Best Practices for Speeding Up Your Web Site

from :http://developer.yahoo.com/performance/rules.html 

Minimize HTTP Requests

最小化http请求

 

tag: content

 

80% of the end-user response time is spent on the front-end. Most of this time is tied up in downloading all the components in the page: images, stylesheets, scripts, Flash, etc. Reducing the number of components in turn reduces the number of HTTP requests required to render the page. This is the key to faster pages.

 

80%的最终用户耗时来自前端。耗时来自页面组件:图片、样式、js脚步、flash等等。通过减少这些组件有助于减少httq请求,让页面更加快的显示。

 

One way to reduce the number of components in the page is to simplify the page's design. But is there a way to build pages with richer content while also achieving fast response times? Here are some techniques for reducing the number of HTTP requests, while still supporting rich page designs.

 

一种方式是让页面设计得更加简单。富页面内容是否能达到更快的速度?这里有一些技巧,去减少http请求。

 

Combined files are a way to reduce the number of HTTP requests by combining all scripts into a single script, and similarly combining all CSS into a single stylesheet. Combining files is more challenging when the scripts and stylesheets vary from page to page, but making this part of your release process improves response times.

 

合并文件：合并全部js脚步到一个文件,合并css到一个css文件。

 

CSS Sprites are the preferred method for reducing the number of image requests. Combine your background images into a single image and use the CSS background-image and background-position properties to display the desired image segment.

 

css sprites : 合并多个细小背景图片到一个图片,使用css background-image和background-position属性显示期望的图片。

 

Image maps combine multiple images into a single image. The overall size is about the same, but reducing the number of HTTP requests speeds up the page. Image maps only work if the images are contiguous in the page, such as a navigation bar. Defining the coordinates of image maps can be tedious and error prone. Using image maps for navigation is not accessible too, so it's not recommended.

 

image maps:合并页面临近的细小图片,如导航栏的图片。这样需要的大小一致,但是请求却减少了。

 

Inline images use the data: URL scheme to embed the image data in the actual page. This can increase the size of your HTML document. Combining inline images into your (cached) stylesheets is a way to reduce HTTP requests and avoid increasing the size of your pages. Inline images are not yet supported across all major browsers.

 

inline images:图片内联还没得到大部份的浏览器支持。

 

Reducing the number of HTTP requests in your page is the place to start. This is the most important guide line for improving performance for first time visitors. As described in Tenni Theurer's blog post Browser Cache Usage - Exposed! , 40-60% of daily visitors to your site come in with an empty cache. Making your page fast for these first time visitors is key to a better user experience.

 

减少http请求是优化的开始。当用户第一次访问时，这是最关键的优化技巧。在tenni theurer的博客中提到,40-60%的日常访问者都没有使用浏览器缓存。减少页面请求是让第一次访问的用户提速的关键。 

 

Use a Content Delivery Network

使用内容分发网络

 

tag: server

The user's proximity to your web server has an impact on response times. Deploying your content across multiple, geographically dispersed servers will make your pages load faster from the user's perspective. But where should you start?

 

用户体验主要来自响应时间,将内容部署到多个、按地理分布的服务器将使你得页面更快的展现。但如何开始?

 

 

As a first step to implementing geographically dispersed content, don't attempt to redesign your web application to work in a distributed architecture. Depending on the application, changing the architecture could include daunting tasks such as synchronizing session state and replicating database transactions across server locations. Attempts to reduce the distance between users and your content could be delayed by, or never pass, this application architecture step.

 

第一步,按区域分散部署内容,不应该重新设计你的应用。改变应用将涉及session状态同步,数据库事务等问题。

 

Remember that 80-90% of the end-user response time is spent downloading all the components in the page: images, stylesheets, scripts, Flash, etc. This is the Performance Golden Rule . Rather than starting with the difficult task of redesigning your application architecture, it's better to first disperse your static content. This not only achieves a bigger reduction in response times, but it's easier thanks to content delivery networks.

 

记住80-90%的用户响应时间主要来自下载页面组件:图片、样式、js脚步、flash等。这是优化的黄金规则。比起重新涉及你的应用,更好得方式是分散你的静态内容。不紧紧能最大的减少响应时间,而且也是最容易的内容分散。

 

A content delivery network (CDN) is a collection of web servers distributed across multiple locations to deliver content more efficiently to users. The server selected for delivering content to a specific user is typically based on a measure of network proximity. For example, the server with the fewest network hops or the server with the quickest response time is chosen.

 

一个内容分发网络是一个web服务器集合,通过多个路径分发内容。服务器选择分发内容的路径是基于网络距离的。比如,会选择更近的服务器或更快的服务器。

 

Some large Internet companies own their own CDN, but it's cost-effective to use a CDN service provider, such as Akamai Technologies , Mirror Image Internet , or Limelight Networks . For start-up companies and private web sites, the cost of a CDN service can be prohibitive, but as your target audience grows larger and becomes more global, a CDN is necessary to achieve fast response times.

 

一些大型的互联网公司有自己的内容分发网络。不过更有效益的是使用内容分发服务,如akamai technologies,mirror image internet,limelight networks。

 

At Yahoo!, properties that moved static content off their application web servers to a CDN improved end-user response times by 20% or more. Switching to a CDN is a relatively easy code change that will dramatically improve the speed of your web site.

 

在yahoo,20%甚至更多的静态内容使用内容分发网络。

 

Add an Expires or a Cache-Control Header

在response header中添加expires 或cahce-control

 

tag: server

 

There are two things in this rule:

• For static components: implement "Never expire" policy by setting far future Expires header
• For dynamic components: use an appropriate Cache-Control header to help the browser with conditional requests

此规则的两项内容:

     对于静态组件:使用"never expire"策略设置永不过期

     对于动态组件:使用适当的Cache-Control

 

Web page designs are getting richer and richer, which means more scripts, stylesheets, images, and Flash in the page. A first-time visitor to your page may have to make several HTTP requests, but by using the Expires header you make those components cacheable. This avoids unnecessary HTTP requests on subsequent page views. Expires headers are most often used with images, but they should be used on all components including scripts, stylesheets, and Flash components.

