XMLHttpRequest Object: Asynchronous vs Synchronous Mode

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  • HugeBob
    #1

    XMLHttpRequest Object: Asynchronous vs Synchronous Mode

    Hi All,

    I've got a question about Asynchronous vs Synchronous mode with the
    XMLHttpRequest object. What are the ramifications of using one mode
    vs the other? If the script uses Asynchronous mode, it sounds as if a
    thread retrieves the data from the supplied URL and the JS function
    that called the open() and send() methods continues on. Where as
    using Synchronous mode the method that called open() and send() waits
    until the data from the URL has been retrieved. Am I correct in my
    thinking?

    Tags: synch xmlhttprequest, xmlhttp asynchronous, xmlhttp synchronous
    • -Lost
      #2
      Re: XMLHttpRequest Object: Asynchronous vs Synchronous Mode

      "HugeBob" <rnurse@gmail.c omwrote in message
      news:1175177691 .794121.187500@ l77g2000hsb.goo glegroups.com.. .
      Hi All,
      >
      I've got a question about Asynchronous vs Synchronous mode with the
      XMLHttpRequest object. What are the ramifications of using one mode
      vs the other? If the script uses Asynchronous mode, it sounds as if a
      thread retrieves the data from the supplied URL and the JS function
      that called the open() and send() methods continues on. Where as
      using Synchronous mode the method that called open() and send() waits
      until the data from the URL has been retrieved. Am I correct in my
      thinking?
      XMLHttpRequest Level 1
      http://www.w3.org/TR/XMLHttpRequest/#dfn-send


      The ramifications of not using the asynchronous aspect of AJAX is having the UA or browser
      block activity until the request returns, finishes, loads, whatever.

      I would not say that the open() and send() methods continue. The onreadystatecha nge
      property still receives a dispatch however (even when blocking). So instead of being
      notified of what open() and send() are doing you will receive an onload event and the
      readyState of 4.

      I would say you are incorrect in thinking that it cannot update its interface during a
      synchronous call (assuming you really meant onreadystatecha nge instead of open() and
      send()).

      I hope I worded all that correctly.

      -Lost


        Comment

        • HugeBob
          #3
          Re: XMLHttpRequest Object: Asynchronous vs Synchronous Mode

          On Mar 29, 10:58 am, "-Lost" <missed-s...@comcast.ne twrote:
          "HugeBob" <rnu...@gmail.c omwrote in message
          >
          news:1175177691 .794121.187500@ l77g2000hsb.goo glegroups.com.. .
          >
          Hi All,
          >
          I've got a question about Asynchronous vs Synchronous mode with the
          XMLHttpRequest object. What are the ramifications of using one mode
          vs the other? If the script uses Asynchronous mode, it sounds as if a
          thread retrieves the data from the supplied URL and the JS function
          that called the open() and send() methods continues on. Where as
          using Synchronous mode the method that called open() and send() waits
          until the data from the URL has been retrieved. Am I correct in my
          thinking?
          >
          XMLHttpRequest Level 1
          http://www.w3.org/TR/XMLHttpRequest/#dfn-send

          >
          The ramifications of not using the asynchronous aspect of AJAX is having the UA or browser
          block activity until the request returns, finishes, loads, whatever.
          >
          I would not say that the open() and send() methods continue. The onreadystatecha nge
          property still receives a dispatch however (even when blocking). So instead of being
          notified of what open() and send() are doing you will receive an onload event and the
          readyState of 4.
          >
          I would say you are incorrect in thinking that it cannot update its interface during a
          synchronous call (assuming you really meant onreadystatecha nge instead of open() and
          send()).
          >
          I hope I worded all that correctly.
          >
          -Lost
          I find myself in the same state as you signature, LOL. Well, sort
          of. I've got an app that uses AJAX. The onreadystatecha nge method
          sets a global variable with XML it retrieves from the server when
          readyState is 4. The global variable is later searched via JS. The
          application works in IE when synchronous mode is used and not at all
          in FireFox 1.5 (the onreadystatecha nge method never seems to run).
          So, take the following function:

          globalVariable = null;
          function ajaxFunction( )
          {
          var xmlHttp;

          xmlHttp = getXMLHTTPReque stObject( );
          alert(xmlHttp);
          xmlHttp.onready statechange=fun ction()
          {
          if(xmlHttp.read yState==4)
          {
          globalVariable = xmlHttp.respons eText;
          }
          }

          xmlHttp.open("G ET","time.asp", true);
          xmlHttp.send(nu ll);
          }

          So, are you saying that even when this function completes,
          globalVariable may not have anything in it when it's accessed later
          regardless of whether I use asynch or synch mode?

