DS3.pdf

Distributed systems has the following significant consequences: Concurrency: In a network of computers, concurrent program execution is the norm. I can do my work on my computer while you do your work on yours, sharing resources such as web pages or files when necessary. The capacity of the system...

Distributed systems has the following significant consequences: Concurrency: In a network of computers, concurrent program execution is the norm. I can do my work on my computer while you do your work on yours, sharing resources such as web pages or files when necessary. The capacity of the system to handle shared resources can be increased by adding more resources (for example. computers) to the network. We will describe ways in which this extra capacity can be usefully deployed at many points in this book. The coordination of concurrently executing programs that share resources is also an important and recurring topic. No global clock: When programs need to cooperate they coordinate their actions by exchanging messages. Close coordination often depends on a shared idea of the time at which the programs’ actions occur. But it turns out that there are limits to the accuracy with which the computers in a network can synchronize their clocks – there is no single global notion of the correct time. This is a direct consequence of the fact that the only communication is by sending messages through a network. Independent failures: All computer systems can fail, and it is the responsibility of system designers to plan for the consequences of possible failures. Distributed systems can fail in new ways. Faults in the network result in the isolation of the computers that are connected to it, but that doesn’t mean that they stop running. In fact, the programs on them may not be able to detect whether the network has failed or has become unusually slow. Similarly, the failure of a computer, or the unexpected termination of a program somewhere in the system (a crash), is not immediately made known to the other components with which it communicates. Each component of the system can fail independently, leaving the others still running. TRENDS IN DISTRIBUTED SYSTEMS Distributed systems are undergoing a period of significant change and this can be traced back to Page | 11 a number of influential trends:  the emergence of pervasive networking technology; the emergence of ubiquitous computing coupled with the desire to support user  mobility in distributed systems;  the increasing demand for multimedia services; the view of distributed systems as a utility. Internet The modern Internet is a vast interconnected collection of computer networks of many different types, with the range of types increasing all the time and now including, for example, a wide range of wireless communication technologies such as WiFi, WiMAX, Bluetooth and third- generation mobile phone networks. The net result is that networking has become a pervasive resource and devices can be connected (if desired) at any time and in any place. A typical portion of the Internet The Internet is also a very large distributed system. It enables users, wherever they are, to make use of services such as the World Wide Web, email and file transfer. (Indeed, the Web is sometimes incorrectly equated with the Internet.) The set of services is open-ended – it can be extended by the addition of server computers and new types of service. The figure shows a collection of intranets – subnetworks operated by companies and other organizations and typically protected by firewalls. The role of a firewall is to protect an intranet by preventing unauthorized messages from leaving or entering. A firewall is implemented by filtering incoming and outgoing messages. Filtering might be done by source or destination, or a firewall might allow only those messages related to email and web access to pass into or out of the intranet that it protects. Internet Service Providers (ISPs) are companies that provide broadband links and other types of connection to individual users and small organizations, enabling them to access services anywhere in the Internet as well as providing local services such as email and web hosting. The intranets are linked together by backbones. A backbone is a network link with a high transmission capacity, employing satellite connections, fibre optic cables and other high- bandwidth circuits Page | 12 Computers vs. Web servers in the Internet Intranet – 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 LANs linked by backbone connections – Be connected to the Internet via a router A typical intranet email server Deskt op compu ters print and otherservers Local area Web server network email server print File server other the rest of the Internet router/fire wall servers Page | 13 Main issues in the design of components for the use in intranet File services Firewall The cost of software installation and support Mobile and ubiquitous computing Technological advances in device miniaturization and wireless networking have led increasingly to the integration of small and portable computing devices into distributed systems. These devices include:  Laptop computers. Handheld devices, including mobile phones, smart phones, GPS-enabled devices, pagers,  personal digital assistants (PDAs), video cameras and digital cameras.  Wearable devices, such as smart watches with functionality similar to a PDA. Devices embedded in appliances such as washing machines, hi-fi systems, cars and refrigerators. The portability of many of these devices, together with their ability to connect conveniently to networks in different places, makes mobile computing possible. Mobile computing is the performance of computing tasks while the user is on the move, or visiting places other than their usual environment. In mobile computing, users who are away from their ‘home’ intranet (the intranet at work, or their residence) are still provided with access to resources via the devices they carry with them. They can continue to access the Internet; they can continue to access resources in their home intranet; and there is increasing provision for users to utilize resources such as printers or even sales points that are conveniently nearby as they move around. The latter is also known as location-aware or context-aware computing. Mobility introduces a number of challenges