 

web页面越来越大,既页面脚本、样式、图片、flash。第一次访问必须有多个http请求,使用expires响应头可以让这些组件缓存。这样可以避免随后发起的无用的http请求。expires头经常使用在图片,不过这应该使用在全部页面组件:脚本、样式和flash.

 

Browsers (and proxies) use a cache to reduce the number and size of HTTP requests, making web pages load faster. A web server uses the Expires header in the HTTP response to tell the client how long a component can be cached.

 

浏览器使用缓存而无需发出http请求。web服务器在http响应中使用expires头告诉浏览器该组件多长时间可以被缓存。

 

This is a far future Expires header, telling the browser that this response won't be stale until April 15, 2010.

      Expires: Thu, 15 Apr 2010 20:00:00 GMT

 

这是一个长的expires头,告诉浏览器这个响应在2010年4月15日前不会改变。

 

If your server is Apache, use the ExpiresDefault directive to set an expiration date relative to the current date. This example of the ExpiresDefault directive sets the Expires date 10 years out from the time of the request.

      ExpiresDefault "access plus 10 years"

 

如果服务器是apache,则使用ExpiresDefault设置相对于今天的过期时间。如这个过期时间是10年后的今天。

 

Keep in mind, if you use a far future Expires header you have to change the component's filename whenever the component changes. At Yahoo! we often make this step part of the build process: a version number is embedded in the component's filename, for example, yahoo_2.0.6.js.

 

记住,如果使用了长时间的将来过期时间,你必须通过修改文件名达到改变组件。在yahoo.我们通常通过添加版本号控制,如yahoo_2.0.6.js.

 

Using a far future Expires header affects page views only after a user has already visited your site. It has no effect on the number of HTTP requests when a user visits your site for the first time and the browser's cache is empty. Therefore the impact of this performance improvement depends on how often users hit your pages with a primed cache. (A "primed cache" already contains all of the components in the page.)

 

使用一个长时间的过期时间头将影响那些用户已经访问过你的网站。将无法影响第一次请求的用户。所以这项优化的性能将取决于多上用户经常访问你的网站。

 

 We measured this at Yahoo! and found the number of page views with a primed cache is 75-85%. By using a far future Expires header, you increase the number of components that are cached by the browser and re-used on subsequent page views without sending a single byte over the user's Internet connection.

 

我们测量过yahoo的。我们发现75-80%的组件被缓存了。

 

Gzip Components

gzip 组件

 

tag: server

The time it takes to transfer an HTTP request and response across the network can be significantly reduced by decisions made by front-end engineers. It's true that the end-user's bandwidth speed, Internet service provider, proximity to peering exchange points, etc. are beyond the control of the development team. But there are other variables that affect response times. Compression reduces response times by reducing the size of the HTTP response.

 

使用gzip传输http请求与响应时间能被显著减少。压缩http响应的大小从而减少了响应时间。

 

Starting with HTTP/1.1, web clients indicate support for compression with the Accept-Encoding header in the HTTP request.

      Accept-Encoding: gzip, deflate

http/1.1协议开始,web客户端通过accept-encoding header支持压缩。

 

If the web server sees this header in the request, it may compress the response using one of the methods listed by the client. The web server notifies the web client of this via the Content-Encoding header in the response.

      Content-Encoding: gzip

如果web服务器看到此请求,将使用gzip压缩响应,web服务器通过Content-Encoding通知客户端使用gzip压缩。

 

Gzip is the most popular and effective compression method at this time. It was developed by the GNU project and standardized by RFC 1952 . The only other compression format you're likely to see is deflate, but it's less effective and less popular.

 

gzip是当前最受欢迎与最显著的压缩方法。是gnu项目开发并且纳入RFC1952的规范。

 

Gzipping generally reduces the response size by about 70%. Approximately 90% of today's Internet traffic travels through browsers that claim to support gzip. If you use Apache, the module configuring gzip depends on your version: Apache 1.3 uses mod_gzip while Apache 2.x uses mod_deflate .

 

通常gzip能减少70%的大小。大部分的浏览器都支持gizp.如果你使用apache.apache1.3使用mod_gizp,apache2.x使用mod_deflate.

 

There are known issues with browsers and proxies that may cause a mismatch in what the browser expects and what it receives with regard to compressed content. Fortunately, these edge cases are dwindling as the use of older browsers drops off. The Apache modules help out by adding appropriate Vary response headers automatically.

 

 

 

Servers choose what to gzip based on file type, but are typically too limited in what they decide to compress. Most web sites gzip their HTML documents. It's also worthwhile to gzip your scripts and stylesheets, but many web sites miss this opportunity. In fact, it's worthwhile to compress any text response including XML and JSON. Image and PDF files should not be gzipped because they are already compressed. Trying to gzip them not only wastes CPU but can potentially increase file sizes.

 

许多web网站压缩其html文档。很多网站都忽略了压缩脚本与样式。事实上。全部text返回都可以被压缩。包括xml与json.图片和pdf则无需进行压缩因为他们已经被压缩过了。gizp紧紧花费cpu却能有效减少文件大小。

 

Gzipping as many file types as possible is an easy way to reduce page weight and accelerate the user experience.

 

gzip是简单的方式减少文件大小并有效的改进用户体验。

Put Stylesheets at the Top

将样式放在页面顶部

 

tag: css

While researching performance at Yahoo!, we discovered that moving stylesheets to the document HEAD makes pages appear to be loading faster. This is because putting stylesheets in the HEAD allows the page to render progressively.

