Apache 网站访问速度提升优化

　在 Yahoo 开发者网站上，Yahoo 提出了优化网站性能的N条规则(Best Practices for Speeding Up Your Web Site)。这些规则是：

　　1. 尽可能的减少 HTTP 的请求数(Minimize HTTP Requests) ——　Tag: content

　　2. 使用 CDN(Use a Content Delivery Network) ——　Tag: server

　　3. 添加 Expires 头(或者 Cache-control ) (Add an Expires or a Cache-Control Header) ——　Tag: server

　　4. Gzip 组件(Gzip Components) ——　Tag: server

　　5. 将 CSS 样式放在页面的上方 (Put Stylesheets at the Top) ——　Tag: css

　　6. 将脚本移动到底部（包括内联的）(Put Scripts at the Bottom) ——　Tag: javascript

　　7. 避免使用 CSS 中的 Expressions(Avoid CSS expressions) ——　Tag: css

　　8. 将 JavaScript 和 CSS 独立成外部文件(Make JS and CSS external) ——　Tag: javascript, css

　　9. 减少 DNS 查询 (Reduce DNS lookups) ——　Tag: content

　　10. 压缩 JavaScript 和 CSS ( Minify JS) ——　Tag: javascript, css

　　11. 避免重定向(Avoid redirects) ——　Tag: server

　　12. 移除重复的脚本(Remove duplicate scripts) ——　Tag: javascript

　　13. 配置实体标签(Configure ETags) ——　Tag: css

　　14. 使 AJAX 缓存(Make Ajax Cacheable) ——　Tag: content

　　15. 提前刷新输出缓冲(Flush the Buffer Early) ——　Tag: server

　　16. Use GET for AJAX Requests ——　Tag: server

　　17. Post-load Components ——　Tag: content

　　18. ……

 

 

 

 

　以 apache 为例，在 httpd.conf 中做以下修改即可：

1. 启用 expires_module 模块

LoadModule expires_module modules/mod_expires.so

2. 打开 Expires 控制

ExpiresActive On

3. 设置 Expires 规则

我的 Expire/Cache-Control 规则是：将 IMAGE 图片(JPEG, GIF, PNG)、CSS 文件、JS 文件缓存 14 天。因为这些文件基本上很少修改。我的配置语法如下：

ExpiresByType image/gif "access plus 14 days"
ExpiresByType image/jpeg "access plus 14 days"
ExpiresByType image/png "access plus 14 days"
ExpiresByType text/css "access plus 14 days"
ExpiresByType application/x-javascript "access plus 14 days"

4. 重启 Apache 服务器。

 

 

 

下面是英文原文

 

 

Best Practices for Speeding Up Your Web Site

Minimize HTTP Requests

tag: content

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

One way to reduce the number of components in the page is tosimplify the page's design. But is there a way to build pages withricher content while also achieving fast response times? Here are sometechniques for reducing the number of HTTP requests, while stillsupporting rich page designs.

Combined files are a way to reduce the number ofHTTP requests by combining all scripts into a single script, andsimilarly combining all CSS into a single stylesheet. Combining filesis more challenging when the scripts and stylesheets vary from page topage, but making this part of your release process improves responsetimes.

CSS Spritesare 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.

Image mapscombine multiple images into a single image. The overall size is aboutthe 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 asa navigation bar. Defining the coordinates of image maps can be tediousand error prone. Using image maps for navigation is not accessible too,so it's not recommended.

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

Reducing the number of HTTP requests in your page is the place tostart. This is the most important guideline for improving performancefor 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 betteruser experience.

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Use a Content Delivery Network

tag: server

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

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

Remember that 80-90% of the end-user response time is spentdownloading 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 yourapplication architecture, it's better to first disperse your staticcontent. This not only achieves a bigger reduction in response times,but it's easier thanks to content delivery networks.

A content delivery network (CDN) is a collection of web serversdistributed across multiple locations to deliver content moreefficiently to users. The server selected for delivering content to aspecific user is typically based on a measure of network proximity. Forexample, the server with the fewest network hops or the server with thequickest response time is chosen.

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 servicecan be prohibitive, but as your target audience grows larger andbecomes more global, a CDN is necessary to achieve fast response times.At Yahoo!, properties that moved static content off their applicationweb servers to a CDN improved end-user response times by 20% or more.Switching to a CDN is a relatively easy code change that willdramatically improve the speed of your web site.

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Add an Expires or a Cache-Control Header

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

Web page designs are getting richer and richer, which means morescripts, stylesheets, images, and Flash in the page. A first-timevisitor to your page may have to make several HTTP requests, but byusing the Expires header you make those components cacheable. Thisavoids unnecessary HTTP requests on subsequent page views. Expiresheaders are most often used with images, but they should be used on all components including scripts, stylesheets, and Flash components.

