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exam oct 15 > Units 5 & 6 > Flashcards

Units 5 & 6 Flashcards

1
Q

Software archtiectures

A

concerned with the structure of a system

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2
Q

Fault tolerance

A

the ability of a system to respond gracefully to an unexpected hardware or software failure

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3
Q

Client-server style architecture

A

server - service provider role - waits for arrival of requests

client - issues requests to the server + awaits responses

server process is normally placed on a different host than the client process

server typically manages large amounts of data + files

many clients can connect to one server

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4
Q

Load balancing

A

avoiding situations where one component has to do all the work, while other components remain idle

enables the available resources to be better utilised

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5
Q

Multiple servers acting together

A

variation of client server style - 2 ways:

  1. data partitioning
    • resources are distributed among the servers
    • each server is responsible for its own section
    • servers interact with each other to provide replies to
      requests
  2. data replication
    • each server holds an identical copy of the data
    • better performance + availability + improved fault
      tolerance
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6
Q

Proxy servers

A

a server that acts as an intermediary between clients and other servers

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7
Q

Peer-to-peer style

A

an architecture in which computer resources + services are directly exchanged between computer systems without the need for distinct clients and servers

computers communicate among themselves + can act both as clients and as servers

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8
Q

Which factors measure the performance of a distributed system’s software architecture ?

A

throughput

responsiveness

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9
Q

What are the weaknesses of the client-server style?

A

central role of the server forms a bottleneck + thus limits the performance of the system

this central role also forms a single point of failure - if the server goes down the whole system goes down

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10
Q

What is load balancing?

A

a strategy to improve the performance of a distributed system such that the workload is shared among many processes

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11
Q

Two-tier model

A

the clients, taken together, form one tier - the client tier

the database sever forms another tier - the database tier

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12
Q

Layer

A

a layer of software in which implementation details are hidden but functionality of the layer can be invoked through a well defined interface

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13
Q

OSI model

A

defines a modular approach to networking, with each layer responsible for some discrete aspect of the networking process

  1. Application layer
  2. Presentation layer
  3. Session layer
  4. Transport layer
  5. Network layer
  6. Data link layer
  7. Physical layer

each layer able to communicate with layer directly above or below

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14
Q

Software component

A

an independent software unit that can be combined with other components to create a software system

each component has an interface

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15
Q

Interface

A

an abstraction that a component or some other entity provides of itself to the outside

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16
Q

What are thin + thick clients?

A

Thin clients do not carry much processing and leave this to the server

Think clients carry out more processing

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17
Q

Java Platform Enterprise edition (Java EE)

A

EIS tier - holds application data + stores it permanently in some kind of database

business tier - contains the software relating to the business logic

web tier - required in those cases where the clients are web clients, role is to make business logic accessible for we applications

client tier - presentation or user interface of the application as a whole

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18
Q

Asynchronous message passing

A

sending + receiving processes to not have to be at their synchronisation points at the same time

possible that one process is ahead of the other

if receiving process is ahead of the sending process it will have to wait

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19
Q

synchronous message passing

A

both processes are required to be at the point of sending + receiving at the same time

the message goes straight from the sending process to the receiving process

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20
Q

Addressing

A

needed so processes in a distributed system can identify each other in order to communicate

addressing is a standard naming process

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21
Q

IP address

A

a symbolic address of a computer

for communication to be possible the IP address has to be translated + the actual computer located - done via Domain Name Service (DNS)

port is also needed - map incoming data to a particular process running on a computer

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22
Q

What form of communication requires a buffer?

A

asynchronous communication

if a message arrives before the recipient is ready to receive it the message must be stored somewhere - i.e. buffered until the recipient can read it

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23
Q

Marshalling

A

the process of taking a collection of data items + assembling them into a form suitable for transmission in a message

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24
Q

Unmarshalling

A

the process of disassembling a message on arrival in order to produce the equivalent collection of data items at the destination

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25
Q

Socket

A

to communicate client + server processes in Java can use a socket

it provides various streams on which messages can be transferred

associated with a port + IP address

client + server are connected by ‘plugging together’ their respective sockets

each sock has an input stream + an output stream

26
Q

ServerSocket

A

objects of this class are used by the server to listen for connection requests from clients

then establishes a socket connection with each client

associated with a port where it can listen for a request using the method accept

27
Q

Socket (class)

A

object of this class is used to establish a connection between client + server processes

its constructors (more than 1) associate a new socket with a specified host (IP address) + host port

client + server must each construct a socket

methods: getInputStream + getOutputStream

28
Q

Using pseudocode, give an outline design of the code for a server which can communicate with several clients not just one

A

listen for clients on ServerSocket
while(true)
{
create a socket for the newly arrived client
create thread to handle communication with the client
start the thread
}

29
Q

Remote Procedure Call (RPC)

A

aim to make communication distribution transparent

i.e. hide the complexities of a distributed system

(procedure call = method call)

30
Q

Procedure call (i.e. method call)

A

‘normal’ - procedure is called at a certain point in the program + this causes the code associated with the process to be executed

remote location - procedure has to be executed by a process not on local machine

Steps:

  1. set up 2 way communication (sockets + streams)
  2. package message that specifies procedure call
    (name of process + any arguments)
    (processes at bot ends must know how to pack +
    unpack)
  3. send message containing request to remote process
    (calling process now waits)
  4. on receiving request the remote process:
    - unpacks message
    - executes procedure
    - packages any return value
    - sends result back
  5. calling process receives results, unpacks + call in now
    complete
31
Q

Middleware

A

aims to mask heterogeneity of underlying platforms in a distributed system (networks, hardware, operating system)

