Unit 09

Question 1

applet

Answer 1

A Java program that is downloaded along with a web page and run by the browser.

Question 2

application

Answer 2

A Java program that can run independently of a browser.

Question 3

application client

Answer 3

A Java EE client implemented as a Java application, running on the client computer either in its own JVM or in an application client container.

Question 4

application client container

Answer 4

A software environment for application clients that takes care of many of the routine requirements of communication, security and so on. In particular, it facilitates access from an application client to remote EJB session beans running on the server.

Question 5

atomic clock

Answer 5

An extremely accurate timekeeping device, based on the resonant frequency of atoms, usually of the element caesium, and on which official definitions for time units are now based.

Question 6

attribute (HTML)

Answer 6

A named item of additional data in an HTML tag. It is usually paired with a value, such as method=”POST” or type=”HIDDEN”, where method and type are attributes of the tag and the tag respectively.

Question 7

auction sniping (aka sniping)

Answer 7

A practice where potential bidders in a time-limited online auction access the auction for an item just before it closes and submit a bid that is higher than the current highest bid in the hope of securing the item before anyone else can raise their bid.

Question 8

broadcasting

Answer 8

Sending a message to all hosts on a network; normally limited to a local network.

Question 9

causality

Answer 9

The relationship between an event (a cause) and its effect – for our purposes we require a notion of time in a distributed system so that the causes occur at an earlier time than their effects.

Question 10

client polling

Answer 10

An automatic process that causes the browser to reload a web page at regular intervals by including special instructions in the HTML header of the web page.

Question 11

clock skew

Answer 11

The difference in clock readings between any two hosts in a distributed system.

Question 12

Coordinated Universal Time (UTC)

Answer 12

The international standard time based on atomic clocks.

Question 13

drift

Answer 13

The absolute deviation of a clock from perfect time (in seconds or a fraction of a second).

Question 14

drift rate

Answer 14

The clock deviation per unit of time (usually per second), such as typical computer hardware clocks’ drift rate of between 10^-5 and 10^-6 seconds per second.

Question 15

external synchronisation

Answer 15

Keeping a clock accurate by regular comparisons with a more accurate external time source, such as a close approximation to UTC.

Question 16

global time

Answer 16

A common value of the current time in a distributed system, in the sense that all hosts have perfectly synchronised clocks (an ideal that cannot be achieved).

Question 17

hidden form field

Answer 17

A parameter in a web form defined using an tag with the attribute value type=”HIDDEN”, which means it is not displayed by the browser.

Question 18

HTTP header

Answer 18

The first part of an HTTP request or response preceding the body of the request or response. A response header includes information such as the number of bytes and the type of content returned, while the body typically contains web page content or other requested resource.

Question 19

HTTPS

Answer 19

A secure HTTP protocol that uses encryption to protect information sent across the Web.

Question 20

internal synchronisation

Answer 20

A process where a host compares its clock value with the clocks of other hosts in the same system so that all hosts can ensure they have approximately the same clock time, although this time may differ significantly from the correct (UTC) time.

Question 21

Java Naming and Directory Interface (JNDI)

Answer 21

An API for accessing naming and directory services in a vendor-independent way.

Question 22

Java Plug-in

Answer 22

Downloadable software that can be ‘plugged in’ to most browsers to provide a standard and up-to-date JVM and runtime environment enabling them to run applets using current versions of Java.

Question 23

local clock

Answer 23

The clock giving a time value in a particular host in a distributed system – typically local clocks show different values from each other.

Question 24

logical time

Answer 24

A system of recording time values so as to ensure that the order of events is consistent with causality – i.e. that the cause of an event (such as sending a message) occurs at an earlier logical time than the event itself (the arrival of the message).The time values are not normally related to the actual time shown on local or external clocks.

Question 25

message bean

Answer 25

A type of EJB component that can be used to implement a messageoriented communication approach between Java EE components.

Question 26

middle tier

Answer 26

The part of a layered distributed system that sits between the client tier and the EIS or database tier. In Java EE it may have two component tiers, the business tier and an optional web tier.

Question 27

multicast

Answer 27

A form of communication whereby the sender sends a message once onto a network where it can be read by many receivers who have previously registered to receive such messages.

Question 28

multicast socket

Answer 28

An extension of the standard Java socket concept, used to facilitate multicast communication.

Question 29

persistent connection

Answer 29

The approach used by HTTP v1.1 in keeping the TCP connection open after a first request to allow for possible subsequent requests. This contrasts with HTTP v1.0 which requires a new TCP connection for each request.

Question 30

primary time server

Answer 30

A host that has direct access to a standard time source (such as an atomic radio clock) and that makes this time value available to other hosts in a distributed system – in NTP, a stratum 1 host.

Question 31

probabilistic synchronisation

Answer 31

An approach to clock synchronisation in distributed systems that guarantees the accuracy of synchronisation with a certain probability as opposed to a deterministic (100% certain) guarantee – the accuracy can be increased by increasing the number of synchronisation requests.

Question 32

pull technology

Answer 32

An informal description of the request–response nature of HTTP – that information travels from the server to the client only when the client does something to ‘pull’ that information to it.

Question 33

push technology

Answer 33

An informal description of various ways to mitigate the request–response nature of HTTP by enabling the server to send (or ‘push’) information to the client, in order to keep the client up to date, without an explicit request from the client.

Question 34

reserved characters

Answer 34

Characters in a URL that have a special meaning, such as / or #, and so cannot be used as part of identifiers.

Question 35

RSS (Really Simple Syndication)

Answer 35

A technology that allows users to sign up to receive updates from a website whenever something important has changed.

Question 36

sandbox

Answer 36

A restrictive environment in which untrusted Java applets execute.

The word is derived from an American term for a children’s sandy playing area.

Question 37

synchronisation (time)

Answer 37

To cause clocks to indicate the same time or time values as closely as possible.

Question 38

thick client (aka fat or rich client)

Answer 38

Complex client software that carries out significant processing on the client machine in a client–server system.

Question 39

thin client

Answer 39

In a client–server system, a simple client with very limited functionality, mainly limited to display and communication – for example, a web browser.

Question 40

time-stamp

Answer 40

A record of the time of an event (such as the sending of a message) that is attached to that event (such as being appended to the message in a data packet).

Question 41

topic

Answer 41

In a messaging system (such as JMS), a topic is a named area of interest to which clients can subscribe, such that they receive any messages relating to this area.

Question 42

Transmission Control Protocol (TCP) connection

Answer 42

A link agreed between two internet hosts that is established by a ‘handshake’ (an exchange of a number of initial data packets), prior to the hosts sending actual data.

Question 43

unicast

Answer 43

A form of communication involving one sender and one receiver at a time.

Question 44

unsafe character

Answer 44

One of the reserved characters or other characters that cannot be included in a URL without the possibility of confusion about their intended role or meaning.

Question 45

URL encoding

Answer 45

A means of including potentially unsafe characters in a URL by encoding them using the corresponding ASCII code, such as using %2F instead of /.

Question 46

web service

Answer 46

A standards-based web application that interact with other web applications for the purpose of providing a service.