Web-Based Computing 1 Flashcards

Question

What is an application framework? What benefit does the use of a framework provide? What two key things does a framework specify?

Answer 1

The goal of a framework is to simplify the design of complex apps via higher level **reuse**. A framework defines a reusable software architectural design typically composed of: * A group of abstract classes (i.e. parts) * And specification of how their instances collaborate (including **communication**) It is a reusable design for all or, commonly, part of a software system * E.g. a user interface (UI) framework provides a design for just the user interface parts of the system Frameworks specify a context for the components that make up an application and can be applied to any sort of application development.

Answer 2

MVC = Model-View-Controller * The Model - defines the underlying data and [“business”] logic that manipulates it * The View - which defines the view (i.e. the user interface) that the user sees * The Controller - coordinates operations between the other system components Design patterns describe reusable software techniques/”idioms” * Strategies for solving specific software problems that we commonly use repeatedly * There are a multitude of well-defined design patterns

Answer 3

XML = eXtensible Markup Language A markup language that adds meta-information (information about information) to a document (like HTML). XML is focused on document content description NOT about presentation. XML enables effective data structuring and simplified data exchange * You can define your own data “types” (unlike HTML) XML is important for standardization, support for structure data and self description. How XML is used is a matter for the software (XML-aware app) processing the XML documents.

Answer 4

Well-formed = An XML document that is syntactically correct Valid = Valid means the XML document conforms to the specification in the DTD (Document Type Definition) or XML schema definition (XSD).

Answer 5

A DTD describes the structure of a particular type of XML file. * It is a reference when creating XML files that are expected to meet a type of definition * It is used by an XML-aware application that wishes to process such file to know what it should expect * It enumerates all the possible tags in a given XML specification and specifies how they can be used (nesting rules) * In essence it describes the proper use of tags and thereby the valid hierarchical structure of the corresponding documents

Answer 6

The root element is the tag that encompasses all other tags in an XML document. All tags in XML MUST have matching closing tags and tags MUST be properly nested, therefore there must be one tag (the root tag) that encompasses all other tags.

Answer 7

XSL = eXtensible Stylesheet Language It is a stylesheet language used specifically to display an XML document. Very similar to how CSS is used with HTML to associate presentation characteristics with data tags Short for Extensible Style Language, a specification for separating style from content when creating HTML or XML pages. The specifications work much like templates, allowing designers to apply single style documents to multiple pages. XSL is the second style specification to be offered by the World Wide Web Consortium (W3C )(www.w3c.org). The first, called Cascading Style Sheets(CSS), is similar to XSL but does not include two major XSL's innovations -- allowing developers to dictate the way Web pages are printed, and specifications allowing one to transfer XML documents across different applications. W3C released the first draft of XSL in August 1998, and promotes the specifications as helpful to the Web's speed, accessibility, and maintenance.

Answer 8

One centralized server, maybe we could use a bit of DNS round robin style... Maybe all the clients could keep the information Maybe we bould broadcast across the network

Answer 9

Flooding is asking all machines on the network for a resource (used in broadcast). Often restricted to LANS. When broadcasting a request on the local network the resource provider(s) that recieve the request can respond. Advantages: * If a packet can be delivered, it will (probably mult times) * Since flooding naturally utilizes every path through a network, it will also use the shortest path * Simple to implement Disadvantages * Costly in wasted bandwidth (and DNS attacks use this to slow down a network) * Messages can become duplicated and further slow the network * Duplicates may circulate forevery (unless addressed in code