Browsers (and proxies) use a cache to reduce the number and size ofHTTP requests, making web pages load faster. A web server uses theExpires header in the HTTP response to tell the client how long acomponent can be cached. This is a far future Expires header, tellingthe browser that this response won't be stale until April 15, 2010.

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

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

      ExpiresDefault "access plus 10 years"

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

Using a far future Expires header affects page views only after auser has already visited your site. It has no effect on the number ofHTTP requests when a user visits your site for the first time and thebrowser's cache is empty. Therefore the impact of this performanceimprovement depends on how often users hit your pages with a primedcache. (A "primed cache" already contains all of the components in thepage.) We measured this at Yahoo!and found the number of page views with a primed cache is 75-85%. Byusing a far future Expires header, you increase the number ofcomponents that are cached by the browser and re-used on subsequentpage views without sending a single byte over the user's Internetconnection.

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Gzip Components

tag: server

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

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

      Accept-Encoding: gzip, deflate

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

      Content-Encoding: gzip

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.

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

There are known issues with browsers and proxies that may cause amismatch in what the browser expects and what it receives with regardto compressed content. Fortunately, these edge cases are dwindling asthe use of older browsers drops off. The Apache modules help out byadding appropriate Vary response headers automatically.

Servers choose what to gzip based on file type, but are typicallytoo limited in what they decide to compress. Most web sites gzip theirHTML documents. It's also worthwhile to gzip your scripts andstylesheets, but many web sites miss this opportunity. In fact, it'sworthwhile to compress any text response including XML and JSON. Imageand PDF files should not be gzipped because they are alreadycompressed. Trying to gzip them not only wastes CPU but can potentiallyincrease file sizes.

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

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Put Stylesheets at the Top

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.

Front-end engineers that care about performance want a page to loadprogressively; that is, we want the browser to display whatever contentit has as soon as possible. This is especially important for pages witha lot of content and for users on slower Internet connections. Theimportance of giving users visual feedback, such as progressindicators, has been well researched and documented.In our case the HTML page is the progress indicator! When the browserloads the page progressively the header, the navigation bar, the logoat the top, etc. all serve as visual feedback for the user who iswaiting for the page. This improves the overall user experience.

The problem with putting stylesheets near the bottom of the documentis that it prohibits progressive rendering in many browsers, includingInternet Explorer. These browsers block rendering to avoid having toredraw elements of the page if their styles change. The user is stuckviewing a blank white page.

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

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Put Scripts at the Bottom

tag: javascript

The problem caused by scripts is that they block parallel downloads. The HTTP/1.1 specificationsuggests that browsers download no more than two components in parallelper hostname. If you serve your images from multiple hostnames, you canget more than two downloads to occur in parallel. While a script isdownloading, however, the browser won't start any other downloads, evenon different hostnames.

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

An alternative suggestion that often comes up is to use deferred scripts. The DEFERattribute 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 DEFERattribute. In Internet Explorer, the script may be deferred, but not asmuch as desired. If a script can be deferred, it can also be moved tothe bottom of the page. That will make your web pages load faster.

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Avoid CSS Expressions

tag: css

CSS expressions are a powerful (and dangerous) way to set CSSproperties 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 expressionmethod is ignored by other browsers, so it is useful for settingproperties in Internet Explorer needed to create a consistentexperience across browsers.

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

One way to reduce the number of times your CSS expression isevaluated is to use one-time expressions, where the first time theexpression is evaluated it sets the style property to an explicitvalue, which replaces the CSS expression. If the style property must beset dynamically throughout the life of the page, using event handlersinstead of CSS expressions is an alternative approach. If you must useCSS expressions, remember that they may be evaluated thousands of timesand could affect the performance of your page.

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Make JavaScript and CSS External

tag: javascript, css

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

Using external files in the real world generally produces fasterpages because the JavaScript and CSS files are cached by the browser.JavaScript and CSS that are inlined in HTML documents get downloadedevery time the HTML document is requested. This reduces the number ofHTTP requests that are needed, but increases the size of the HTMLdocument. On the other hand, if the JavaScript and CSS are in externalfiles cached by the browser, the size of the HTML document is reducedwithout increasing the number of HTTP requests.

The key factor, then, is the frequency with which externalJavaScript and CSS components are cached relative to the number of HTMLdocuments requested. This factor, although difficult to quantify, canbe gauged using various metrics. If users on your site have multiplepage views per session and many of your pages re-use the same scriptsand stylesheets, there is a greater potential benefit from cachedexternal files.

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

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

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Reduce DNS Lookups

tag: content

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

DNS lookups are cached for better performance. This caching canoccur on a special caching server, maintained by the user's ISP orlocal area network, but there is also caching that occurs on theindividual user's computer. The DNS information remains in theoperating system's DNS cache (the "DNS Client service" on MicrosoftWindows). Most browsers have their own caches, separate from theoperating system's cache. As long as the browser keeps a DNS record inits own cache, it doesn't bother the operating system with a requestfor the record.