            Comment

            • -Lost
              #4
              Re: XMLHttpRequest Object: Asynchronous vs Synchronous Mode

              "HugeBob" <rnurse@gmail.c omwrote in message
              news:1175183260 .060842.183960@ d57g2000hsg.goo glegroups.com.. .
              On Mar 29, 10:58 am, "-Lost" <missed-s...@comcast.ne twrote:
              >"HugeBob" <rnu...@gmail.c omwrote in message
              >>
              >news:117517769 1.794121.187500 @l77g2000hsb.go oglegroups.com. ..
              >>
              Hi All,
              >>
              I've got a question about Asynchronous vs Synchronous mode with the
              XMLHttpRequest object. What are the ramifications of using one mode
              vs the other? If the script uses Asynchronous mode, it sounds as if a
              thread retrieves the data from the supplied URL and the JS function
              that called the open() and send() methods continues on. Where as
              using Synchronous mode the method that called open() and send() waits
              until the data from the URL has been retrieved. Am I correct in my
              thinking?
              >>
              >http://www.w3.org/TR/XMLHttpRequest/#dfn-send
              >>
              >The ramifications of not using the asynchronous aspect of AJAX is having the UA or
              >browser
              >block activity until the request returns, finishes, loads, whatever.
              >>
              >I would not say that the open() and send() methods continue. The onreadystatecha nge
              >property still receives a dispatch however (even when blocking). So instead of being
              >notified of what open() and send() are doing you will receive an onload event and the
              >readyState of 4.
              >>
              >I would say you are incorrect in thinking that it cannot update its interface during a
              >synchronous call (assuming you really meant onreadystatecha nge instead of open() and
              >send()).
              >>
              >I hope I worded all that correctly.
              >>
              >-Lost
              >
              I find myself in the same state as you signature, LOL. Well, sort
              of. I've got an app that uses AJAX. The onreadystatecha nge method
              sets a global variable with XML it retrieves from the server when
              readyState is 4. The global variable is later searched via JS. The
              application works in IE when synchronous mode is used and not at all
              in FireFox 1.5 (the onreadystatecha nge method never seems to run).
              So, take the following function:
              >
              globalVariable = null;
              function ajaxFunction( )
              {
              var xmlHttp;
              >
              xmlHttp = getXMLHTTPReque stObject( );
              alert(xmlHttp);
              xmlHttp.onready statechange=fun ction()
              {
              if(xmlHttp.read yState==4)
              {
              globalVariable = xmlHttp.respons eText;
              }
              }
              >
              xmlHttp.open("G ET","time.asp", true);
              xmlHttp.send(nu ll);
              }
              >
              So, are you saying that even when this function completes,
              globalVariable may not have anything in it when it's accessed later
              regardless of whether I use asynch or synch mode?
              No, globalVariable will have the responseText specified by the XMLHttpRequest. However,
              if asynchronous is set to false, depending on the size of the server's output (e.g. how
              long it takes to load) globalVariable will still have the responseText, it may not be
              available as quickly.

              Assuming of course that getXMLHTTPReque stObject() returns the proper XMLHttpRequest.

              To support the greatest number of possible browsers first have a look at:

              http://jibbering.com/2002/4/httprequest.html


              ....the implementation there is fairly solid.

              -Lost


                Comment

                • HugeBob
                  #5
                  Re: XMLHttpRequest Object: Asynchronous vs Synchronous Mode