 

在yahoo的性能研究中,我们发现将样式防止在documet的head使页面出现得更快。因为在head可以使页面逐步呈现样式。

 

Front-end engineers that care about performance want a page to load progressively; that is, we want the browser to display whatever content it has as soon as possible. This is especially important for pages with a lot of content and for users on slower Internet connections. The importance of giving users visual feedback, such as progress indicators, has been well researched and documented . In our case the HTML page is the progress indicator! When the browser loads the page progressively the header, the navigation bar, the logo at the top, etc. all serve as visual feedback for the user who is waiting for the page. This improves the overall user experience.

 

浏览器希望装载得更快,当浏览器装载先显示内容,再逐步显示header,滚动条、logo等。

 

The problem with putting stylesheets near the bottom of the document is that it prohibits progressive rendering in many browsers, including Internet Explorer. These browsers block rendering to avoid having to redraw elements of the page if their styles change. The user is stuck viewing a blank white page.

 

 

The HTML specification clearly states that stylesheets are to be included in the HEAD of the page: "Unlike A, [LINK] may only appear in the HEAD section of a document, although it may appear any number of times." Neither of the alternatives, the blank white screen or flash of unstyled content, are worth the risk. The optimal solution is to follow the HTML specification and load your stylesheets in the document HEAD.

 

http协议中定义了样式包含在head中。

Put Scripts at the Bottom

将脚本放在底部

 

tag: javascript

The problem caused by scripts is that they block parallel downloads. The HTTP/1.1 specification suggests that browsers download no more than two components in parallel per hostname. If you serve your images from multiple hostnames, you can get more than two downloads to occur in parallel. While a script is downloading, however, the browser won't start any other downloads, even on different hostnames.

 

这个问题原因是script阻塞了并行下载。http协议中建议浏览器并行下载两个组件。如果将图片在多个hostname,你能有2个并行下载。当script下载时,浏览器无法进行多个并行下载。

 

In some situations it's not easy to move scripts to the bottom. If, for example, the script uses document.write to insert part of the page's content, it can't be moved lower in the page. There might also be scoping issues. In many cases, there are ways to workaround these situations.

An alternative suggestion that often comes up is to use deferred scripts. The DEFER attribute indicates that the script does not contain document.write, and is a clue to browsers that they can continue rendering. Unfortunately, Firefox doesn't support the DEFER attribute. In Internet Explorer, the script may be deferred, but not as much as desired. If a script can be deferred, it can also be moved to the bottom of the page. That will make your web pages load faster.

 

在某些情况下无法将脚本移到底部。如脚本使用了document.write去插入内容。使用DEFER属性延迟script下载。

 

Avoid CSS Expressions

避免使用css expression

 

tag: css

CSS expressions are a powerful (and dangerous) way to set CSS properties dynamically. They're supported in Internet Explorer, starting with version 5 . As an example, the background color could be set to alternate every hour using CSS expressions.

      background-color: expression( (new Date()).getHours()%2 ? "#B8D4FF" : "#F08A00" );

 

As shown here, the expression method accepts a JavaScript expression. The CSS property is set to the result of evaluating the JavaScript expression. The expression method is ignored by other browsers, so it is useful for setting properties in Internet Explorer needed to create a consistent experience across browsers.

The problem with expressions is that they are evaluated more frequently than most people expect. Not only are they evaluated when the page is rendered and resized, but also when the page is scrolled and even when the user moves the mouse over the page. Adding a counter to the CSS expression allows us to keep track of when and how often a CSS expression is evaluated. Moving the mouse around the page can easily generate more than 10,000 evaluations.

One way to reduce the number of times your CSS expression is evaluated is to use one-time expressions, where the first time the expression is evaluated it sets the style property to an explicit value, which replaces the CSS expression. If the style property must be set dynamically throughout the life of the page, using event handlers instead of CSS expressions is an alternative approach. If you must use CSS expressions, remember that they may be evaluated thousands of times and could affect the performance of your page.

 

Make JavaScript and CSS External

使javascript和css独立于外部文件

 

tag: javascript, css

Many of these performance rules deal with how external components are managed. However, before these considerations arise you should ask a more basic question: Should JavaScript and CSS be contained in external files, or inlined in the page itself?

 

Using external files in the real world generally produces faster pages because the JavaScript and CSS files are cached by the browser. JavaScript and CSS that are inlined in HTML documents get downloaded every time the HTML document is requested. This reduces the number of HTTP requests that are needed, but increases the size of the HTML document. On the other hand, if the JavaScript and CSS are in external files cached by the browser, the size of the HTML document is reduced without increasing the number of HTTP requests.

 

通常使用外部文件而不放在html中因为js和css可以被缓存。如果在html中，则每次请求都需要返回。

如果被缓存了。那么既可以减少html的大小,也无需发起http请求。

 

The key factor, then, is the frequency with which external JavaScript and CSS components are cached relative to the number of HTML documents requested. This factor, although difficult to quantify, can be gauged using various metrics. If users on your site have multiple page views per session and many of your pages re-use the same scripts and stylesheets, there is a greater potential benefit from cached external files.

 

这里的关键是,频繁的访问多个页面,包含的是相同的js和css,则他们将会组件被cache.这样是非常显著的。

 

Many web sites fall in the middle of these metrics. For these sites, the best solution generally is to deploy the JavaScript and CSS as external files. The only exception where inlining is preferable is with home pages, such as Yahoo!'s front page and My Yahoo! . Home pages that have few (perhaps only one) page view per session may find that inlining JavaScript and CSS results in faster end-user response times.

