How many concurrent AJAX XmlHttpRequest requests are allowed in popular browsers

Question

In Firefox 3  the answer is 6 per domain  as soon as a 7th XmlHttpRequest  on any tab  to the same domain is fired  it is queued until one of the other 6 finish   What are the numbers for the other major browsers   Also  are there ways around these limits without having my users modify their browser settings   For example  are there limits to the number of jsonp requests  which use script tag injection rather than an XmlHttpRequest object    Background  My users can make XmlHttpRequests from a web page to the server  asking the server to run ssh commands on remote hosts   If the remote hosts are down  the ssh command takes a few minutes to fail  eventually preventing my users from performing any further commands

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I have writen a single file AJAX tester  Enjoy it    Just because I have had problems with my hosting provider   lt  php     Author    Luis Siquot Purpose   Check ajax performance and errors License   GPL site5     Please don t drop json requests  nor delay            r    int   GET  r     w    int   GET  w    if  r        sleep  w      echo json encode   GET      die          else   gt  lt head gt   lt script type  text javascript  src  http   ajax googleapis com ajax libs jquery 1 4 2 jquery min js  gt  lt  script gt   lt script type  text javascript  gt   var  settimer  var  timer  var  waiting     function      clearTable          boton   bind  click   donow       function donow      var w    var estim   0     waiting       total   0  value   1    clearTable      for var r 1 r lt   waiting r            w   Math floor Math random   6  2         estim    w         dodebug  r r  w w             ajax  url    lt  php echo   SERVER  SCRIPT NAME      gt                   data      r r  w w                  dataType   json         html                   type   GET                  success  function CBdata  status                      CBdebug CBdata                                     doStat estim     timer estim 10      function doStat what           stat   replaceWith           lt table border  0  id  stat  gt  lt tr gt  lt td gt Request Time Sum  lt th gt   what           lt td gt  amp nbsp  amp nbsp  2  lt th gt   Math ceil what 2            lt td gt  amp nbsp  amp nbsp  3  lt th gt   Math ceil what 3            lt td gt  amp nbsp  amp nbsp  4  lt th gt   Math ceil what 4            lt td gt  amp nbsp  amp nbsp  6  lt th gt   Math ceil what 6            lt td gt  amp nbsp  amp nbsp  8  lt th gt   Math ceil what 8            lt td gt   amp nbsp   seconds  lt  table gt             function timer what     if what            timer   0   settimer   what     if  waiting  0             showTimer   0  innerHTML    completed in  lt b gt      timer     seconds lt  b gt   aprox        return         if  timer lt  settimer            showTimer   0  innerHTML    timer       setTimeout  timer    1000         timer         return            showTimer   0  innerHTML     lt b gt don  t wait any more    lt  b gt        function CBdebug what        waiting--          req  what r  0  innerHTML    x       function dodebug what       var tt     lt tr gt  lt td gt     what r     lt td gt     what w     lt td id req    what r     gt  amp nbsp           debug   append tt       function clearTable            debug   replaceWith   lt table border  1  id  debug  gt  lt tr gt  lt td gt Request   lt td gt Wait Time lt td gt Done lt  table gt          lt  script gt   lt  head gt   lt body gt   lt center gt   lt input type  button  value  start  id  boton  gt   lt input type  text  value  80  id  total  size  2  gt  concurrent json requests  lt table id  stat  gt  lt tr gt  lt td gt  amp nbsp  lt  table gt  Elapsed Time   lt span id  showTimer  gt  lt  span gt   lt table id  debug  gt  lt  table gt   lt  center gt   lt  body gt    Edit  r means row and w waiting time  When you initially press start button 80  or any other number  of concurrent ajax request are launched by javascript  but as is known they are spooled by the browser   Also they are requested to the server in parallel  limited to certain number  this is the fact of this question    Here the requests are solved server side with a random delay  established by w   At start time all the time needed to solve all ajax calls is calculated   When test is finished  you can see if it took half  took third  took a quarter  etc of the total time  deducting which was the parallelism on the calls to the server  This is not strict  nor precise  but is nice to see in real time how ajaxs calls are completed  seeing the incoming cross   And is a very simple self contained script to show ajax basics  Of course  this assumes  that server side is not introducing any extra limit  Preferably use in conjunction with firebug net panel  or your browser s equivalent

