Cookie
它是標准的客戶端浏覽器狀態保存方式，可能在浏覽器誕生不久就有Cookie了，為什麼需要Cookie 這個東東？由於HTTP協議沒有狀態，所以需要一個標志/存儲來記錄客戶浏覽器當前的狀態，保證客戶浏覽器和服務器通訊時可以知道客戶浏覽器當前的狀態。Cookie就是記錄這個狀態的容器，Cookie在每次請求的時候都被帶回到服務器，從而保證了Server可以知道浏覽器當前的狀態，由於Cookie會被帶回到Server，所以Cookie的內容不能存太多，最多不能超過4K，4K 限制的介紹 http://ec.europa.eu/ipg/standards/cookies/index_en.htm
其中一段內容為：
A browser is only required to store up to 300 cookies overall and maintain only the last 20 from each domain. The maximum size of a cookie is 4K of disk space.
但是在一些場景下可能需要存儲超過4K或者更多的數據，但是這些數據不用在每次請求的時候被帶回到服務器，只要能在客戶的浏覽器上保存住，並且可以方便的被Javascript讀寫就可以了，這種需求尤為在中大型RIA的應用場景下更加的迫切，部分數據放在客戶浏覽器，節約帶寬，提高浏覽速度。HTML5標准已經替我們想到了滿足這種需求的方案：sessionStorage , webSqlDatabase, 微軟的IE 有 userData 方案。
userData
微軟對USERDATA的介紹: http://msdn2.microsoft.com/en-us/library/ms531424(VS.85).aspx
其中一段內容為：
Security Alert：For security reasons, a UserData store is available only in the same directory and with the same protocol used to persist the store.
Security Alert：Using this behavior incorrectly can compromise the security of your application. Data in a UserData store is not encrypted and therefore not secure. Any application that has access to the drive where UserData is saved has access to the data. Therefore, it is recommended that you not persist sensitive data like credit card numbers. For more information, see Security Considerations: DHTML and Default Behaviors.
……
The userData behavior persists data across sessions, using one UserData store for each object. The UserData store is persisted in the cache using the save and load methods. Once the UserData store has been saved, it can be reloaded even if Microsoft Internet Explorer has been closed and reopened.
Setting the userData behavior class on the html, head, title, or style object causes an error when the save or load method is called.
userData可以在同目錄同協議下相互訪問，長期存儲在客戶機器上。最大存儲空間也增大了很多。userData需要綁定到一個Dom元素上使用。在userData的method中有removeAttribute方法。經過測試代碼發現removeAttribute方法好像不是很管用，需要使用像cookie過期的方式，才可以徹底的刪除一個userData Attribute。
在 http://www.itwen.com/04web/11skill/skill20060918/60588.html 中介紹說userData存儲在X:\Documents and Settings\當前用戶\UserData\ 目錄下。具體細節MS在userData說明文檔中沒有具體說明。
sessionStorage
HTML5 標准對 sessionStorage的介紹： http://www.whatwg.org/specs/web-apps/current-work/
其中對 sessionStorage 的介紹：
This specification introduces two related mechanisms, similar to HTTP session cookies [RFC2965], for storing structured data on the client side.
The first is designed for scenarios where the user is carrying out a single transaction, but could be carrying out multiple transactions in different windows at the same time.
Cookies dont really handle this case well. For example, a user could be buying plane tickets in two different windows, using the same site. If the site used cookies to keep track of which ticket the user was buying, then as the user clicked from page to page in both windows, the ticket currently being purchased would "leak" from one window to the other, potentially causing the user to buy two tickets for the same flight without really noticing.
To address this, this specification introduces the sessionStorage DOM attribute. Sites can add data to the session storage, and it will be accessible to any page from that origin opened in that window.
Html5 sessionStorage Demo: http://html5demos.com/storage
下面是根據 http://www.blogjava.net/emu/archive/2006/10/04/73385.html 中提到的IE FF 兼容userData的測試代碼：
復制代碼代碼如下:
function isIE() {
return !!document.all;
}
function initUserData() {
if (isIE()) document.documentElement.addBehavior("#default#userdata");
}
function saveUserData(key, value) {
var ex;
if (isIE()) {
//IE
with (document.documentElement) try {
load(key);
setAttribute("value", value);
save(key);
return getAttribute("value");
} catch (ex) {
alert(ex.message)
}
} else if (window.sessionStorage) {
//FF 2.0+
try {
sessionStorage.setItem(key, value)
} catch (ex) {
alert(ex);
}
} else {
alert("Error occured in user data saving. your browser do not support user data.");
}
}
function loadUserData(key) {
var ex;
if (isIE()) {
//IE
with (document.documentElement) try {
load(key);
return getAttribute("value");
} catch (ex) {
alert(ex.message); return null;
}
} else if (window.sessionStorage) {
//FF 2.0+
try {
return sessionStorage.getItem(key)
} catch (ex) {
alert(ex)
}
} else {
alert("Error occured in user data loading. your browser do not support user data.")
}
}
function deleteUserData(key) {
var ex;
if (isIE()) {
//IE
with (document.documentElement) try {
load(key);
expires = new Date(315532799000).toUTCString();
save(key);
}
catch (ex) {
alert(ex.message);
}
} else if (window.sessionStorage) {
//FF 2.0+
try {
sessionStorage.removeItem(key)
} catch (ex) {
alert(ex)
}
} else {
alert("Error occured in user data deleting. your browser do not support user data.")
}
}
userData和sessionStorage共同的特點就是：這兩個對象都可以存儲比cookie大的多的多內容。並且不會隨每次請求帶回到服務器端。但是根據Html5標准和測試發現userData和sessionStorage有很多地方是不同的。
下面是一個測試頁面：
其中的 SetInsurance link 會操作javascript 在IE下用userData寫數據, 在FF下用sessionStore寫數據。在IE下的情況是：關閉IE或者重啟機器寫入的值都不會丟失。在FF下的情況很有意思：在本頁面寫入的值在本頁面可以訪問，在由本頁面所打開的其它頁面可以訪問。但是就算本頁面開著，在導航欄裡輸入地址，打開本頁面，存入的值就不能訪問了。在本頁面存入的值，在它的父頁面(打開這個頁面的頁面)是訪問不到的。又看了看Html5標准。sessionStorage 的全名是：Client-side session and persistent storage of name/value pairs 意思估計是存儲在Client的內容是有session 會話的，存儲的值由session會話所維系，一旦session會話中斷或者丟失，存入的值也就隨之消失了。所以當頁面沒有session(父頁面，由地址欄打開的頁面)，是取不到值的。當FF關閉或者重啟機器必然也就取不到值了。
webSqlDatabase
webSqlDatabase在HTML5 標准中是非常Cool的一個東東， 用Javascript寫SQL查詢，數據庫就在浏覽器裡，這在以前幾乎不敢想象。不過今天Safari, Chrome, Opera 都已經支持了，兩個webSqlDatabase 的 Demo 頁面： http://html5demos.com/database http://html5demos.com/database-rollback
W3C 對WEBSQLDATABASE 的介紹頁面： http://dev.w3.org/html5/webdatabase/
WiKi上一個簡明的說明： http://en.wikipedia.org/wiki/Web_SQL_Database
From W3C: "...an API for storing data in databases that can be queried using a variant of SQL"
Web SQL Database is supported by Google Chrome[1], Opera and Safari but will not be implemented by Mozilla(Firefox)[2] who instead propone Indexed Database API access.
不知道 HTML 5 的 SQLDB 會被浏覽器支持的怎麼樣， 不過sessionStorage看上去已經可以基本滿足需求了。