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INTRODUCTION TO DISTRIBUTED SYSTEM (DEFINITION (Examples (Internet,…

- - - - Variety and differences in
         Networks
         Computer hardware
         Operating systems
         Programming languages
         Implementations by different developers
      - Middleware as software layers to provide a programming abstraction as well as masking the heterogeneity of the underlying networks, hardware, OS, and programming languages (e.g., CORBA).
      - Mobile Code to refer to code that can be sent from one computer to another and run at the destination (e.g., Java applets and Java virtual machine).
    - - concerned with extensions and improvements of distributed systems
      - Detailed interfaces of components need to be published
      - New components have to be integrated with existing components
      - Systems that are designed to support resource sharing in this way are termed as open distributed system 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 introducing of the new services and the re-implementation of the old one, enabling application to share resources.
    - - clients send requests to access data managed by servers, resources in the networks: each links required security measures.  Doctors requesting records from hospitals  Users purchase products through electronic commerce
      - Security is required for:  Concealing the contents of messages: security and privacy  Identifying a remote user or other agent correctly (authentication)
      - New challenges:  Denial of service attack  Security of mobile code
    - - must be scalable as the number of user increases
      - A system is said to be scalable if it can handle the addition of users and resources without suffering a noticeable loss of performance or increase in administrative complexity
      - dimensions
        
        • Size – Number of users and resources to be processed. Problem associated is overloading
        • Geography – Distance between users and resources. Problem associated is communication reliability
        • Administration – As the size of distributed systems increases, many of the system needs to be controlled.
    - - involves a lot of collaborating components (hardware, software, communication). So there is a huge possibility of partial or total failure.
      - Hardware, software and networks fail!
      - Distributed systems must maintain availability even at low levels of hardware/software/network reliability
      - Fault tolerance is achieved by – recovery – redundancy
    - - Distributed Systems usually is multi-users environment. In order to maximize concurrency, resource handling components should be anticipate as they will be accessed by competing users. Concurrency is a tricky challenges, then we must avoid the system’s state from becoming unstable when users compete to view or update data.
    - - Hide the fact that its processes and resources are physically distributed across multiple computers.
      - Distributed systems designers must hide the complexity of the systems as much as they can
      - While users hit search in google.com, they never notice that their query goes through a complex process before google shows them a result. Some terms of transparency in distributed systems
        
        • Access: Hide the differences in data representation and how a resource is accessed
        • Location: Hide where a resource is located
        • Migration: Hide that a resource may move to another location •Relocation: Hide that a resource may be moved to another location while in use
        • Replication: Hide that a resources is replicated
        • Concurrency: Hide that a resource may be shared by several competitive users
        • Failure: Hide the failure and recovery of a resource.
- - - - Google, the market leader in web search technology, has put significant effort into the design of a sophisticated distributed system infrastructure to support search (and indeed other Google applications and services such as Google Earth)
      - Highlights of this infrastructure
        
        an underlying physical infrastructure consisting of very large numbers of networked computers located at data centres all around the world
        
        a distributed file system designed to support very large files and heavily optimized for the style of usage required by search and other Google applications (especially reading from files at high and sustained rates)
        
        an associated structured distributed storage system that offers fast access to very large datasets
        
        a lock service that offers distributed system functions such as distributed locking and agreement
        
        a programming model that supports the management of very large parallel and distributed computations across the underlying physical infrastructure
    - - Massively multiplayer online games offer an immersive experience whereby very large numbers of users interact through the Internet with a persistent virtual world.
      - The engineering of MMOGs represents a major challenge for distributed systems technologies, particularly because of the need for fast response times to preserve the user experience of the game.
      - Other challenges include the real-time propagation of events to the many players and maintaining a consistent view of the shared world.
      - Therefore provides an excellent example of the challenges facing modern distributed systems designers.
    - - The financial industry has long been at the cutting edge of distributed systems technology with its need, in particular, for real-time access to a wide range of information sources (for example, current share prices and trends, economic and political developments). The industry employs automated monitoring and trading applications
      - Note that the emphasis in such systems is on the communication and processing of items of interest, known as events in distributed systems, with the need also to deliver events reliably and in a timely manner to potentially very large numbers of clients who have a stated interest in such information items.
      - Examples of such events include a drop in a share price, the release of the latest unemployment figures, and so on
- - - - Laptops • Handheld devices (personal digital assistants, mobile phones, • video cameras, digital cameras • Wearable devices (smart watches) • Location-aware or content-aware computing
    - - the harnessing of many small cheap computational devices that are presents in the users physical environment including the home office and even natural settings
      - present, appearing, or found everywhere
      - method of enhancing computer use by making many computers available throughout the physical environment, but making them effectively invisible to the user.
      - Thus, mobile and ubiquitous networks is another example for distributed system
      - The presence of computer everywhere can only be useful when they can communicate (share resources) with one another.
      - For instance, it will be very convenient for the users to control their washing machine through universal remote control
    - - Contextual change: The system is part of an environment in which changes should be immediately accounted for.
      - Ad hoc composition: Each node may be used in a very different ways by different users. Requires ease-of-configuration
      - Sharing is the default: Nodes come and go, providing sharable services and information.