 

在主页,可以使用js内联,而其他页面。则独立成文件。

 

For front pages that are typically the first of many page views, there are techniques that leverage the reduction of HTTP requests that inlining provides, as well as the caching benefits achieved through using external files. One such technique is to inline JavaScript and CSS in the front page, but dynamically download the external files after the page has finished loading. Subsequent pages would reference the external files that should already be in the browser's cache.

 

主页一般特殊于其他页面。

 

Reduce DNS Lookups

减少dns查询

 

tag: content

 

The Domain Name System (DNS) maps hostnames to IP addresses, just as phonebooks map people's names to their phone numbers. When you type www.yahoo.com into your browser, a DNS resolver contacted by the browser returns that server's IP address. DNS has a cost. It typically takes 20-120 milliseconds for DNS to lookup the IP address for a given hostname. The browser can't download anything from this hostname until the DNS lookup is completed.

 

dns将hostname映射到ip地址。当打开www.yahoo.com ,一个dns查询返回域名的ip地址。dns需要耗费时间。大概需要20-120毫秒。浏览器无法下载内容知道hostname完成映射。

 

DNS lookups are cached for better performance. This caching can occur on a special caching server, maintained by the user's ISP or local area network, but there is also caching that occurs on the individual user's computer. The DNS information remains in the operating system's DNS cache (the "DNS Client service" on Microsoft Windows). Most browsers have their own caches, separate from the operating system's cache. As long as the browser keeps a DNS record in its own cache, it doesn't bother the operating system with a request for the record.

 

dns查询被缓存。dns信息在操作系统的dns中缓存。许多浏览器有他们自己的缓存。

 

Internet Explorer caches DNS lookups for 30 minutes by default, as specified by the DnsCacheTimeout registry setting. Firefox caches DNS lookups for 1 minute, controlled by the network.dnsCacheExpiration configuration setting. (Fasterfox changes this to 1 hour.)

 

ie默认缓存dns信息30分钟。通过DnsCacheTImeout设置。firefox缓存1分钟。通过network.dnsCacheExpiration配置。

 

When the client's DNS cache is empty (for both the browser and the operating system), the number of DNS lookups is equal to the number of unique hostnames in the web page. This includes the hostnames used in the page's URL, images, script files, stylesheets, Flash objects, etc. Reducing the number of unique hostnames reduces the number of DNS lookups.

 

当dns缓存为空时。dns请求将发起。包括页面url、图片、脚本等。减少独一的hostname有助于优化。

 

Reducing the number of unique hostnames has the potential to reduce the amount of parallel downloading that takes place in the page. Avoiding DNS lookups cuts response times, but reducing parallel downloads may increase response times. My guideline is to split these components across at least two but no more than four hostnames. This results in a good compromise between reducing DNS lookups and allowing a high degree of parallel downloads.

 

 

Minify JavaScript and CSS

缩小js和css

 

tag: javascript, css

 

Minification is the practice of removing unnecessary characters from code to reduce its size thereby improving load times. When code is minified all comments are removed, as well as unneeded white space characters (space, newline, and tab). In the case of JavaScript, this improves response time performance because the size of the downloaded file is reduced. Two popular tools for minifying JavaScript code are JSMin and YUI Compressor . The YUI compressor can also minify CSS.

Obfuscation is an alternative optimization that can be applied to source code. It's more complex than minification and thus more likely to generate bugs as a result of the obfuscation step itself. In a survey of ten top U.S. web sites, minification achieved a 21% size reduction versus 25% for obfuscation. Although obfuscation has a higher size reduction, minifying JavaScript is less risky.

In addition to minifying external scripts and styles, inlined <script> and <style> blocks can and should also be minified. Even if you gzip your scripts and styles, minifying them will still reduce the size by 5% or more. As the use and size of JavaScript and CSS increases, so will the savings gained by minifying your code.

 

通过删除无用的js与css代码从而减少其大小。如空白,换行、tab等。

两个最受欢迎的工具是JSMin与yui compressor.

淘宝兄弟的可视化工具:http://lifesinger.org/blog/2008/10/tbcompressor-240/

Avoid Redirects

避免重定向

 

tag: content

 

Redirects are accomplished using the 301 and 302 status codes. Here's an example of the HTTP headers in a 301 response:

      HTTP/1.1 301 Moved Permanently
      Location: http://example.com/newuri
      Content-Type: text/html

重定向使用301与302状态码。

以下是例子。

 

The browser automatically takes the user to the URL specified in the Location field. All the information necessary for a redirect is in the headers. The body of the response is typically empty. Despite their names, neither a 301 nor a 302 response is cached in practice unless additional headers, such as Expires or Cache-Control , indicate it should be. The meta refresh tag and JavaScript are other ways to direct users to a different URL, but if you must do a redirect, the preferred technique is to use the standard 3xx HTTP status codes, primarily to ensure the back button works correctly.

 

The main thing to remember is that redirects slow down the user experience. Inserting a redirect between the user and the HTML document delays everything in the page since nothing in the page can be rendered and no components can start being downloaded until the HTML document has arrived.

 

重定向降低了用户的体验。页面延迟,直到页面重定向后才能展现页面。

 

One of the most wasteful redirects happens frequently and web developers are generally not aware of it. It occurs when a trailing slash (/) is missing from a URL that should otherwise have one. For example, going to http://astrology.yahoo.com/astrology results in a 301 response containing a redirect to http://astrology.yahoo.com/astrology/ (notice the added trailing slash). This is fixed in Apache by using Alias or mod_rewrite , or the DirectorySlash directive if you're using Apache handlers.

 

一个最频繁发生的重定向浪费,并且许多web开发者并不知道。他发生在url少了/结尾。比如http://yahoo.com/astology 返回301重定向到http://yahoo.com/astology/ 。这个apache通过使用Alias或mod_rewrite.或者DIrectorySlash.

