Comet Server Push Over Web
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Comet allows web servers to push data to browsers in real-time without requiring the browser to send a request first. It uses techniques like long polling, iframe streaming, and server-sent events to mimic bidirectional communication over HTTP. While challenging to implement across browsers, Comet enables real-time web applications by reducing the need for frequent polling from the browser. Future standards like WebSocket aim to make real-time web functionality easier to build with more consistent browser support.
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Comet Server Push Over Web
- 1. Comet: Server-Push in Web @morgancheng March 2nd, 2011
- 3. Pull with HTTP GET / HTTP/1.1 Host: www.yahoo.com Connection: keep-alive Request HTTP/1.1 200 OK Content-Type: text/html;charset=utf-8 Transfer-Encoding: chunked <!DOCTYPE html> <html> … Response
- 4. What If Web Server Want To Push?
- 5. Comet
- 6. Actually, it is named after…
- 7. Who is using Comet?
- 11. First Browser War VS
- 12. NetScape: multipart/x-mixed-replace HTTP/1.1 200 OK Content-Type: multipart/x-mixed-replace;boundary=ABCXYZ --ABCXYZ Content-Type:application/octet-stream Hello --ABCXYZ Content-Type:application/octet-stream World --ABCXYZ—
- 13. Fallout Firefox inherits multipart/x-mixed-replace Safari & Chrome supports this type as well, but not in XMLHttpRequest
- 14. Today Poll Flash, Java & SilverLight Long Poll HTTP Streaming
- 15. Poll setInterval( function() { send_request_to_server_for_update(); }, 1000)
- 16. How Poll Works Server Any Update? NO Any Update? Yes. Here you are Browser
- 17. Poll: Pros and Cons Simple Not Real Time Too Many Requests
- 18. What if server holds HTTP response if no update available?
- 19. Long Poll function long_poll () { send_request_to_server(function(data) { process(data); long_poll(); }); } long_poll();
- 20. How Long Poll Works Server Here you are Give me update when you have it Give me update when you have it Browser
- 21. Long Poll: Pros and Cons More Efficient Than Poll Easy to Implement Cross-Browser Still Too Many Requests Not so real time
- 22. Can we reuse single HTTP channel for multiple pushing?
- 23. Server Streaming Server Here you are Here you are, again Give me update when you have it Browser
- 24. Time to get your hands dirty
- 25. Ideally, XHR streaming it is
- 26. Create XHR function createXHR() { return window.XMLHttpRequest? new XMLHttpRequest() : new ActiveXObject(‘Microsoft.XMLHTTP’); } IE failed us again in XHR Streaming
- 27. Create XHR function createXHR() { return new XMLHttpRequest() ; }
- 28. xhr.readyState 0 open() has not been called yet send() has not been called yet 2 send() has been called, headers and status are available 3 Downloading, responseText holds the partial data 4 Finished with all operations
- 29. readystatechange event xhr = createXHR(); xhr.onreadystatechange = function() { if ((xhr.readyState >= 3) && (xhr.status === 200)) { // xhr.responseText is all pushed data } }; xhr.open(‘GET’, streamUrl, true); xhr.send();
- 30. Need a format protocol of xhr.responseText
- 31. Opera trigger readyState ===3 only once!
- 32. But, it is called “readystatechange” event, right?
- 33. Polling xhr.responseText for Opera Browser
- 34. xhr.responseText is empty till readyState === 4
- 35. iframe Streaming
- 36. Tales of Two frames The Parent Window Server Pushed Stream The Hidden iframe Invoke JavaScript Functions
- 37. Progressive Rendering Response … 1K padding … <script type=“text/javascript”> parent.push(‘msg1’); </script> <script type=“text/javascript”> parent.push(‘msg2’); </script> …
- 38. Never Ending Loading Throbber
- 39. Trick for IE vartransDoc = new window.ActiveXObject('htmlfile'); transDoc.open(); transDoc.write('<html></html>'); transDoc.close(); transDoc.parentWindow.push = push_callback; variframeDiv = transDoc.createElement('div'); transDoc.body.appendChild(this.iframeDiv); iframeDiv.innerHTML = '<iframesrc="' + url_to_stream + '"></iframe>';
- 40. Can we do iframe streaming for other browser?
- 41. Hack There is one way for Firefox 2.x When one message is pushed, add one empty iframe to body and then remove it Not work for Firefox 3.x
- 43. Complete HTTP Streaming Solution XHR Streaming for all non-IE browsers polling xhr.responseText for Opera iFraming Streaming for IE with “htmlfile” hack YUI Gallery Comet Stream http://yuilibrary.com/gallery/show/comet-stream
- 44. HTTP Streaming: Pros and Cons Real Time Fewer HTTP Request Complex HTTP Proxy could bust it
- 45. How to Cross Domain?
- 46. How to Identify Duplicate Pushed Message?
- 47. Future SPDY EventSource WebSocket
- 48. SPDY Replacement of HTTP Only for Chrome now Server can push data
- 49. EventSource Server push message HTML5 feature Only Opera support it for the time being Broken in Opera 10.51
- 50. WebSocket Dual Way Cross-Domain HTML5 feature
- 51. Security Hole!!!
- 52. Firefox 4 and Opera 11 disable WebSocket by default http://www.adambarth.com/experimental/websocket.pdf
- 53. Server Side
- 56. Don’t Use Thread-Based Server
- 57. Do Use Event-Based Server
- 58. Bayeux Protocol
- 59. Ask Yourself Several Questions Before Proceed With Comet
- 60. Do You Really Need Comet?
- 61. Which Browsers Do You Want Support?
- 62. How to define real-time?
- 63. How About Your Server Capability?
- 64. Choose Comet Solution for Your App
- 65. Resources Comet Daily http://cometdaily.com Socket.IO http://socket.io YUI Gallery Comet Stream http://yuilibrary.com/gallery/show/comet-stream
- 66. ThanksQ & A
Editor's Notes
- HTTP 1.1 RFC2616 in 1999 is still widely used.