User · Answer

One trick you can use to increase the number of concurrent connections is to host your images from a different sub domain  These will be treated as separate requests  each domain is what will be limited to the concurrent maximum    IE6  IE7 - have a limit of two  IE8 is 6 if you have a broadband - 2  if it s a dial up

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The network results at Browserscope will give you both Connections per Hostname and Max Connections for popular browsers   The data is gathered by running tests on users  in the wild   so it will stay up to date

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According to IE 9     What   s Changed  on the HttpWatch blog  IE9 still has a 2 connection limit when over VPN      Using a VPN Still Clobbers IE 9 Performance      We previously reported   about the scaling back of the maximum   number of concurrent connections in IE   8 when your PC uses a VPN connection    This happened even if the browser   traffic didn   t go over that   connection       Unfortunately  IE 9 is affected by VPN   connections in the same way

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Wrote my own test  tested the code on stackoverflow  works fine tells me that chrome FF can do 6  var change   0  var simultanius   0  var que   20     number of tests  Array que  join 0  split 0  forEach function a i       var xhr   new XMLHttpRequest      xhr open  GET        i      cacheBust     xhr onreadystatechange   function             if xhr readyState    2               change                simultanius   Math max simultanius  change                     if xhr readyState    4               change--              que--              if  que                   console log simultanius                                      xhr send          it works for most websites that can trigger readystate change event at different times   aka  flushing   I notice on my node js server that i had to output at least 1025 bytes to trigger the event flush  otherwise the events would just trigger all three state at once when the request is complete so here is my backend   var app   require  express       app get      function req res        res write Array 1025  join  a         setTimeout function             res end  a          500        app listen 80     Update  I notice that You can now have up to 2x request if you are using both xhr and fetch api at the same time   x000D   x000D  var change   0  x000D  var simultanius   0  x000D  var que   30     number of tests x000D   x000D  Array que  join 0  split 0  forEach function a i   x000D      fetch    b  i  then r   gt    x000D          change    x000D          simultanius   Math max simultanius  change   x000D          return r text   x000D         then r   gt    x000D          change--  x000D          que--  x000D          if  que   x000D              console log simultanius   x000D            x000D          x000D      x000D   x000D  Array que  join 0  split 0  forEach function a i   x000D      var xhr   new XMLHttpRequest  x000D      xhr open  GET      a  i      cacheBust x000D      xhr onreadystatechange   function     x000D          if xhr readyState    2   x000D              change    x000D              simultanius   Math max simultanius  change   x000D            x000D          if xhr readyState    4   x000D              change--  x000D              que--  x000D              if  que   x000D                  document body innerHTML   simultanius  x000D                x000D            x000D         x000D      xhr send    x000D      x000D   x000D   x000D

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With IE6   IE7 one can tweak the number of concurrent requests in the registry   Here s how to set it to four each    HKEY CURRENT USER Software Microsoft Windows CurrentVersion Internet Settings   MaxConnectionsPerServer  dword 00000004  MaxConnectionsPer1 0Server  dword 00000004

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I just checked with www browserscope org and with IE9 and Chrome 24 you can have 6 concurrent connections to a single domain  and up to 17 to multiple ones

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I believe there is a maximum number of concurrent http requests that browsers will make to the same domain  which is in the order of 4-8 requests depending on the user s settings and browser   You could set up your requests to go to different domains  which may or may not be feasible  The Yahoo guys did a lot of research in this area  which you can read about  here   Remember that every new domain you add also requires a DNS lookup  The YSlow guys recommend between 2 and 4 domains to achieve a good compromise between parallel requests and DNS lookups  although this is focusing on the page s loading time  not subsequent AJAX requests   Can I ask why you want to make so many requests  There is good reasons for the browsers limiting the number of requests to the same domain  You will be better off bundling requests if possible

[ajax] How many concurrent AJAX (XmlHttpRequest) requests are allowed in popular browsers?

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