 

Connecting an old web site to a new one is another common use for redirects. Others include connecting different parts of a website and directing the user based on certain conditions (type of browser, type of user account, etc.). Using a redirect to connect two web sites is simple and requires little additional coding. Although using redirects in these situations reduces the complexity for developers, it degrades the user experience. Alternatives for this use of redirects include using Alias and mod_rewrite if the two code paths are hosted on the same server. If a domain name change is the cause of using redirects, an alternative is to create a CNAME (a DNS record that creates an alias pointing from one domain name to another) in combination with Alias or mod_rewrite .

 

访问旧系统跳转到新系统是另外经常出现的重定向。

 

Remove Duplicate Scripts

删除重复脚本

 

tag: javascript

 

It hurts performance to include the same JavaScript file twice in one page. This isn't as unusual as you might think. A review of the ten top U.S. web sites shows that two of them contain a duplicated script. Two main factors increase the odds of a script being duplicated in a single web page: team size and number of scripts. When it does happen, duplicate scripts hurt performance by creating unnecessary HTTP requests and wasted JavaScript execution.

 

一个页面的重复脚本影响了性能。你可能认为这不常出现。美国最高的10家网站的审查表明,出现重复脚本的两个因素:团队大小与脚本数量。当出现重复脚本时,无用的请求影响了性能。

 

Unnecessary HTTP requests happen in Internet Explorer, but not in Firefox. In Internet Explorer, if an external script is included twice and is not cacheable, it generates two HTTP requests during page loading. Even if the script is cacheable, extra HTTP requests occur when the user reloads the page.

In addition to generating wasteful HTTP requests, time is wasted evaluating the script multiple times. This redundant JavaScript execution happens in both Firefox and Internet Explorer, regardless of whether the script is cacheable.

 

无效的http请求出现在ie浏览器,在firefox中不会出现。在ie中,如果外部脚本出现两次并且未被缓存,页面装载时产生了两次请求。就算是脚本被缓存,额外的http请求在reload时发生。增加了请求,所以耗费了时间。重复的javascrip会被重复执行。

 

One way to avoid accidentally including the same script twice is to implement a script management module in your templating system. The typical way to include a script is to use the SCRIPT tag in your HTML page.

      <script type="text/javascript" src="menu_1.0.17.js"></script>

一种避免的方法是实现自己的scirpt管理模块在你的模板系统。

 

An alternative in PHP would be to create a function called insertScript .

      <?php insertScript("menu.js") ?>

 

In addition to preventing the same script from being inserted multiple times, this function could handle other issues with scripts, such as dependency checking and adding version numbers to script filenames to support far future Expires headers.

 

Configure ETags

配置ETags

 

tag: server

Entity tags (ETags) are a mechanism that web servers and browsers use to determine whether the component in the browser's cache matches the one on the origin server. (An "entity" is another word a "component": images, scripts, stylesheets, etc.) ETags were added to provide a mechanism for validating entities that is more flexible than the last-modified date. An ETag is a string that uniquely identifies a specific version of a component. The only format constraints are that the string be quoted. The origin server specifies the component's ETag using the ETag response header.

      HTTP/1.1 200 OK
      Last-Modified: Tue, 12 Dec 2006 03:03:59 GMT
      ETag: "10c24bc-4ab-457e1c1f"
      Content-Length: 12195

entity tags实体标签是这样一种机制:浏览器与服务器通过etag判断缓存组件与原服务器匹配。etags比last-modify提供更灵活机制判断是否有效。etag通过一个字符串定义实体的版本标识。服务器在返回头指定组件的etag。

 

Later, if the browser has to validate a component, it uses the If-None-Match header to pass the ETag back to the origin server. If the ETags match, a 304 status code is returned reducing the response by 12195 bytes for this example.

      GET /i/yahoo.gif HTTP/1.1
      Host: us.yimg.com
      If-Modified-Since: Tue, 12 Dec 2006 03:03:59 GMT
      If-None-Match: "10c24bc-4ab-457e1c1f"
      HTTP/1.1 304 Not Modified

然后,浏览器验证组件,使用if-none-match 头传输Etag给服务器。如果ETags匹配了,304状态码将返回。

 

The problem with ETags is that they typically are constructed using attributes that make them unique to a specific server hosting a site. ETags won't match when a browser gets the original component from one server and later tries to validate that component on a different server, a situation that is all too common on Web sites that use a cluster of servers to handle requests. By default, both Apache and IIS embed data in the ETag that dramatically reduces the odds of the validity test succeeding on web sites with multiple servers.

 

ETags的问题是,他是通过属性被构造,etags无法匹配一个组件到另外一个服务器。一种解决方案是全部web服务器使用集群的请求处理。默认的,apache和iis内置etag数据,极大的降低了出错可能。

 

The ETag format for Apache 1.3 and 2.x is inode-size-timestamp . Although a given file may reside in the same directory across multiple servers, and have the same file size, permissions, timestamp, etc., its inode is different from one server to the next.

 

etag格式在1.3和2.x是inode-size-timestamp。不过一个文件可能从一个服务器拷贝到其他服务器,有相同的大小、权限、时间戳,它的inode是不同的。

 

IIS 5.0 and 6.0 have a similar issue with ETags. The format for ETags on IIS is Filetimestamp:ChangeNumber . A ChangeNumber is a counter used to track configuration changes to IIS. It's unlikely that the ChangeNumber is the same across all IIS servers behind a web site.

The end result is ETags generated by Apache and IIS for the exact same component won't match from one server to another. If the ETags don't match, the user doesn't receive the small, fast 304 response that ETags were designed for; instead, they'll get a normal 200 response along with all the data for the component. If you host your web site on just one server, this isn't a problem. But if you have multiple servers hosting your web site, and you're using Apache or IIS with the default ETag configuration, your users are getting slower pages, your servers have a higher load, you're consuming greater bandwidth, and proxies aren't caching your content efficiently. Even if your components have a far future Expires header, a conditional GET request is still made whenever the user hits Reload or Refresh.