                  On Mar 29, 12:21 pm, "-Lost" <missed-s...@comcast.ne twrote:
                  "HugeBob" <rnu...@gmail.c omwrote in message
                  >
                  news:1175183260 .060842.183960@ d57g2000hsg.goo glegroups.com.. .
                  >
                  >
                  >
                  On Mar 29, 10:58 am, "-Lost" <missed-s...@comcast.ne twrote:
                  "HugeBob" <rnu...@gmail.c omwrote in message
                  >
                  >news:117517769 1.794121.187500 @l77g2000hsb.go oglegroups.com. ..
                  >
                  Hi All,
                  >
                  I've got a question about Asynchronous vs Synchronous mode with the
                  XMLHttpRequest object. What are the ramifications of using one mode
                  vs the other? If the script uses Asynchronous mode, it sounds as if a
                  thread retrieves the data from the supplied URL and the JS function
                  that called the open() and send() methods continues on. Where as
                  using Synchronous mode the method that called open() and send() waits
                  until the data from the URL has been retrieved. Am I correct in my
                  thinking?
                  >
                  >http://www.w3.org/TR/XMLHttpRequest/#dfn-send
                  >
                  The ramifications of not using the asynchronous aspect of AJAX is having the UA or
                  browser
                  block activity until the request returns, finishes, loads, whatever.
                  >
                  I would not say that the open() and send() methods continue. The onreadystatecha nge
                  property still receives a dispatch however (even when blocking). So instead of being
                  notified of what open() and send() are doing you will receive an onload event and the
                  readyState of 4.
                  >
                  I would say you are incorrect in thinking that it cannot update its interface during a
                  synchronous call (assuming you really meant onreadystatecha nge instead of open() and
                  send()).
                  >
                  I hope I worded all that correctly.
                  >
                  -Lost
                  >
                  I find myself in the same state as you signature, LOL. Well, sort
                  of. I've got an app that uses AJAX. The onreadystatecha nge method
                  sets a global variable with XML it retrieves from the server when
                  readyState is 4. The global variable is later searched via JS. The
                  application works in IE when synchronous mode is used and not at all
                  in FireFox 1.5 (the onreadystatecha nge method never seems to run).
                  So, take the following function:
                  >
                  globalVariable = null;
                  function ajaxFunction( )
                  {
                  var xmlHttp;
                  >
                  xmlHttp = getXMLHTTPReque stObject( );
                  alert(xmlHttp);
                  xmlHttp.onready statechange=fun ction()
                  {
                  if(xmlHttp.read yState==4)
                  {
                  globalVariable = xmlHttp.respons eText;
                  }
                  }
                  >
                  xmlHttp.open("G ET","time.asp", true);
                  xmlHttp.send(nu ll);
                  }
                  >
                  So, are you saying that even when this function completes,
                  globalVariable may not have anything in it when it's accessed later
                  regardless of whether I use asynch or synch mode?
                  >
                  No, globalVariable will have the responseText specified by the XMLHttpRequest. However,
                  if asynchronous is set to false, depending on the size of the server's output (e.g. how
                  long it takes to load) globalVariable will still have the responseText, it may not be
                  available as quickly.
                  >
                  Assuming of course that getXMLHTTPReque stObject() returns the proper XMLHttpRequest.
                  >
                  To support the greatest number of possible browsers first have a look at:
                  >
                  http://jibbering.com/2002/4/httprequest.html

                  >
                  ...the implementation there is fairly solid.
                  >
                  -Lost
                  Oh, my getXMLHTTPReque stObject() method handles that. I borrowed it
                  from W3Schools (http://www.w3schools.com/ajax/ajax_server.asp):

                  function getXMLHTTPReque stObject( )
                  {
                  var xmlHttp = null;
                  try
                  {
                  // Firefox, Opera 8.0+, Safari
                  xmlHttp=new XMLHttpRequest( );
                  }
                  catch (e)
                  {
                  // Internet Explorer
                  try
                  {
                  xmlHttp=new ActiveXObject(" Msxml2.XMLHTTP" );
                  }
                  catch (e)
                  {
                  try
                  {
                  xmlHttp=new ActiveXObject(" Microsoft.XMLHT TP");
                  }
                  catch (e)
                  {
                  alert("Your browser does not support AJAX!");
                  }
                  }
                  }
                  return xmlHttp;
                  }

                  Though, I'm still confused as to why the onreadystatecha nge method
                  isn't being called in Firefox. Excuse me for being so dense. I'm
                  trying to wrap my brain around this. So, basically, if I reach the
                  portion of my JS that wants to parse the retrieved XML before
                  xmlHttp.readySt ate==4, I'm hosed? I was thinking that using
                  synchronous mode would avoid this.