for distributed systems, including the need to deal with variable connectivity and indeed disconnection, and the need to maintain operation in the face of device mobility. Portable and handheld devices in a distributed system Internet Host intranet WAP Home intranet Wireless LAN gateway Mobile phone Printer Laptop Camera Host site Ubiquitous computing is the harnessing of many small, cheap computational devices that are present in users’ physical environments, including the home, office and even natural settings. The term ‘ubiquitous’ is intended to suggest that small computing devices will eventually Page | 14 become so pervasive in everyday objects that they are scarcely noticed. That is, their computational behaviour will be transparently and intimately tied up with their physicalfunction. The presence of computers everywhere only becomes useful when they can communicate with one another. For example, it may be convenient for users to control their washing machine or their entertainment system from their phone or a ‘universal remote control’ device in the home. Equally, the washing machine could notify the user via a smart badge or phone when the washing is done. Ubiquitous and mobile computing overlap, since the mobile user can in principle benefit from computers that are everywhere. But they are distinct, in general. Ubiquitous computing could benefit users while they remain in a single environment such as the home or a hospital. Similarly, mobile computing has advantages even if it involves only conventional, discrete computers and devices such as laptops and printers. RESOURCE SHARING Is the primary motivation of distributed computing Resources types – Hardware, e.g. printer, scanner, camera – Data, e.g. file, database, web page – More specific functionality, e.g. search engine, file Service – manage a collection of related resources and present their functionalities to users and applications Server – a process on networked computer that accepts requests from processes on other computers to perform a service and responds appropriately Client – the requesting process Remote invocation A complete interaction between client and server, from the point when the client sends its request to when it receives the server’s response Motivation of WWW – Documents sharing between physicists of CERN – Web is an open system: it can be extended and implemented in new ways without disturbing its existing functionality. – Its operation is based on communication standards and document standards – Respect to the types of ‘resource’ that can be published and shared on it. HyperText Markup Language – A language for specifying the contents and layout of pages Uniform Resource Locators – Identify documents and other resources A client-server architecture with HTTP – By with browsers and other clients fetch documents and other resources from web servers HTML Page | 15 HTML text is stored in a file of a web server.  A browser retrieves the contents of this file from a web server. -The browser interprets the HTML text -The server can infer the content type from the filename extension. URL   HTTP URLs are the most widely   used An HTTP URL has two main jobs to do: To identify which web server maintains the resource To identify which of the resources at that server Web servers and web browsers HTTP URLs Page | 16 HTTP Defines the ways in which browsers and any other types of client interact with web servers (RFC2616) Main features – Request-replay interaction – Content types. The strings that denote the type of content are called MIME (RFC2045,2046) – One resource per request. HTTP version 1.0 – Simple access control More features-services and dynamic pages Dynamic content – Common Gateway Interface: a program that web servers run to generate content for their clients Downloaded code – JavaScript – Applet Discussion of Web   Dangling: a resource is deleted or moved, but links to it may still remain   Description Framework which standardize the format Find information easily: e.g. Resource  of metadata about web resources  Exchange information easily: e.g. XML – a self  describing language  Scalability: heavy load on popular web servers  More applets or many images in pages increase in the download time Page | 17 THE CHALLENGES IN DISTRIBUTED SYSTEM: Q7.Heterogeneity The Internet enables users to access services and run applications over a heterogeneous collection of computers and networks. Heterogeneity (that is, variety and difference) applies to all of the following:  networks;  computer hardware;  operating systems;  programming languages; implementations by different developers Although the Internet consists of many different sorts of network, their differences are masked by the fact that all of the computers attached to them use the Internet protocols to communicate with one another. For example, a computer attached to an Ethernet has an implementation of the Internet protocols over the Ethernet, whereas a computer on a different sort of network will need an implementation of the Internet protocols for that network. Data types such as integers may be represented in different ways on different sorts of hardware – for example, there are two alternatives for the byte ordering of integers. These differences in representation must be dealt with if messages are to be exchanged between programs running on different hardware. Although the operating systems of all computers on the Internet need to include an implementation of the Internet protocols, they do not necessarily all provide the same application programming interface to these protocols. For example, the calls for exchanging messages in UNIX are different from the calls in Windows. Different programming languages use different representations for characters and data structures such as arrays and records. These differences must be addressed if programs written in different languages are to be able to communicate with one another. Programs written by different developers cannot communicate with one another unless they use common standards, for example, for network communication and the representation of primitive data items and data structures in messages. For this to happen, standards need to be agreed and adopted – as have the Internet protocols. Middleware The term 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. The Common Object