 

 

If you're not taking advantage of the flexible validation model that ETags provide, it's better to just remove the ETag altogether. The Last-Modified header validates based on the component's timestamp. And removing the ETag reduces the size of the HTTP headers in both the response and subsequent requests. This Microsoft Support article describes how to remove ETags. In Apache, this is done by simply adding the following line to your Apache configuration file:

      FileETag none

如果你不考虑etag,更好的方式是移除ETag.使用Last-Modified头通过组件的tmiestamp验证。删除ETag较少http头的大小。在apache中,使用FileETag none设置。

 

 

Make Ajax Cacheable

缓存ajax返回

tag: content

 

One of the cited benefits of Ajax is that it provides instantaneous feedback to the user because it requests information asynchronously from the backend web server. However, using Ajax is no guarantee that the user won't be twiddling his thumbs waiting for those asynchronous JavaScript and XML responses to return. In many applications, whether or not the user is kept waiting depends on how Ajax is used. For example, in a web-based email client the user will be kept waiting for the results of an Ajax request to find all the email messages that match their search criteria. It's important to remember that "asynchronous" does not imply "instantaneous".

 

想起ajax是它提供马上的展现。因为他异步获得信息。然而,使用ajax无法肯定用户无需等待ajax的异步返回。在许多应用,用户是否等待取决于如何使用ajax.比如。email客户端将等待ajax查询全部email信息的返回。记住异步不代表马上。

 

To improve performance, it's important to optimize these Ajax responses. The most important way to improve the performance of Ajax is to make the responses cacheable, as discussed in Add an Expires or a Cache-Control Header .

 

提高性能,关键是优化这些ajax返回,最关键的ajax优化是让返回值缓存。在Add_an_expires or a Cache-Control Header中讨论的。

 

Some of the other rules also apply to Ajax:

 

一些其他的规则应用在ajax:

• Gzip Components gzip组件
  
• Reduce DNS Lookups 减少dns查询
  
• Minify JavaScript 最小化javascript
  
• Avoid Redirects 避免重定向
  
• Configure ETags 配置ETags
  

Let's look at an example. A Web 2.0 email client might use Ajax to download the user's address book for autocompletion. If the user hasn't modified her address book since the last time she used the email web app, the previous address book response could be read from cache if that Ajax response was made cacheable with a future Expires or Cache-Control header. The browser must be informed when to use a previously cached address book response versus requesting a new one. This could be done by adding a timestamp to the address book Ajax URL indicating the last time the user modified her address book, for example, &t=1190241612 . If the address book hasn't been modified since the last download, the timestamp will be the same and the address book will be read from the browser's cache eliminating an extra HTTP roundtrip. If the user has modified her address book, the timestamp ensures the new URL doesn't match the cached response, and the browser will request the updated address book entries.

 

看一个例子,web 2.0 email客户端可能使用ajax自动下载用户的地址本。如果用户没有修改他们的地址本,那么之前返回的请求可以被缓存。使用Expires或Cache-control。浏览器必须保存之前请求响应的信息。这可以通过添加一个时间戳在url中表明时间。如&t=1190241612.如果地址本没有修改,时间戳将一致。浏览器将从缓存中读取。如果修改了地址本。新的地址本将无法命中缓存的响应。浏览器将发起一个新的更新请求。

 

Even though your Ajax responses are created dynamically, and might only be applicable to a single user, they can still be cached. Doing so will make your Web 2.0 apps faster.

 

尽管你的ajax响应是动态输出的。并且只对一个用户有效。但它将提升你应用的速度。

 

Flush the Buffer Early

尽早刷缓冲

 

tag: server

 

When users request a page, it can take anywhere from 200 to 500ms for the backend server to stitch together the HTML page. During this time, the browser is idle as it waits for the data to arrive. In PHP you have the function flush() . It allows you to send your partially ready HTML response to the browser so that the browser can start fetching components while your backend is busy with the rest of the HTML page. The benefit is mainly seen on busy backends or light frontends.

 

A good place to consider flushing is right after the HEAD because the HTML for the head is usually easier to produce and it allows you to include any CSS and JavaScript files for the browser to start fetching in parallel while the backend is still processing.

Example:

      ... <!-- css, js -->
    </head>
    <?php flush(); ?>


    <body>
      ... <!-- content -->

Yahoo! search pioneered research and real user testing to prove the benefits of using this technique.

 

一个好的地方是刷出head信息。因为html head更容易输出。并且可以让浏览器尽早的展现页面。

 

Use GET for AJAX Requests

使用get进行ajax请求

 

tag: server

 

The Yahoo! Mail team found that when using XMLHttpRequest , POST is implemented in the browsers as a two-step process: sending the headers first, then sending data. So it's best to use GET, which only takes one TCP packet to send (unless you have a lot of cookies). The maximum URL length in IE is 2K, so if you send more than 2K data you might not be able to use GET.

 

yahoo email团队发现使用XMLHttpRequest,post比在浏览器被实现为两个过程:首先发送 header,然后发送数据。所以这里使用get更好,只有一次tcp包请求(除非你有很多cookies)。最大的URL在ie是2k,所以大于2k的数据将无法使用get.

 

An interesting side affect is that POST without actually posting any data behaves like GET. Based on the HTTP specs , GET is meant for retrieving information, so it makes sense (semantically) to use GET when you're only requesting data, as opposed to sending data to be stored server-side.

有趣的是,post发送方式与get不同。在http协议中,get是取得信息,所以明显是使用get去获得信息。而post是发送需要存储在server端的数据。

 

Post-load Components

请求装载组件

 

tag: content

 

You can take a closer look at your page and ask yourself: "What's absolutely required in order to render the page initially?". The rest of the content and components can wait.

