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High Performance Ajax Applications


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Day 6 of 7-days "JavaScript and Rich User Interfaces" training for my colleagues. It covers ways how to speed up your application.

Day 6 of 7-days "JavaScript and Rich User Interfaces" training for my colleagues. It covers ways how to speed up your application.

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  • 1. High Performance Ajax Applications Siarhei Barysiuk
  • 2. Our roadmap
  • 3. Introduction: What will we cover today? • How to speed up site loading time? • How to speed up JavaScript performance? • Which tools use to do it?
  • 4. Best Practices from Yahoo! 34 best practices divided into 7 categories. • Content • Server • Cookie • CSS • JavaScript • Images • Mobile
  • 5. Server
  • 6. Server 1. Use a Content Delivery Network 2. Add an Expires or a Cache-Control Header 3. Gzip Components 4. Configure ETags 5. Flush the Buffer Early 6. Use GET for AJAX Requests
  • 7. Server: Use a CDN Performance Golden Rule. “80-90% of the end-user response time is spent downloading all the components in the page: images, stylesheets, scripts, Flash, etc.” 20% end-user response time improvement at Yahoo!
  • 8. Server: Add an Expires or a Cache-Control Header Static content: Implement quot;Never expirequot; policy by setting far future Expires header. For dynamic components: Use an appropriate Cache-Control header to help the browser with conditional requests.
  • 9. Server: Add an Expires or a Cache-Control Header A web server uses the Expires header in the HTTP response to tell the client how long a component can be cached. Expires: Thu, 15 Apr 2010 20:00:00 GMT If you use a far future Expires header you have to change the component's filename whenever the component changes. yahoo_2.0.6.js yahoo.js?ver=2.0.6
  • 10. Server: Add an Expires or a Cache-Control Header Loading with an empty cache a full cache
  • 11. Server: Gzip Components Accept-Encoding: gzip, deflate Content-Encoding: gzip
  • 12. Server: Gzip Components Gzipping generally reduces the response size by about 70%. Apache 1.3 mod-gzip Apache 2.0 mod-deflate IIS ...
  • 13. Server: Configure ETags GET /i/yahoo.gif HTTP/1.1 HTTP/1.1 200 OK Last-Modified: Tue, 12 Dec 2006 03:03:59 GMT ETag: quot;10c24bc-4ab-457e1c1fquot; Content-Length: 12195 GET /i/yahoo.gif HTTP/1.1 Host: If-Modified-Since: Tue, 12 Dec 2006 03:03:59 GMT If-None-Match: quot;10c24bc-4ab-457e1c1fquot; HTTP/1.1 304 Not Modified
  • 14. Server: Configure ETags Not very good for cluster environment. The end result is ETags generated by Apache and IIS for the exact same component won't match from one server to another.
  • 15. Server: Use GET for AJAX Requests POST through XMLHttpRequest is a two-step process: • Sending headers • 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.
  • 16. Questions?
  • 17. Content
  • 18. Content 1. Make Fewer HTTP Requests 2. Reduce DNS Lookups 3. Avoid Redirects 4. Make Ajax Cacheable 5. Post-load Components 6. Preload Components 7. Reduce the Number of DOM Elements 8. Split Components Across Domains 9. Minimize the Number of iframes 10. No 404s
  • 19. Content: Make fewer HTTP Requests 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. One way to reduce the number of components in the page is to simplify the page's design. Other techniques: • Combined files • CSS Sprites • Image maps • Inline Images
  • 20. Content: Make fewer HTTP Requests Very interesting statistics: 40-60% of Yahoo!’s users have an empty cache experience and ~20% of all page views are done with an empty cache.
  • 21. Content: Make Ajax Cacheable “asynchronous” != “instantaneous” Do not need to request some parts each time, for example region-city list. Use Expires and Cache-Control headers Use additional parameter e.g. timestamp.
  • 22. Content: Post-load Components You can take a closer look at your page and ask yourself: quot;What's absolutely required in order to render the page initially?quot; The rest of the content and components can wait. JavaScript: • Separate out a core • Add other components as extensions • Load them on demand Good for loading hidden content and images.
  • 23. Content: Preload 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. Several types of preloading: • Unconditional ( and sprites) • Conditional • Anticipated (redesign)
  • 24. Content: Reduce the Number of DOM Elements complex page = more bytes for download = slower DOM access 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.
  • 25. Content: Reduce the Number of DOM Elements
  • 26. Content: Split Components Across Domains 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.
