A blog for reviewing system design in about 1 month, highly recommended.

System Design Study Group

Ajax (programming)

From Wikipedia, the free encyclopedia

"AJAX" redirects here. For other uses, see Ajax.

Ajax

Paradigm	Web Application, HTML, JavaScript,DHTML, Domain Object Model,XMLHttpRequest, JSON, XML, XSLT,XHTML, Cascading Style Sheets
Designed by	Jesse James Garrett
First appeared	February 18, 2005; 11 years ago

Ajax (also AJAX; /ˈeɪdʒæks/; short for asynchronous JavaScriptand XML)^[1][2][3] is a set of web development techniques using many web technologies on the client-side to create asynchronous Web applications. With Ajax, web applications can send data to and retrieve from a server asynchronously (in the background) without interfering with the display and behavior of the existing page. By decoupling the data interchange layer from the presentation layer, Ajax allows for web pages, and by extension web applications, to change content dynamically without the need to reload the entire page. Data can be retrieved using the XMLHttpRequest object. Despite the name, the use of XML is not required (JSON is often used in the AJAJ variant), and the requests do not need to be asynchronous.^[4]

Ajax is not a technology, but a group of technologies. HTML and CSScan be used in combination to mark up and style information. The DOM is accessed with JavaScript to dynamically display – and allow the user to interact with – the information presented. JavaScript and the XMLHttpRequest object provide a method for exchanging data asynchronously between browser and server to avoid full page reloads.

Contents

  [hide] 

• 1History
• 2Technologies
• 3Drawbacks
• 4Example
• 5See also
• 6References
• 7External links

History[edit]

In the early 1990s, most Web sites were based on complete HTML pages. Each user action required that a complete page be loaded from the server. This process was inefficient, as reflected by the user experience: all page content disappeared, then reappeared. Each time the browser reloaded a page because of a partial change, all of the content had to be re-sent, even though only some of the information had changed. This placed additional load on the server and used excessivebandwidth.

In 1996, the iframe tag was introduced by Internet Explorer to load or to fetch content asynchronously.

In 1998, Microsoft Outlook Web App team implemented the first component XMLHTTP by client script.

In 1999, Microsoft used its iframe technology to dynamically update the news stories and stock quotes on the default page for Internet Explorer,^[5] and created the XMLHTTP ActiveX control in Internet Explorer 5, which was later adopted byMozilla, Safari, Opera and other browsers as the XMLHttpRequest JavaScript object.^[6] Microsoft has adopted the native XMLHttpRequest model as of Internet Explorer 7. The ActiveX version is still supported in Internet Explorer, but not inMicrosoft Edge. The utility of background HTTP requests to the server and asynchronous Web technologies remained fairly obscure until it started appearing in full scale online applications such as Outlook Web App (2000)^[7] and Oddpost(2002).

Google made a wide deployment of standards-compliant, cross browser Ajax with Gmail (2004) and Google Maps(2005).^[8] In October 2004 Kayak.com's public beta release was among the first large-scale e-commerce uses of what their developers at that time called "the xml http thing".^[9]

The term "Ajax" was publicly stated on 18 February 2005 by Jesse James Garrett in an article titled "Ajax: A New Approach to Web Applications", based on techniques used on Google pages.^[3]

On 5 April 2006, the World Wide Web Consortium (W3C) released the first draft specification for the XMLHttpRequest object in an attempt to create an official Web standard.^[10][11] The latest draft of the XMLHttpRequest object was published on 30 January, 2014.^[12]

Technologies[edit]

The conventional model for a Web Application versus an application using Ajax

The term Ajax has come to represent a broad group of Web technologies that can be used to implement a Web application that communicates with a server in the background, without interfering with the current state of the page. In the article that coined the term Ajax,^[3][4] Jesse James Garrett explained that the following technologies are incorporated:

• HTML (orXHTML) andCSS for presentation
• The Document Object Model(DOM) for dynamic display of and interaction with data
• XML for the interchange of data, and XSLT for its manipulation
• The XMLHttpRequest object for asynchronous communication
• JavaScript to bring these technologies together

Since then, however, there have been a number of developments in the technologies used in an Ajax application, and in the definition of the term Ajax itself. XML is no longer required for data interchange and, therefore, XSLT is no longer required for the manipulation of data. JavaScript Object Notation (JSON) is often used as an alternative format for data interchange,^[13] although other formats such as preformatted HTML or plain text can also be used.^[14]

Asynchronous HTML and HTTP (AHAH) involves using XMLHTTPRequest to retrieve (X)HTML fragments, which are then inserted directly into the Web page.