你可以问自己:那些需要马上显示,那些是可以等待的组件?

 

JavaScript is an ideal candidate for splitting before and after the onload event. For example if you have JavaScript code and libraries that do drag and drop and animations, those can wait, because dragging elements on the page comes after the initial rendering. Other places to look for candidates for post-loading include hidden content (content that appears after a user action) and images below the fold.

 

javascript是可以在onload事件前后拆分,如你的javascript代码实现拖拽,这是可以等待的。因为拖拽必须等待页面展现后。另外的条件是如请求后隐藏内容与图片折叠。

 

Tools to help you out in your effort: YUI Image Loader allows you to delay images below the fold and the YUI Get utility is an easy way to include JS and CSS on the fly. For an example in the wild take a look at Yahoo! Home Page with Firebug's Net Panel turned on.

It's good when the performance goals are inline with other web development best practices. In this case, the idea of progressive enhancement tells us that JavaScript, when supported, can improve the user experience but you have to make sure the page works even without JavaScript. So after you've made sure the page works fine, you can enhance it with some post-loaded scripts that give you more bells and whistles such as drag and drop and animations.

 

/*

此处的意思是发送post请求后获得其他script代码?

*/

 

Preload Components

预装载组件

 

tag: content

 

Preload may look like the opposite of post-load, but it actually has a different goal. By preloading components you can take advantage of the time the browser is idle and request components (like images, styles and scripts) you'll need in the future. This way when the user visits the next page, you could have most of the components already in the cache and your page will load much faster for the user.

 

预装载组件像是post-load的相反,然而它有其他不同的目标。通过预装载需要的组件。当用户浏览下个页面时,访问的是缓存,所以页面会更加快。

 

There are actually several types of preloading:

• Unconditional preload - as soon as onload fires, you go ahead and fetch some extra components. Check google.com for an example of how a sprite image is requested onload. This sprite image is not needed on the google.com homepage, but it is needed on the consecutive search result page.
• Conditional preload - based on a user action you make an educated guess where the user is headed next and preload accordingly. On search.yahoo.com you can see how some extra components are requested after you start typing in the input box.
• Anticipated preload - preload in advance before launching a redesign. It often happens after a redesign that you hear: "The new site is cool, but it's slower than before". Part of the problem could be that the users were visiting your old site with a full cache, but the new one is always an empty cache experience. You can mitigate this side effect by preloading some components before you even launched the redesign. Your old site can use the time the browser is idle and request images and scripts that will be used by the new site

无条件预装载:如google.com的sprite图片无需在首页中使用,不过它会在查询结果集页面出现。故装载了。

有条件预装载:如search.yahoo.com你可以看到一些额外的组件在你停留在输入栏时被请求。

预期的预装载:

 

Reduce the Number of DOM Elements

减少dom元素个数

 

tag: content

 

A complex page means more bytes to download and it also means slower DOM access in JavaScript. It makes a difference if you loop through 500 or 5000 DOM elements on the page when you want to add an event handler for example.

 

A high number of DOM elements can be a symptom that there's something that should be improved with the markup of the page without necessarily removing content. Are you using nested tables for layout purposes? Are you throwing in more <div> s only to fix layout issues? Maybe there's a better and more semantically correct way to do your markup.

A great help with layouts are the YUI CSS utilities : grids.css can help you with the overall layout, fonts.css and reset.css can help you strip away the browser's defaults formatting. This is a chance to start fresh and think about your markup, for example use <div> s only when it makes sense semantically, and not because it renders a new line.

 

The number of DOM elements is easy to test, just type in Firebug's console:
document.getElementsByTagName('*').length

 

And how many DOM elements are too many? Check other similar pages that have good markup. For example the Yahoo! Home Page is a pretty busy page and still under 700 elements (HTML tags).

 

 

Split Components Across Domains

组件分到不同域

 

tag: content

 

Splitting components allows you to maximize parallel downloads. Make sure you're using not more than 2-4 domains because of the DNS lookup penalty. For example, you can host your HTML and dynamic content on www.example.org and split static components between static1.example.org and static2.example.org

 

组件分到不同域可以最大化并行下载。确保没有超过2-4个dns查询。如你可以显示动态页面在www.example.org .将静态组件拆分到static1.example.org与static2.example.org。

 

For more information check "Maximizing Parallel Downloads in the Carpool Lane " by Tenni Theurer and Patty Chi.

 

 

Minimize the Number of iframes

最小化iframe个数

 

tag: content

Iframes allow an HTML document to be inserted in the parent document. It's important to understand how iframes work so they can be used effectively.

 

 

<iframe> pros:

• Helps with slow third-party content like badges and ads 允许显示第三方内容
• Security sandbox 安全的沙箱
• Download scripts in parallel 并行下载脚本

<iframe> cons:

• Costly even if blank 如果空白页面是代价高昂的
• Blocks page onload 阻塞页面onload
• Non-semantic 非语义的

 

No 404s

不要404错误

 

tag: content

HTTP requests are expensive so making an HTTP request and getting a useless response (i.e. 404 Not Found) is totally unnecessary and will slow down the user experience without any benefit.

Some sites have helpful 404s "Did you mean X?", which is great for the user experience but also wastes server resources (like database, etc). Particularly bad is when the link to an external JavaScript is wrong and the result is a 404. First, this download will block parallel downloads. Next the browser may try to parse the 404 response body as if it were JavaScript code, trying to find something usable in it.

 

404是没必要并且降低用户体验。一些网站404时显示"Did you mean X" ,更好的用户体验。最不好的是请求的javascript是404.它阻塞并发下载,让浏览器解析404的javascript脚本。

 

Reduce Cookie Size

减少cookie大小

 

tag: cookie

 

HTTP cookies are used for a variety of reasons such as authentication and personalization. Information about cookies is exchanged in the HTTP headers between web servers and browsers. It's important to keep the size of cookies as low as possible to minimize the impact on the user's response time.