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.)

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

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

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Minify JavaScript and CSS

tag: javascript, css

Minification is the practice of removing unnecessary characters fromcode to reduce its size thereby improving load times. When code isminified all comments are removed, as well as unneeded white spacecharacters (space, newline, and tab). In the case of JavaScript, thisimproves response time performance because the size of the downloadedfile 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 tosource code. It's more complex than minification and thus more likelyto generate bugs as a result of the obfuscation step itself. In asurvey of ten top U.S. web sites, minification achieved a 21% sizereduction versus 25% for obfuscation. Although obfuscation has a highersize 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 scriptsand styles, minifying them will still reduce the size by 5% or more. Asthe use and size of JavaScript and CSS increases, so will the savingsgained by minifying your code.

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Avoid Redirects

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

The browser automatically takes the user to the URL specified in the Locationfield. 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 unlessadditional headers, such as Expires or Cache-Control,indicate it should be. The meta refresh tag and JavaScript are otherways 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 userexperience. Inserting a redirect between the user and the HTML documentdelays everything in the page since nothing in the page can be renderedand no components can start being downloaded until the HTML documenthas arrived.

One of the most wasteful redirects happens frequently and webdevelopers are generally not aware of it. It occurs when a trailingslash (/) is missing from a URL that should otherwise have one. Forexample, 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.

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

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Remove Duplicate Scripts

tag: javascript

It hurts performance to include the same JavaScript file twice inone page. This isn't as unusual as you might think. A review of the tentop 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 asingle web page: team size and number of scripts. When it does happen,duplicate scripts hurt performance by creating unnecessary HTTPrequests and wasted JavaScript execution.

Unnecessary HTTP requests happen in Internet Explorer, but not inFirefox. In Internet Explorer, if an external script is included twiceand is not cacheable, it generates two HTTP requests during pageloading. Even if the script is cacheable, extra HTTP requests occurwhen the user reloads the page.

In addition to generating wasteful HTTP requests, time is wastedevaluating the script multiple times. This redundant JavaScriptexecution happens in both Firefox and Internet Explorer, regardless ofwhether the script is cacheable.

One way to avoid accidentally including the same script twice is toimplement a script management module in your templating system. Thetypical way to include a script is to use the SCRIPT tag in your HTMLpage.

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

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 insertedmultiple times, this function could handle other issues with scripts,such as dependency checking and adding version numbers to scriptfilenames to support far future Expires headers.

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Configure ETags

tag: server

Entity tags (ETags) are a mechanism that web servers and browsersuse to determine whether the component in the browser's cache matchesthe one on the origin server. (An "entity" is another word a"component": images, scripts, stylesheets, etc.) ETags were added toprovide a mechanism for validating entities that is more flexible thanthe last-modified date. An ETag is a string that uniquely identifies aspecific version of a component. The only format constraints are thatthe string be quoted. The origin server specifies the component's ETagusing 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

Later, if the browser has to validate a component, it uses the If-None-Matchheader 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 forthis 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

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

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 multipleservers, and have the same file size, permissions, timestamp, etc., itsinode is different from one server to the next.

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 theexact same component won't match from one server to another. If theETags don't match, the user doesn't receive the small, fast 304response that ETags were designed for; instead, they'll get a normal200 response along with all the data for the component. If you hostyour web site on just one server, this isn't a problem. But if you havemultiple servers hosting your web site, and you're using Apache or IISwith the default ETag configuration, your users are getting slowerpages, your servers have a higher load, you're consuming greaterbandwidth, and proxies aren't caching your content efficiently. Even ifyour 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 modelthat ETags provide, it's better to just remove the ETag altogether. TheLast-Modified header validates based on the component'stimestamp. And removing the ETag reduces the size of the HTTP headersin 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

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Make Ajax Cacheable

tag: content

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

To improve performance, it's important to optimize these Ajaxresponses. The most important way to improve the performance of Ajax isto make the responses cacheable, as discussed in Add an Expires or a Cache-Control Header. Some of the other rules also apply to Ajax:

• Gzip Components
• Reduce DNS Lookups
• Minify JavaScript
• Avoid Redirects
• Configure ETags

Let's look at an example. A Web 2.0 email client might use Ajax todownload the user's address book for autocompletion. If the user hasn'tmodified her address book since the last time she used the email webapp, the previous address book response could be read from cache ifthat Ajax response was made cacheable with a future Expires orCache-Control header. The browser must be informed when to use apreviously cached address book response versus requesting a new one.This could be done by adding a timestamp to the address book Ajax URLindicating the last time the user modified her address book, forexample, &t=1190241612. If the address book hasn'tbeen modified since the last download, the timestamp will be the sameand the address book will be read from the browser's cache eliminatingan extra HTTP roundtrip. If the user has modified her address book, thetimestamp ensures the new URL doesn't match the cached response, andthe browser will request the updated address book entries.