Drawbacks[edit]

• Any user whose browser does not support JavaScript or XMLHttpRequest, or has this functionality disabled, will not be able to properly use pages that depend on Ajax. Simple devices (such as smartphones and PDAs) may not support the required technologies. The only way to let the user carry out functionality is to fall back to non-JavaScript methods. This can be achieved by making sure links and forms can be resolved properly and not relying solely on Ajax.^[15]
• Similarly, some Web applications that use Ajax are built in a way that cannot be read by screen-reading technologies, such as JAWS. The WAI-ARIA standards provide a way to provide hints in such a case.^[16]
• Screen readers that are able to use Ajax may still not be able to properly read the dynamically generated content.^[17]
• The same origin policy prevents some Ajax techniques from being used across domains,^[10] although the W3C has a draft of the XMLHttpRequest object that would enable this functionality.^[18] Methods exist to sidestep this security feature by using a special Cross Domain Communications channel embedded as an iframe within a page,^[19] or by the use of JSONP.
• The asynchronous callback-style of programming required can lead to complex code that is hard to maintain, to debug^[20] and to test.^[21]
• Because of the asynchronous nature of Ajax, each chunk of data that is sent or received by the client occurs in a connection established specifically for that event. This creates a requirement that for every action, the client must poll the server, instead of listening, which incurs significant overhead. This overhead leads to several times higher latency with Ajax than what can be achieved with a technology such as websockets.^[22]
• In pre-HTML5 browsers, pages dynamically created using successive Ajax requests did not automatically register themselves with the browser's history engine, so clicking the browser's "back" button may not have returned the browser to an earlier state of the Ajax-enabled page, but may have instead returned to the last full page visited before it. Such behavior — navigating between pages instead of navigating between page states — may be desirable, but if fine-grained tracking of page state is required, then a pre-HTML5 workaround was to use invisible iframes to trigger changes in the browser's history. A workaround implemented by Ajax techniques is to change the URL fragment identifier (the part of a URL after the "#") when an Ajax-enabled page is accessed and monitor it for changes.^[23][24]HTML5 provides an extensive API standard for working with the browser's history engine.^[25]
• Dynamic Web page updates also make it difficult to bookmark and return to a particular state of the application. Solutions to this problem exist, many of which again use the URL fragment identifier.^[23][24] The solution provided by HTML5 for the above problem also applies for this.^[25]
• Depending on the nature of the Ajax application, dynamic page updates may disrupt user interactions, particularly if the internet connection is slow or unreliable. For example, editing a search field may trigger a query to the server for search completions, but the user may not know that a search completion popup is forthcoming, and if the internet connection is slow, the popup list may show up at an inconvenient time, when the user has already proceeded to do something else.
• Excluding Google,^[26] most major Web crawlers do not execute JavaScript code,^[27] so in order to be indexed bysearch engines, a Web application must provide an alternative means of accessing the content that would normally be retrieved with Ajax. It has been suggested that a headless browser may be used to index content provided by Ajax-enabled websites, although Google is no longer recommending the Ajax crawling proposal they made back in 2009.^[28]

What happens when you type a URL in brower

A classical question for system design. This blog gives a comprehensive answer:
What happens when you type a URL in brower

Please refer to:

Crack the System Design Interview

http VS. https

HTTP, or hypertext transfer protocol, is the way a Web server communicates with browsers like Internet Explorer® and Mozilla Firefox®. HTTP lets visitors view a site and send information back to the Web server.

HTTPS, hypertext transfer protocol secure, is HTTP through a secured connection. Communications through an HTTPS server are encrypted by a secure certificate known as an SSL. The encryption prevents third-parties from eavesdropping on communications to and from the server.

Only servers that have their own SSL can create HTTPS connections. A site's visitor cannot encrypt the connection.

If you have a hosting account with an SSL encrypting its connection, you can automatically redirect visitors to the HTTPS version of your site (more info).