Request Broker (CORBA), is an example. Some middleware, such as Java Remote Method Invocation (RMI), supports only a single programming language. Most middleware is implemented over the Internet protocols, which themselves mask the differences of the underlying networks, but all middleware deals with the differences in operating systems and hardware. Heterogeneity and mobile code The term mobile code is used to refer to program code that can be transferred from one computer to another and run at the destination – Java applets are an example. Code suitable for running on one computer is not necessarily suitable for running on another because executable programs are normally specific both to the instruction set and to the host operating system. The virtual machine approach provides a way of making code executable on a variety of host computers: the compiler for a particular language generates code for a virtual machine instead of Page | 18  - a particular hardware order code. For example, the Java compiler produces code for a Java virtual machine, which executes it by interpretation. The Java virtual machine needs to be implemented once for each type of computer to enable Java programs to run. Today, the most commonly used form of mobile code is the inclusion Javascript programs in some web pages loaded into client browsers. Q7.Openness The openness of a computer system is the characteristic that determines whether the system can be extended and reimplemented in various ways. The 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. Openness cannot be achieved unless the specification and documentation of the key software interfaces of the components of a system are made available to software developers. In a word, the key interfaces are published. This process is akin to the standardization of interfaces, but it often bypasses official standardization procedures, which are usually cumbersome and slow-moving. However, the publication of interfaces is only the starting point for adding and extending services in a distributed system. The challenge to designers is to tackle the complexity of distributed systems consisting of many components engineered by different people. The designers of the Internet protocols introduced a series of documents called ‘Requests For Comments’, or RFCs, each of which is known by a number. The specifications of the Internet communication protocols were published in this series in the early 1980s, followed by specifications for applications that run over them, such as file transfer, email and telnet by the mid-1980s. Systems that are designed to support resource sharing in this way are termed open distributed systems to emphasize the fact that they are extensible. They may be extended at the hardware level by the addition of computers to the network and at the software level by the introduction of new services and the reimplementation of old ones, enabling application programs to share resources. To summarize: Open systems are characterized by the fact that their key interfaces are published. Open distributed systems are based on the provision of a uniform communication mechanism and published interfaces for access to shared resources. Open distributed systems can be constructed from heterogeneous hardware and software, possibly from different vendors. But the conformance of each component to the published standard must be carefully tested and verified if the system is to work correctly. Security Many of the information resources that are made available and maintained in distributed systems have a high intrinsic value to their users. Their security is therefore of considerable importance. Security for information resources has three components: confidentiality (protection against disclosure to unauthorized individuals), integrity Page | 19  - (protection against alteration or corruption), and availability (protection against interference with the means to access the resources). In a distributed system, clients send requests to access data managed by servers, which involves sending information in messages over a network. For example: A doctor might request access to hospital patient data or send additions to that data. In electronic commerce and banking, users send their credit card numbers across the Internet. In both examples, the challenge is to send sensitive information in a message over a network in a secure manner. But security is not just a matter of concealing the contents of messages – it also involves knowing for sure the identity of the user or other agent on whose behalf a message was sent. However, the following two security challenges have not yet been fully met: Denial of service attacks: Another security problem is that a user may wish to disrupt a service for some reason. This can be achieved by bombarding the service with such a large number of pointless requests that the serious users are unable to use it. This is called a denial of service attack. There have been several denial of service attacks on well-known web services. Currently such attacks are countered by attempting to catch and punish the perpetrators after the event, but that is not a general solution to the problem. Security of mobile code: Mobile code needs to be handled with care. Consider someone who receives an executable program as an electronic mail attachment: the possible effects of running the program are unpredictable; for example, it may seem to display an interesting picture but in reality it may access local resources, or perhaps be part of a denial of service attack. Scalability Distributed systems operate effectively and efficiently at many different scales, ranging from a small intranet to the Internet. A system is described as scalable if it will remain effective when there is a significant increase in the number of resources and the number of users. The number of computers and servers in the Internet has increased dramatically. Figure 1.6 shows the increasing number of computers and web servers during the 12-year history of the Web up to 2005 [zakon.org]. It is interesting to note the significant growth in both computers and web servers in this period, but also that the relative percentage is flattening out – a trend that is explained by the growth of fixed and mobile personal computing. One web server may also increasingly be hosted on multiple computers.