  • 27. Content: Split Components Across Domains HTTP/1.1 spec suggests that browsers download two components in parallel per hostname. Downloading 2 Downloading 4 components in components in parallel parallel
  • 28. Content: Split Components Across Domains What if we use additional aliases to increase parallel downloads in our pages? Loading an Empty HTML Document with 20 images using Various Number of Aliases
  • 29. Content: Minimize the Number of iframes It's important to understand how iframes work so they can be used effectively. Pros: • Helps with slow third-party content like badges and ads • Security sandbox • Download scripts in parallel Cons: • Costly even if blank • Blocks page onload • Non-semantic
  • 30. Content: Minimize the Number of iframes Demo
  • 31. Questions?
  • 32. Cookie
  • 33. Cookie 1. Reduce Cookie Size 2. Use Cookie-free Domains for Components
  • 34. Cookie: Reduce Cookie Size HTTP cookies are used for a variety of reasons such as authentication and personalization. GET HTTP/1.1 HTTP/1.1 200 OK Content-Type: text/html; charset=utf-8 Set-Cookie: C=abcde; path=/; GET / HTTP/1.1 Host: User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; ... Cookie: C=abcde;
  • 35. Cookie: Reduce Cookie Size 2x Response times for various cookie sizes
  • 36. Cookie: Reduce Cookie Size Analysis of Cookie Sizes across the Web Total Cookie Sizes *(for home page)
  • 37. Cookie: Reduce Cookie Size Conclusion: • 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.
  • 38. Cookie: Use Cookie-free Domains for Components 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. Create a subdomain and host all your static components there.
  • 39. Questions?
  • 40. CSS
  • 41. CSS 1. Put Stylesheets at the Top 2. Avoid CSS Expressions 3. Choose <link> over @import 4. Avoid Filters
  • 42. CSS: Put Stylesheets at the Top 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. 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.
  • 43. CSS: Avoid CSS Expressions CSS expressions are a powerful (and dangerous) way to set CSS properties dynamically. background-color: expression( (new Date()).getHours()%2 ? quot;#B8D4FFquot; : quot;#F08A00quot; ); The problem with expressions is that they are evaluated more frequently than most people expect. Moving the mouse around the page can easily generate more than 10,000 evaluations.
  • 44. CSS: Choose <link> over @import 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.
  • 45. CSS: Avoid Filters The IE-proprietary AlphaImageLoader filter aims to fix a problem with semi- transparent true color PNGs in IE versions < 7. The problems: • blocks rendering • freezes the browser while the image is being downloaded • increases memory consumption and is applied per element
  • 46. Questions?
  • 47. JavaScript
  • 48. JavaScript 1. Put Scripts at the Bottom 2. Make JavaScript and CSS External 3. Minify JavaScript and CSS 4. Remove Duplicate Scripts 5. Minimize DOM Access 6. Develop Smart Event Handlers
  • 49. JavaScript: Put Scripts at the Bottom 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. DEFER
  • 50. JavaScript: Make JavaScript and CSS External Should JavaScript and CSS be contained in external files, or inlined in the page itself? Pages which reuse External: the same scrips and styles • Cached by the browser • Parallel download Inline: Yahoo!'s front page • Less HTTP responses
  • 51. JavaScript: Minify JavaScript and CSS Minification is the practice of removing unnecessary characters from code to reduce its size thereby improving load times. // is.js // (c) 2001 Douglas Crockford // 2001 June 3 // is // The -is- object is used to identify the browser. Every browser edition // identifies itself, but there is no standard way of doing it, and some of // the identification is deceptive. This is because the authors of web // browsers are liars. For example, Microsoft's IE browsers claim to be var is={ie:navigator.appName=='Microsoft Internet // Mozilla 4. Netscape 6 claims to be version 5. Explorer',java:navigator.javaEnabled(),ns:navigator.appName=='Net scape',ua:navigator.userAgent.toLowerCase(),version:parseFloat(na var is = { vigator.appVersion.substr(21))|| ie: navigator.appName == 'Microsoft Internet Explorer', parseFloat(navigator.appVersion),win:navigator.platform=='Win32'} java: navigator.javaEnabled(),'mac')>=0;if('opera')>=0) ns: navigator.appName == 'Netscape', {;is.opera=true;} ua: navigator.userAgent.toLowerCase(), if('gecko')>=0){;is.gecko=true;} version: parseFloat(navigator.appVersion.substr(21)) || parseFloat(navigator.appVersion), win: navigator.platform == 'Win32' } is.mac ='mac') >= 0; if ('opera') >= 0) { = is.ns = false; is.opera = true; } if ('gecko') >= 0) { = is.ns = false; is.gecko = true; }
  • 52. JavaScript: Minify JavaScript and CSS Tools: Client Remove Remove white Rename local safe performance comments spaces variables impact Dan Edward’s negative packer (uses eval) YUI Compressor neutral JSMin neutral qooxdoo positive (string optimizer)
  • 53. JavaScript: Remove Duplicate Scripts It hurts performance to include the same JavaScript file twice in one page. • Extra HTTP request in IE • Time to process duplicated scripts Sounds stupid? A review of the ten top U.S. web sites shows that two of them contain a duplicated script.