TCP VS UDP

Q. Can you explain the difference between UDP and TCP internet protocol (IP) traffic and its usage with an example?
A. Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)is a transportation protocol that is one of the core protocols of the Internet protocol suite. Both TCP and UDP work at transport layer TCP/IP model and both have very different usage.

Difference between TCP and UDP

TCP	UDP
Reliability: TCP is connection-oriented protocol. When a file or message send it will get delivered unless connections fails. If connection lost, the server will request the lost part. There is no corruption while transferring a message.	Reliability: UDP is connectionless protocol. When you a send a data or message, you don’t know if it’ll get there, it could get lost on the way. There may be corruption while transferring a message.
Ordered: If you send two messages along a connection, one after the other, you know the first message will get there first. You don’t have to worry about data arriving in the wrong order.	Ordered: If you send two messages out, you don’t know what order they’ll arrive in i.e. no ordered
Heavyweight: – when the low level parts of the TCP “stream” arrive in the wrong order, resend requests have to be sent, and all the out of sequence parts have to be put back together, so requires a bit of work to piece together.	Lightweight: No ordering of messages, no tracking connections, etc. It’s just fire and forget! This means it’s a lot quicker, and the network card / OS have to do very little work to translate the data back from the packets.
Streaming: Data is read as a “stream,” with nothing distinguishing where one packet ends and another begins. There may be multiple packets per read call.	Datagrams: Packets are sent individually and are guaranteed to be whole if they arrive. One packet per one read call.
Examples: World Wide Web (Apache TCP port 80), e-mail (SMTP TCP port 25 Postfix MTA), File Transfer Protocol (FTP port 21) and Secure Shell (OpenSSH port 22) etc.	Examples: Domain Name System (DNS UDP port 53), streaming media applications such as IPTV or movies, Voice over IP (VoIP), Trivial File Transfer Protocol (TFTP) and online multiplayer games etc

[系统设计/OOD] 吐血总结：面试知识点

Please refer to the original post at:
http://www.1point3acres.com/bbs/forum.php?mod=viewthread&tid=169243&extra=page%3D1%26filter%3Dtypeid%26typeid%3D200%26typeid%3D200

语言知识点：以java为例

• Java 语言特性：Java 与 c ++ 区别; abstract class vs interface; shallow copy/deep copy; pass by reference/ pass by value; hashcode()/ equals(); Java 一些关键字：final, static; final/finalize/finally; checked/unchecked exception； java 8种primitive type；overriding vs overload；public static void main(string args[])每个关键字的作用
• Java Collections：stack/queue/deque; hashset/treeset； String vs StringBuffer;Hashmap/TreeMap/Hashtable/LinkedHashMap/ ConcurrentHashMap; Array/ArrayList/LinkedList; PriorityQueue(heap); comparable/comparator; iterator

3.  拓展一些问题： Java memory leak/JVM/ garbage collection,  Object class method: getclass()/ hashcode(); java: heap/stack存什么; Java 8/Java 7

数据结构和算法

• 二叉树：超级重点： 收集所有二叉树的题
• 链表： 会翻转／快慢指针
• Binary Deduction/Search: sorted/rotated array/ Sqrt()
• 实现基本数据结构： hashmap， stack和queue
• Array/ String： shuffle an array， java big integer实现
• dfs vs bfs  word ladder/ topological  sorting

7.  简单dp，不需要很复杂: paint house/stock price/


计算机网络
1. TCP 三次握手，TCP/UDP 区别；
2.  http/https 区别；http request：post／get ；http port 80 ssl;
3.输入www.google.com 会发生什么；What happens when you type [url]www.google.com in your browser?[/url]
4.Public key/Private key;
5. HTTP 401, 403, or 404 Error／ client/server模型


数据库
1. SQL vs NoSql 区别
2. select／update／delete／insert
3.primary key；join（四种）和index 原理和作用
4.简单的sql语句：从table中找出成绩第二好的学生姓名； group by
5.简单了解几种nosql数据库： MangoDB/ Cassandra/HBase


系统及其它

• ACID/CAP 分布式系统
• Java多线程： extend Tread类，Implement Runnable接口/blockingqueue
• 序列化的几种方式：JSON／Object Serialize／ProtoBuf
• what is dead lock?死锁问题／如何解决
• Design Pattern 设计模式（singleton，factory, builder, decorator）
• Linux command: kill -9   / scp / telnet / ps