32
Q

Messaging system paradigm

A

messages do not go straight from sender to receiver - go through intermediary object

sender deposits message with the messaging system + it looks after the message from that point onwards

job of delivering message to appropriate receiver is that of the messaging system - forwards message to the queue associated with the appropriate receiver - will hold until receiver is ready to receive it

33
Q

Publish/subscribe messaging model

A

each message associated with a topic or event

an object that generates certain events will publish the event + other processes are allowed to observe

applications interested in the event may subscribe to messages for that event

when the awaited event occurs, the process publishes a message announcing the event or topic - known as the notification

middleware messaging system distributes the message to all subscribers for that event

34
Q

Multicast

A

sender can send info to a group of destinations simultaneously

35
Q

Unicast

A

sender + receiver are in a one-to-one relationship

sender sends message to each individual recipient

36
Q

Database

A

an organised collection of data

used when dealing with large amounts of data that need to be stored independent of any running program

37
Q

Persistent storage

A

data that is held in volatile memory will disappear when the device is switched off

persistent data outlives the execution of the software program that created it

38
Q

Transaction

A

a sequence of atomic actions where we want the whole sequence to be performed in its entirety or not performed at all

39
Q

Failure atomicity

A

if a transaction fails, it is as if nothing had happened

40
Q

ACID properties

A

desirable properties of a transaction

Atomicity
all operations in transaction performed or not at all

Consistency
transforms the system from one consistent state to
another

Isolation
cannot reveal any part of its state until committed

Durability
once transaction committed the system must
guarantee the result were persist even if there are
subsequent failures

41
Q

Transaction states (5 states)

A

Active - initial state

        - while executing remains in this state
        - transaction.start()

Partially Committed - state after final operation has been
executed

Committed - state are successfully completed transaction
- transaction.commit()

Failed - state when discovery made that normal execution
cannot proceed

Aborted - state when transaction has been rolled back +
its effects are undone
- transaction.rollback()

42
Q

Transaction Processing System (TP System) aka TP monitor

A

manage the correct + efficient execution of transactions

located between clients + the database

43
Q

Components of TP monitor

Transaction Processing System

A

Transaction manager - provides interface

                                 - validates requests
                                 - passes operations to scheduler

Scheduler - makes sure all the operations are executed in
the right order

Recovery manager - deals with recoveries from crashes

Cache manager - organises objects in main memory +
interfaces to persistent memory

44
Q

Schedule

A

a sequence of operations that specifies the order in which operations of concurrent transactions are executed

45
Q

Serial scheduler

A

transactions executed in a strict serial order

non-serial - operations interleaved

46
Q

Precedence graphs

A

used to test whether a given schedule shows a serialisable execution of the transaction - must show no cycles

read + write operations that relate to different data objects do not affect each other

conflicting operations - only conflict if one is a write on same object by different transactions (2 reads ok)

47
Q

Pessimistic algorithms (transactions)

A

assume that there are likely to be conflicts + they take measures to avoid them

48
Q

Optimistic algorithms (transactions)

A

assume conflicts are likely to be rare + can therefore be dealt with after the event

49
Q

Two-phase locking (2PL)

A

aim of the 2PL algorithm is to ensure the schedules for transactions are such that effects of transactions on shared data are serially equivalent

2 phases:

acquire - transaction gradually acquires locks it requires
as + when it needs them

release - transaction unlocks the locks by:

            1. strict 2PL - releases all locks on commit

                                      or

            2. non-strict - transaction can let go of locks 
                                   gradually after it has finished
                                   with them
50
Q

Write lock

A

transaction can only acquire a write lock for an object if no other transaction currently holds a write or read lock for that object

51
Q

Read lock

A

transaction can only acquire a read lock for an object if no other transaction currently holds a write lock for that object

52
Q

Time-stamp ordering

A

assigning a unique time-stamp to each transaction

enforces that pairs of conflicting operations can only be executed in the order of the time-stamps

prevents cycles

53
Q

Advantages + disadvantages of time-stamp ordering

A

efficient + fewer deadlocks
few overheads

can suffer from high levels of aborts
more restrictive than 2PL

54
Q

Optimistic currency control

A

goal to avoid overheads + problems associated with locking mechanisms

based on likelihood that 2 clients want to access the same data is minimal

Phases:
1. Working phase - transaction works with a copy of
the most recently committed
version of the object

 2.  Validation phase - closeTransaction issued a check
                                      for conflicts is carried out
                                    - if succeeds the transaction can
                                      commit
                                    - if fails, this or another transaction
                                       will need to be aborted

 3.  Update phase - if validated the tentative objects that
                                 the transaction has worked on will      
                                 be made permanent
55
Q

What is a shared lock?

A

useful when non-conflicting operations are using the same object

increases concurrency

56
Q

What is a simple alternative to deadlock detention?

A

Timeouts

57
Q

What are cascading aborts?

A

occur when one transaction has to abort which leads to other transactions having to abort

bad for performance as much work may need to be repeated

58
Q

Crash resilience

A

system is able to continue processing by making use of recovery techniques

aka fault transparency

create illusion that a failure never happened

59
Q

Approaches for supporting rollback

A

logging - for each update a record is kept (transaction ID,
before image, after image, whether committed
or aborted
- when system restarts after a crash the log is
analysed + partially complete transactions are
undone

shadowing - makes use of duplicate disk pages in stable
storage
- one page used to make updates + one
page (shadow page) is left untouched
- when transaction is committed the new
page replaces the old page

60
Q

Rational databases

A

data is conceptually organised in the form of a table

requests for information from db are made in the form of queries ( e.g. SQL)

exam oct 15 (4 decks)

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