servers run continuously, whereas clients last only as the applications of which they form a part. Web browser is an example of a client that communicates with a web server, to request a web page. The World Wide Web ▪ Is an evolving system for publishing and accessing resources and services across the Internet. ▪ Purpose of WWW ▪ ▪ Exchanging documents between physicists of European center for nuclear research (CERN), 1989. Provides hypertext structure among the documents that it stores to link to other documents and resources in the web. The World Wide Web ▪ The Web is an open system that can be extended and implemented in new ways without disturbing its existing functionality: 1. 2. Its operation is based on communication standards and document standards that are freely published and widely implemented. With respect to the types of ‘resource’ that can be published and shared on it. The Web components: The 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 using Hyper Text Transfer Protocol (HTTP) ▪ ▪ By which browsers and other clients fetch documents and other resources from web servers. The Web components: HyperText Markup Language (HTML) ⚫ ⚫ ⚫ HTML is used to specify the text and images that make up the contents of a web page, and to specify how they are laid out and formatted for presentation to the user. HTML is stored in a file that web server can access. A browser retrieves the contents of this file from a web server. -The browser reads the contents returned by the server and renders it into formatted text and images on the web page. -The server can infer the content type from the filename extension ‘.html’. HyperText Markup Language (HTML) <IMG SRC = http://www.cdk3.net/WebExample/Images/earth.jpg> <P> Welcome to Earth! Visitors may also be interested in taking a look at the <A HREF = “http://www.cdk3.net/WebExample/moon.html>Moon</A>. <P> (etcetera) ▪ A browser retrieves the contents of this file (earth.html) from a web server. ▪ The browser reads the contents returned by the server and renders it into formatted text and images on the web page. ▪ The server can infer the content type from the filename extension ‘.html’. Uniform Resource Locator (URL) To identify a resource in such away as to enable the browser to locate the resource. Every URL has two top-level components : ▪ ▪ ▪ ▪ ▪ Scheme: scheme-specific-location Scheme declares which type of URL this is. ◼mailto:joe@anISP.net : a user email address ftp://ftp.downloadIt.com/software/a9Prog.exe : a file that is to be retrieved using File Transfer Protocol (FTP) nntp : usenet news group, telnet : to log in to a computer HTTP URLs are the most widely used protocol. ▪ ▪ ▪ ▪ Identify which web server maintains the resource. Which of the resources at the server is required. http://servername [:port][PathNameOnServer][?arguments] HyperText Transfer Protocol (HTTP) Defines the ways in which browsers and any other types of client interact with web servers. HTTP features : ▪ ▪ ▪ ▪ ▪ ▪ Request-reply interactions Content types. (GIF,JPEG, Text/html) the strings that denote the type of content are called MIME One resource per request. (HTTP version 1.0) ▪ 9 images = 10 requests. Simple access control : e.g. Password Advanced Features – Services and Dynamic pages. Dynamic content ▪ ▪ Common Gateway Interface (CGI): a program that web servers run to generate content for their clients. Downloaded code ▪ ▪ ▪ JavaScript : immediate feedback Applet : Chat applications Discussion of the Web ▪ Web is a Hypertext Model that is lacking in some respects : 1. A resource is deleted or moved, but links to it may still remain (dangling). 2. Resource Description Framework is to standardize the format of metadata about web resources to find data easily. 3. Extensible Markup Language (XML) : A way of representing data in standard, structured, application- specific forms rather than HTML. 4. Scalability: popular web servers may experience many ‘hits’ per second, which results in slow response. Challenges: ▪ ▪ ▪ ▪ ▪ ▪ ▪ Heterogeneity Openness Security Scalability Failure handling Concurrency Transparency Challenges : Heterogeneity Networks ▪ ▪ IP over Ethernet, token ring Computer hardware ▪ ▪ Different representation for Data types (byte ordering of Integers) Operating systems ▪ ▪ Exchanging messages in Unix are different from Windows Programming languages ▪ ▪ Different representations for characters and data structures such as arrays and records. Implementations by different developers ▪ ▪ No common standards in writing applications. Challenges : Heterogeneity ◼ Middleware ◼ ◼ ◼ Applies to a software layer that provides a programming abstraction as well as masking the heterogeneity of the underlying networks, hardware, operating systems and programming languages. It provides a uniform computational model for use by the programmers of servers and distributed application to solve the problem of heterogeneity. Mobile code ◼ is used to refer to code that can be sent from one computer to another and run at the destination (Java applet, Java Virtual Machine) Challenges : Openness Openness of a computer system ▪ ▪ Is the characteristic that determines whether the system can be extended and reimplemented in various ways. Openness of distributed systems ▪ ▪ Is 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. How to deal with openness? ▪ ▪ Key interfaces are published, e.g. Request for Comments (RFC) Challenges : Security ▪ Confidentiality ▪ protection against disclosure to unauthorized individuals, ▪ ACL in Unix File System ▪ Integrity ▪ protection against alteration or corruption, ▪ Checksum, encryption ▪ Availability ▪ protection against interference with the means to access the resources, ▪ Denial of service, security of mobile code Challenges : Scalability ▪ A system is described as scalable if will remain effective when there is a significant increase in the number of resources and the number of users ▪ The Internet provides an illustration of scalable system Challenges : Scalability ▪ Design challenges ▪Controlling the cost of physical resources, system with (n) users requires at most O(n) physical resources. ▪Controlling the performance loss, DNS no worse than O(logn) ▪Prevent software resources running out, e.g., IP address ▪Avoid performance bottlenecks, e.g., partitioning name table of DNS, cache and replication Challenges : Failure handling Failure in distributed systems are partial, some components fail while others continue to function. ▪ Detecting failure ▪ ▪ ▪ Checksum for corrupted data Sometimes impossible to detect, e.g. a remote crashed server in the Internet Masking failure ▪ ▪ Retransmit message when they fail to arrive. Tolerating failure ▪ ▪ A web browser cannot contact a web server Recovery from failure ▪ ▪ State of permanent data can be recovered or Rolled back Redundancy ▪ ▪ Routes in the Internet, replicated name table of DNS Challenges : Concurrency • Correctness – ensure the operations on shared resource correct in a concurrent environment e.g. records bids for an auction • Performance – Ensure the high performance of concurrent operations Challenges : Transparency ▪ The concealment from the user and the application programmer of the separation of components in a distributed system. ▪ Access transparency ▪ Enable local and remote resources to be accessed using identical operations, e.g. GUI ▪ Location transparency ▪ resources to be accessed without knowledge of their location, e.g. URL ▪ Concurrency transparency ▪ several processed operate concurrently using shared resources without interference between them. Challenges : Transparency ▪ Replication transparency ▪ multiple instances of resources to be used to increase reliability and performance without knowledge of the replicas by users or application programmers, ▪ Failure transparency ▪ users and applications to complete their tasks despite the failure of hardware and software components, e.g., email ▪ Mobility transparency ▪ movement of resources and clients within a system without affecting the operation of users and programs, e.g., mobile phone ▪ 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 Summary • Distributed systems are pervasive • Resource sharing is the primary motivation for constructing distributed systems • Characterization of Distributed System – Concurrency – No global clock – Independent failures Summary • Challenges to construct distributed system – – – – – – – Heterogeneity Openness Security Scalability Failure handling Concurrency Transparency
