DistSyst

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Zidanny5
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291626
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DistSyst
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2014-12-16 16:32:03
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DystSyst
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  1. Name the two problems caused by increase in end users for two-tier client system?
    1. A ‘fat’ client, requiringconsiderable resources on the client’s computer to run effectively (resources include disk space, RAM, and CPU power).

    2. A significant client side administration overhead.
  2. In 1995, what 3 layers were proposed for a 3-tier architecture and what was different about them?
    1. The user interface layer, which runs on the client computer.

    2. The business logic and data processing layer. This middle tier runs on a server and is often called the application server. One application server is designed to server multiple clients.

    3. A DBMS, which stores the data required by the middle tier. This tier may runa on a seperate server called the database server.
  3. Advantages of a 3-tier design over 2-tier?
    A 'thin' client, which requires less expensive hardware.

    Simplified application maintenance, as a result of centralising the business logic for many end-users into a single application server. This eliminates the concerns of software distribution that are problematic in the tratitional two-tier client/server architecture.

    Added modularity, which makes it easier to modify or replace one tier without affecting the other tiers.

    Easier load balancing, again as a result of separating the core business logic from the database functions. For example, a Transaction Processing Monitor (TPM) can be used to reduce the number of connections to the database server. (A TPM is a program that controls data transfer between clients and servers in order to provide a consistent environment for Online Transaction Processing (OLTP).

    An aditional advantage is that three-tier architecture maps naturally to the Web environment, with the Web browser acting as a 'thin' client and a Web Server acting as the application server.
  4. What is a web service?
    A web service is a web page that's meant to be consumed by an autonomous program as opposed to a Web browser or similar UI tool
  5. What is REST
    REpresentational State Transfer

    An architectural style, not a toolkit.

    Resources are identified by uniform resource identifiers (URIs)
  6. Identify the transparency requirements of a distributed file system. Your answer should discuss the relative importance of the various forms of transparency and provide application examples to illustrate and justify your answer.
    Access and Location transparency are the most important in this context. It is vital that users can access their files regardless of whether they are held locally or remotely, and the user should not have to know where the files are actually held, or to do anything different to access the files if they are remote.

    Failure, Concurrency, Performance and Scalability transparencies are also very important. Users should have reliable error-free access to their files and should not be corrupted (in particular, multi-user updates should be protected from the lost-update problem). Performance should be predictable (not sporadic) and should be maintained (or degrade gracefully) as the system scale increases.

    Replication transparency is desirable, as it is a means of increasing robustness, access and performance.
  7. Explain,the need for clock synchronisation services in distributed systems. Provide application examples to illustrate your answer.
    Each node must have a clock and these need to be kept synchronised (i.e. telling the same time). Clocks are used to timestamp events and this is important in transactions. It is sometimes very important to know the order in which events occurred. 

    Examples situations of the use of time-stamping to determine event order would include financial transactions (e.g stock buying and selling, bank account updates, deposits and withdrawals.)

     

    • Replicated database updates require that all replicas remain consistent – one node must not perform event A before event B whilst another performs event B before event A. A
    • telemetry application (at a factory) might ignore the situation where valve X opens before the temperature in tank Y reaches 100 degrees, but signal an alarm if the events are seen to occur in opposite order.
  8. Show with the aid of diagrams, how phantom deadlocks can occur, and discuss the consequences of this.
    • It takes time to gather the ‘state’ of processes and to determine if deadlock is present. During the gathering process the state of processes is constantly running, thus it is not possible to achieve a completely consistent global view.

    The diagram shows that if state-collection spans both time T0 and time T1 it will ‘detect’  a deadlock that does not exist. There is no complete cycle at either time T1 or time T2.

    The consequence of phantom deadlocks are wasting valuable resources, needlessly stopping processes and restarting processes.
  9. Explain the difficulties of detecting deadlock in a distributed environment.
    • The difficulties arise when processes, resources and thus, process’ state are distributed across several computers. The local operating system (i.e. on a specific computer) can only see a subset of the processes, resources and their state.

    The diagram shows two processes on two different computers that are deadlocked because they are competing for two resources. The resources are located on separate computers and the deadlock itself is on three computers. No computer can ‘see’ the entire deadlock.
  10. Identify the transparency requirements of a distributed file system. Your answer should discuss the relative importance of the various forms of transparency and provide application examples to illustrate and justify your answer.
    Access and Location transparency are the most important in this context. It is vital that users can access their files regardless of whether they are held locally or remotely, and the user should not have to know where the files are actually held, or to have to do anything different to access the files if they are remote.

    Failure, Concurrency, Performance and Scalability transparencies are also very important. Users should have reliable error-free access to their files and files should not be corrupted (in particular, multi-user updates should be protected from the lost-update problem). Performance should be predictable (not sporadic) and should be maintained (or degrade gracefully) as the system scale increases.



    Replication transparency is desirable, as it is a means of increasing robustness, access and performance.
  11. Explain,the need for clock synchronisation services in distributed systems. Provide application examples to illustrate your answer.
    Each node must have a clock and these need to be kept synchronised (i.e. telling the same time). Clocks are used to timestamp events and this is important in transactions. It is sometimes very important to know the order in which events occurred. 

    Examples situations of the use of time-stamping to determine event order would include financial transactions (e.g stock buying and selling, bank account updates, deposits and withdrawals.)

    Replicated database updates require that all replicas remain consistent – one node must not perform event A before event B whilst another performs event B before event A.  A telemetry application (at a factory) might ignore the situation where valve X opens before the temperature in tank Y reaches 100 degrees, but signal an alarm if the events are seen to occur in opposite order.
  12. Critically discuss ways in which a clock synchronisation service could be implemented. Your answer should highlight the strengths and weaknesses of at least two approaches.
    Updates are subject to message delays – thus absolute synchronisation is not possible. Problems associated with adjusting clocks immediately – better to speed them up or slow them down a small amount and adjust them over a period. 

    • Problems associated with setting a fast clock back in time – some timestamps that have
    • already been used will now be in the future - bad. Also timestamps that have been used can be re-used – bad. 

    • Cristian’s method Strengths and weaknesses are; 
    •        
    • Intended mainly for use within an Intranet and is not particularly scalable (being server-based and not hierarchical), and not
    • particularly suited to use over networks that hve very large delays, or highly variable delays.
    •        
    • Simple for use in LAN environments.
    •        
    • Takes into account round-trip time (RTT).  

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