                    Comment

                    • -Lost
                      #6
                      Re: XMLHttpRequest Object: Asynchronous vs Synchronous Mode

                      "HugeBob" <rnurse@gmail.c omwrote in message
                      news:1175193279 .258648.203270@ l77g2000hsb.goo glegroups.com.. .
                      On Mar 29, 12:21 pm, "-Lost" <missed-s...@comcast.ne twrote:
                      >"HugeBob" <rnu...@gmail.c omwrote in message
                      >>
                      >news:117518326 0.060842.183960 @d57g2000hsg.go oglegroups.com. ..
                      >>
                      >>
                      >>
                      On Mar 29, 10:58 am, "-Lost" <missed-s...@comcast.ne twrote:
                      >"HugeBob" <rnu...@gmail.c omwrote in message
                      >>
                      >>news:11751776 91.794121.18750 0@l77g2000hsb.g ooglegroups.com ...
                      >>
                      Hi All,
                      >>
                      I've got a question about Asynchronous vs Synchronous mode with the
                      XMLHttpRequest object. What are the ramifications of using one mode
                      vs the other? If the script uses Asynchronous mode, it sounds as if a
                      thread retrieves the data from the supplied URL and the JS function
                      that called the open() and send() methods continues on. Where as
                      using Synchronous mode the method that called open() and send() waits
                      until the data from the URL has been retrieved. Am I correct in my
                      thinking?
                      >>
                      >>http://www.w3.org/TR/XMLHttpRequest/#dfn-send
                      >>
                      >The ramifications of not using the asynchronous aspect of AJAX is having the UA or
                      >browser
                      >block activity until the request returns, finishes, loads, whatever.
                      >>
                      >I would not say that the open() and send() methods continue. The onreadystatecha nge
                      >property still receives a dispatch however (even when blocking). So instead of
                      >being
                      >notified of what open() and send() are doing you will receive an onload event and
                      >the
                      >readyState of 4.
                      >>
                      >I would say you are incorrect in thinking that it cannot update its interface during
                      >a
                      >synchronous call (assuming you really meant onreadystatecha nge instead of open() and
                      >send()).
                      >>
                      >I hope I worded all that correctly.
                      >>
                      >-Lost
                      >>
                      I find myself in the same state as you signature, LOL. Well, sort
                      of. I've got an app that uses AJAX. The onreadystatecha nge method
                      sets a global variable with XML it retrieves from the server when
                      readyState is 4. The global variable is later searched via JS. The
                      application works in IE when synchronous mode is used and not at all
                      in FireFox 1.5 (the onreadystatecha nge method never seems to run).
                      So, take the following function:
                      >>
                      globalVariable = null;
                      function ajaxFunction( )
                      {
                      var xmlHttp;
                      >>
                      xmlHttp = getXMLHTTPReque stObject( );
                      alert(xmlHttp);
                      xmlHttp.onready statechange=fun ction()
                      {
                      if(xmlHttp.read yState==4)
                      {
                      globalVariable = xmlHttp.respons eText;
                      }
                      }
                      >>
                      xmlHttp.open("G ET","time.asp", true);
                      xmlHttp.send(nu ll);
                      }
                      >>
                      So, are you saying that even when this function completes,
                      globalVariable may not have anything in it when it's accessed later
                      regardless of whether I use asynch or synch mode?
                      >>
                      >No, globalVariable will have the responseText specified by the XMLHttpRequest.
                      >However,
                      >if asynchronous is set to false, depending on the size of the server's output (e.g. how
                      >long it takes to load) globalVariable will still have the responseText, it may not be
                      >available as quickly.
                      >>
                      >Assuming of course that getXMLHTTPReque stObject() returns the proper XMLHttpRequest.
                      >>
                      >To support the greatest number of possible browsers first have a look at:
                      >>
                      >http://jibbering.com/2002/4/httprequest.html
                      >>
                      >...the implementation there is fairly solid.
                      >>
                      >-Lost
                      >
                      Oh, my getXMLHTTPReque stObject() method handles that. I borrowed it
                      from W3Schools (http://www.w3schools.com/ajax/ajax_server.asp):
                      >
                      <snip w3schools edited AJAX function>
                      Though, I'm still confused as to why the onreadystatecha nge method
                      isn't being called in Firefox. Excuse me for being so dense. I'm
                      trying to wrap my brain around this. So, basically, if I reach the
                      portion of my JS that wants to parse the retrieved XML before
                      xmlHttp.readySt ate==4, I'm hosed? I was thinking that using
                      synchronous mode would avoid this.
                      Bearing in mind a flawless (near flawless) XHR implementation will not suffer this error.

                      Could you perhaps offer a page of your implementation for further debugging?

                      -Lost


                        Comment

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