Answer 10

Why gossip vs flooding: Flooding can slow down an entire network. Gossip shares information in an exponential way, where instead of sending the same message to every machine at once, information is shared to one machine who shares that information with others and grows in that manner. It has less potential slow down the entire network (imagine if all machines where flooding at the same time). **Flooding:** simple routing technique in computer networks where a source or node sends packets through every outgoing link. This is often used as a denial of service attack by flooding network traffic. This is not something that is typically done on a WAN due to it's ability to slow down a network. **Gossip:** A gossip protocol is a procedure or process of computer peer-to-peer communication that is based on the way that epidemics spread. Some distributed systems use peer-to-peer gossip to ensure that data is routed to all members of an ad-hoc network. Some ad-hoc networks have no central registry and the only way to spread common data is to rely on each member to pass it along to their neighbors. The term epidemic protocol is sometimes used as a synonym for a gossip protocol, because gossip spreads information in a manner similar to the spread of a virusin a biological community. Suppose that we want to find the object that most closely matches some search pattern, within a network of unknown size, but where the computers are linked to one another and where each machine is running a small agent program that implements a gossip protocol. To start the search, a user would ask the local agent to begin to gossip about the search string. (We're assuming that agents either start with a known list of peers, or retrieve this information from some kind of a shared store.) Periodically, at some rate (let's say ten times per second, for simplicity), each agent picks some other agent at random, and gossips with it. Search strings known to A will now also be known to B, and vice versa. In the next "round" of gossip A and B will pick additional random peers, maybe C and D. This round-by-round doubling phenomenon makes the protocol very robust, even if some messages get lost, or some of the selected peers are the same or already know about the search string. On receipt of a search string for the first time, each agent checks its local machine for matching documents. Agents also gossip about the best match, to date. Thus, if A gossips with B, after the interaction, A will know of the best matches known to B, and vice versa. Best matches will "spread" through the network.

Answer 11

DHT = Distributed Hash Table A DHT gives you a dictionary-like interface, but the nodes are distributed across the network. The trick with DHTs is that the node that gets to store a particular key is found by hashing that key, so in effect your hash-table buckets are now independent nodes in a network. This gives a lot of fault-tolerance and reliability, and possibly some performance benefit. Responsibility for maintaining the mapping from keys to values is distributed among the nodes, in such a way that a change in the set of participants causes a minimal amount of disruption. This allows a DHT to scale to extremely large numbers of nodes and to handle continual node arrivals, departures, and failures. DHTs form an infrastructure that can be used to build more complex services, such as anycast, cooperative Web caching, distributed file systems, domain name services, instant messaging, multicast, and also peer-to-peer file sharing and content distribution systems. Notable distributed networks that use DHTs include BitTorrent's distributed tracker, the Coral Content Distribution Network, the Kad network, the Storm botnet, the Tox instant messenger, Freenet, the YaCy search engine, and the InterPlanetary File System.

Answer 12

The M in MVC is the data model layer. The data model layer can be whatever you want it to be (assuming it still conforms to the rules of modeling the data and not the business or view), but most people end up using an ORM to model their data. Object-relational mapping (ORM) is a mechanism that makes it possible to address, access and manipulate objects without having to consider how those objects relate to their data sources. ORM lets programmers maintain a consistent view of objects over time, even as the sources that deliver them, the sinks that receive them and the applications that access them change. Based on abstraction, ORM manages the mapping details between a set of objects and underlying relational databases, XML repositories or other data sources and sinks, while simultaneously hiding the often changing details of related interfaces from developers and the code they create. ORM hides and encapsulates change in the data source itself, so that when data sources or their APIs change, only ORM needs to change to keep up—not the applications that use ORM to insulate themselves from this kind of effort. This capacity lets developers take advantage of new classes as they become available and also makes it easy to extend ORM-based applications. In many cases, ORM changes can incorporate new technology and capability without requiring changes to the code for related applications.

Answer 13

In short: JSON is typically smaller, faster and easier than XML and is probably preferable for most applications moving object-based data JSON is Like XML Because * Both JSON and XML are "self describing" (human readable) * Both JSON and XML are hierarchical (values within values) * Both JSON and XML can be parsed and used by lots of programming languages * Both JSON and XML can be fetched with an XMLHttpRequest JSON is Unlike XML Because * JSON doesn't use end tag * JSON is shorter * JSON is quicker to read and write * JSON can use arrays The biggest difference is: XML has to be parsed with an XML parser. JSON can be parsed by a standard JavaScript function. Why JSON is Better Than XML * XML is much more difficult to parse than JSON. * JSON is parsed into a ready-to-use JavaScript object. * For AJAX applications, JSON is faster and easier than XML: * Using XML * Fetch an XML document * Use the XML DOM to loop through the document * Extract values and store in variables * Using JSON * Fetch a JSON string * JSON.Parse the JSON string

Answer 14

It's object data-type. In the instance data model JSON has six primitive data types, Objects being one of them. You can access nested JSON objects by using the dot notation or bracket notation: ``` x = myObj.cars.car2; // or: x = myObj.cars["car2"]; ```