  • 54. JavaScript: Minimize DOM Access Accessing DOM elements with JavaScript is slow so in order to have a more responsive page: • Cache references to accessed elements • Update nodes quot;offlinequot; and then add them to the tree • Avoid fixing layout with JavaScript
  • 55. JavaScript: Develop Smart Event Handlers 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. <div class=quot;containerquot; id=quot;containerquot;> <div class=quot;colorquot; style=quot;background-color:#000033quot;></div> <div class=quot;colorquot; style=quot;background-color:#000066quot;></div> <div class=quot;colorquot; style=quot;background-color:#000099quot;></div> <div class=quot;colorquot; style=quot;background-color:#0000FFquot;></div> var c = document.getElementById(quot;containerquot;) <div class=quot;colorquot; style=quot;background-color:#003300quot;></div> <div class=quot;colorquot; style=quot;background-color:#006600quot;></div> c.addEventListener(quot;clickquot;, function(e){ <div class=quot;colorquot; style=quot;background-color:#009900quot;></div> <div class=quot;colorquot; style=quot;background-color:#00FF00quot;></div> alert( },false); <div class=quot;colorquot; style=quot;background-color:#330000quot;></div> <div class=quot;colorquot; style=quot;background-color:#660000quot;></div> <div class=quot;colorquot; style=quot;background-color:#990000quot;></div> <div class=quot;colorquot; style=quot;background-color:#FF0000quot;></div> <div class=quot;colorquot; style=quot;background-color:#333333quot;></div> <div class=quot;colorquot; style=quot;background-color:#666666quot;></div> <div class=quot;colorquot; style=quot;background-color:#999999quot;></div> <div class=quot;colorquot; style=quot;background-color:#FFFFFFquot;></div> </div>
  • 56. Questions?
  • 57. ... more JavaScript performance
  • 58. JavaScript: Reduce the amount of symbolic lookups var globalVar = 0; now = new Date(); (function(){ var globalVar = 0; var i,l,localVar; (function(){ l = arr.length; var i=0; for(i-0;i<arr.length;i++) { localVar = globalVar; globalVar++; for(i=0;i<l;i++){ } localVar++; })(); } globalVar = localVar; })(); 100 000 iterations
  • 59. JavaScript: Reduce the amount of symbolic lookups var Foo3 = function() {}; = function() {return false}; var Foo = function() {}; Foo.prototype = new Foo3(); var Foo2 = { bar: function() { return false; } }
  • 60. JavaScript: Reduce the amount of symbolic lookups var foo = new Foo(); for(var j=0;j<iterations;j++) { for(var i=0;i<iterations;i++) {;; } } 10 000 000 iterations
  • 61. JavaScript: String concatenation var i,s=[]; var i,s=quot;quot;; for(i=0;i<10000;i++){ for(i=0;i<10000;i++){ s[i] = quot;xquot;; s+=quot;xquot;; } } s=s.join(quot;quot;); 100 000 iterations 3438 ms 157 ms
  • 62. Questions?
  • 63. Images
  • 64. Images 1. Optimize Images 2. Optimize CSS Sprites 3. Don't Scale Images in HTML 4. Make favicon.ico Small and Cacheable
  • 65. Images: Optimize Images Tools: imagemagick pngcrush jpegtran
  • 66. Images: Optimize CSS Sprites What is CSS Sprite?
  • 67. Images: Optimize CSS Sprites Demo
  • 68. Images: Don't Scale Images in HTML 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=quot;100quot; height=quot;100quot; src=quot;mycat.jpgquot; alt=quot;My Catquot; /> then your image (mycat.jpg) should be 100x100px rather than a scaled down 500x500px image.
  • 69. Images: Make favicon.ico Small and Cacheable The favicon.ico is an image that stays in the root of your server. 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. IE: When you request extra components in the onload, the favicon will be downloaded before these extra components. How to improve: • Make it small (under 1K) • Set appropriate Expires header
  • 70. Questions?
  • 71. Tools
  • 72. Tools: Firebug
  • 73. Tools:YSlow?
  • 74. Tools: Drosera (WebKit)
  • 75. Questions?