 

 

 

For more information check "When the Cookie Crumbles" by Tenni Theurer and Patty Chi. The take-home of this research:

 

• Eliminate unnecessary cookies
• Keep cookie sizes as low as possible to minimize the impact on the user response time
• Be mindful of setting cookies at the appropriate domain level so other sub-domains are not affected
• Set an Expires date appropriately. An earlier Expires date or none removes the cookie sooner, improving the user response time

 

 

Use Cookie-free Domains for Components

为组件使用无cookie域

 

tag: cookie

 

When the browser makes a request for a static image and sends cookies together with the request, the server doesn't have any use for those cookies. So they only create network traffic for no good reason. You should make sure static components are requested with cookie-free requests. Create a subdomain and host all your static components there.

 

 

 

If your domain is www.example.org , you can host your static components on static.example.org . However, if you've already set cookies on the top-level domain example.org as opposed to www.example.org , then all the requests to static.example.org will include those cookies. In this case, you can buy a whole new domain, host your static components there, and keep this domain cookie-free. Yahoo! uses yimg.com , YouTube uses ytimg.com , Amazon uses images-amazon.com and so on.

 

Another benefit of hosting static components on a cookie-free domain is that some proxies might refuse to cache the components that are requested with cookies. On a related note, if you wonder if you should use example.org or www.example.org for your home page, consider the cookie impact. Omitting www leaves you no choice but to write cookies to *.example.org , so for performance reasons it's best to use the www subdomain and write the cookies to that subdomain.

 

 

Minimize DOM Access

最小化dom操作

 

tag: javascript

Accessing DOM elements with JavaScript is slow so in order to have a more responsive page, you should:

• Cache references to accessed elements
• Update nodes "offline" and then add them to the tree
• Avoid fixing layout with JavaScript

For more information check the YUI theatre's "High Performance Ajax Applications" by Julien Lecomte.

 

 

Develop Smart Event Handlers

 

 

tag: javascript

 

Sometimes pages feel less responsive because of too many event handlers attached to different elements of the DOM tree which are then executed too often. That's why using event delegation is a good approach. If you have 10 buttons inside a div , attach only one event handler to the div wrapper, instead of one handler for each button. Events bubble up so you'll be able to catch the event and figure out which button it originated from.

 

You also don't need to wait for the onload event in order to start doing something with the DOM tree. Often all you need is the element you want to access to be available in the tree. You don't have to wait for all images to be downloaded. DOMContentLoaded is the event you might consider using instead of onload, but until it's available in all browsers, you can use the YUI Event utility, which has an onAvailable method.

 

For more information check the YUI theatre's "High Performance Ajax Applications" by Julien Lecomte.

 

 

Choose <link> over @import

tag: css

 

One of the previous best practices states that CSS should be at the top in order to allow for progressive rendering.

In IE @import behaves the same as using <link> at the bottom of the page, so it's best not to use it.

 

 

Avoid Filters

tag: css

The IE-proprietary AlphaImageLoader filter aims to fix a problem with semi-transparent true color PNGs in IE versions < 7. The problem with this filter is that it blocks rendering and freezes the browser while the image is being downloaded. It also increases memory consumption and is applied per element, not per image, so the problem is multiplied.

The best approach is to avoid AlphaImageLoader completely and use gracefully degrading PNG8 instead, which are fine in IE. If you absolutely need AlphaImageLoader , use the underscore hack _filter as to not penalize your IE7+ users.

 

 

Optimize Images

优化图片

 

tag: images

After a designer is done with creating the images for your web page, there are still some things you can try before you FTP those images to your web server.

• You can check the GIFs and see if they are using a palette size corresponding to the number of colors in the image. Using imagemagick it's easy to check using
  identify -verbose image.gif
  When you see an image useing 4 colors and a 256 color "slots" in the palette, there is room for improvement.
• Try converting GIFs to PNGs and see if there is a saving. More often than not, there is. Developers often hesitate to use PNGs due to the limited support in browsers, but this is now a thing of the past. The only real problem is alpha-transparency in true color PNGs, but then again, GIFs are not true color and don't support variable transparency either. So anything a GIF can do, a palette PNG (PNG8) can do too (except for animations). This simple imagemagick command results in totally safe-to-use PNGs:
  convert image.gif image.png
  "All we are saying is: Give PiNG a Chance!"
• Run pngcrush (or any other PNG optimizer tool) on all your PNGs. Example:
  pngcrush image.png -rem alla -reduce -brute result.png
• Run jpegtran on all your JPEGs. This tool does lossless JPEG operations such as rotation and can also be used to optimize and remove comments and other useless information (such as EXIF information) from your images. <br
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  1 楼 yongyuan.jiang 2010-02-12  

  use get for ajax requests
  
  http://developer.yahoo.com/performance/rules.html
  
  yahoo的博客文章，使用get获得请求，里面提到两点，一个是post请求为两步，第一部发送头信息，第二步发送消息数据。而get仅需要一步即可。所以get优于post.第二点是，http协议中定义,get是用于获得数据，而非发送数据保存在服务端。
  
  The Yahoo! Mail team found that when using XMLHttpRequest, POST is implemented in the browsers as a two-step process: sending the headers first, then sending data. So it's best to use GET, which only takes one TCP packet to send (unless you have a lot of cookies). The maximum URL length in IE is 2K, so if you send more than 2K data you might not be able to use GET.
  
  An interesting side affect is that POST without actually posting any data behaves like GET. Based on the HTTP specs, GET is meant for retrieving information, so it makes sense (semantically) to use GET when you're only requesting data, as opposed to sending data to be stored server-side.