The Discard Late Strategy

Web applications typically update data from servers (eg. whether, time, streaming media, dollar rates, thread posts). Developers have to overcome the issue of using obsolete data due to a late response of a server. For example, consider a website that shows an streaming media using several Ajax requests. If server responses of Ajax requests are retrieved in wrong order, the streaming media cannot be observed correctly. A solution could be to wait for all frames, however, this solution could delay to much the streaming media. A better solution is to discard the late frames (a.k.a frame skip) to show the streaming media. The DiscardLate strategy discards the late server responses. As an example, we present below a streaming media about a fight (the streaming media is finite by simplicity). This example shows the effects that reproduce in a streaming media when frames are retrieved in a random order.

When the “activate order” button is clicked, the streaming media is shown in the correct flow. When the “deactivate order” button is clicked, the streaming media is shown a random flow. NOTE: Please first watch the application without clicking the “activate order” button to appreciate clearly the aforementioned issue.

The implementation follows: Toggle between the code and example

The Discard Late strategy is implemented as a piece of advice, which is a JavaScript function, that is executed every time a server response from getstreamingvideo.com is received:

var discardLate = function(jp ,env) {
  var request = jp.target ;
  if (this.idServerRespExpected <= request.idServerResp) {
    jp.proceed();
    this.idServerRespExpected = request.idServerResp + 1;
  }
};
 
WeCa.deployStrategy (discardLate, function(jp) {
  var request = jp.target ;
  return request.url == ”http://getstreamingvideo.com” ;
});

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