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

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Flush the Buffer Early

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.

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Use GET for AJAX Requests

tag: server

The Yahoo! Mail team found that when using XMLHttpRequest,POST is implemented in the browsers as a two-step process: sending theheaders first, then sending data. So it's best to use GET, which onlytakes one TCP packet to send (unless you have a lot of cookies). Themaximum URL length in IE is 2K, so if you send more than 2K data youmight 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 opposedto sending data to be stored server-side.

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Post-load Components

tag: content

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

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

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 webdevelopment best practices. In this case, the idea of progressiveenhancement tells us that JavaScript, when supported, can improve theuser experience but you have to make sure the page works even withoutJavaScript. So after you've made sure the page works fine, you canenhance it with some post-loaded scripts that give you more bells andwhistles such as drag and drop and animations.

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Preload Components

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.

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.
• Anticipatedpreload - preload in advance before launching a redesign. It oftenhappens after a redesign that you hear: "The new site is cool, but it'sslower than before". Part of the problem could be that the users werevisiting your old site with a full cache, but the new one is always anempty cache experience. You can mitigate this side effect by preloadingsome components before you even launched the redesign. Your old sitecan use the time the browser is idle and request images and scriptsthat will be used by the new site

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Reduce the Number of DOM Elements

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).

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Split Components Across Domains

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

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

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Minimize the Number of iframes

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
• Security sandbox
• Download scripts in parallel

<iframe> cons:

• Costly even if blank
• Blocks page onload
• Non-semantic

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No 404s

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.

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Reduce Cookie Size

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

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Use Cookie-free Domains for Components

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-freedomain is that some proxies might refuse to cache the components thatare requested with cookies. On a related note, if you wonder if youshould use example.org or www.example.org for your home page, considerthe cookie impact. Omitting www leaves you no choice but to writecookies to *.example.org, so for performance reasons it's best to use the www subdomain and write the cookies to that subdomain.

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Minimize DOM Access

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.

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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 onehandler for each button. Events bubble up so you'll be able to catchthe event and figure out which button it originated from.

You also don't need to wait for the onload event in order to startdoing something with the DOM tree. Often all you need is the elementyou want to access to be available in the tree. You don't have to waitfor 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.

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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.

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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.

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Optimize Images

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 operationssuch as rotation and can also be used to optimize and remove commentsand other useless information (such as EXIF information) from yourimages.
  jpegtran -copy none -optimize -perfect src.jpg dest.jpg

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Optimize CSS Sprites

tag: images

• Arranging the images in the sprite horizontally as opposed to vertically usually results in a smaller file size.
• Combining similar colors in a sprite helps you keep the color count low, ideally under 256 colors so to fit in a PNG8.
• "Be mobile-friendly" and don't leave big gaps between the images in a sprite. This doesn't affect the file size as much but requires less memory for the user agent to decompress the image into a pixel map. 100x100 image is 10 thousand pixels, where 1000x1000 is 1 million pixels

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Don't Scale Images in HTML

tag: images

Don't use a bigger image than you need just because you can set the width and height in HTML. If you need
<img width="100" height="100" src="mycat.jpg" alt="My Cat" />
then your image (mycat.jpg) should be 100x100px rather than a scaled down 500x500px image.

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Make favicon.ico Small and Cacheable

tag: images

The favicon.ico is an image that stays in the root of your server. It's a necessary evil because even if you don't care about it the browser will still request it, so it's better not to respond with a 404 Not Found. Also since it's on the same server, cookies are sent every time it's requested. This image also interferes with the download sequence, for example in IE when you request extra components in the onload, the favicon will be downloaded before these extra components.

So to mitigate the drawbacks of having a favicon.ico make sure:

• It's small, preferably under 1K.
• Set Expires header with what you feel comfortable (since you cannot rename it if you decide to change it). You can probably safely set the Expires header a few months in the future. You can check the last modified date of your current favicon.ico to make an informed decision.

Imagemagick can help you create small favicons

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Keep Components under 25K

tag: mobile

This restriction is related to the fact that iPhone won't cache components bigger than 25K. Note that this is the uncompressed size. This is where minification is important because gzip alone may not be sufficient.

For more information check "Performance Research, Part 5: iPhone Cacheability - Making it Stick" by Wayne Shea and Tenni Theurer.

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Pack Components into a Multipart Document

tag: mobile

Packing components into a multipart document is like an email with attachments, it helps you fetch several components with one HTTP request (remember: HTTP requests are expensive). When you use this technique, first check if the user agent supports it (iPhone does not).