Cache busting via params

Question

We want to cache bust on production deploys  but not waste a bunch of time off the bat figuring out a system for doing so   My thought was to apply a param to the end of css and js files with the current version number    lt link rel  stylesheet  href  base url com file css v 1 123   gt    Two questions   Will this effectively break the cache   Will the param cause the browser to then never cache the response from that url since the param indicates that this is dynamic content

User · Answer

As others have said  cache busting with a query param is usually considered a Bad Idea  tm   and has been for a long time  It s better to reflect the version in the file name  Html5 Boilerplate recommends against using the query string  among others   That said  of the recommendations I have seen which cited a source  all seem to take their wisdom from a 2008 article by Steve Souders  His conclusions are based on the behaviour of proxies at the time  and they may or may not be relevant these days  Still  in the absence of more current information  changing the file name is the safe option

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Hope this should help you to inject external JS file   lt script type  text javascript  gt   var cachebuster   Math round new Date   getTime     1000    document write   lt scr   ipt type  text javascript  src  external js cb    cachebuster    gt  lt  scr     ipt gt      lt  script gt    Source - Cachebuster code in JavaScript

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lt script type  text javascript  gt     front end cache bust  var cacheBust     js StrUtil js    js protos common js    js conf js    bootstrap ECP js init js       for  i 0  i  lt  cacheBust length  i          var el   document createElement  script         el src   cacheBust i    v     Math random         document getElementsByTagName  head   0  appendChild el      lt  script gt

User · Answer

In general this should be fine  but it s possible for this to not work if there is an intermediate cache  a proxy  that is configured to ignore the request parameters   For example  if you are serving static content through Akamai CDN  it can be configured to ignore request parameters to prevent cache busting using this method

User · Answer

The param  v 1 123 indicates a query string  and the browser will therefore think it is a new path from  say   v 1 0  Thus causing it to load from file  not from cache  As you want    And  the browser will assume that the source will stay the same next time you call  v 1 123 and should cache it with that string  So it will remain cached  however your server is set up  until you move to  v 1 124 or so on

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Two questions  Will this effectively break the cache     Yes  Even Stack Overflow use this method  although I remember that they  with their millions of visitors per day and zillions of different client and proxy versions and configurations  have had some freak edge cases where even this was not enough to break the cache  But the general assumption is that this will work  and is a suitable method to break caching on clients      Will the param cause the browser to then never cache the response from that url since the param indicates that this is dynamic content    No  The parameter will not change the caching policy  the caching headers sent by the server still apply  and if it doesn t send any  the browser s defaults

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Found a comparison of the 2 techniques  query string vs file name  here  Version as a querystring has two problems   First  it may not always be a browser that implements caching through which we need to bust  It is said that certain  possibly older  proxies do ignore the querystring with respect to their caching behavior  Second  in certain more complex deployment scenarios  where you have multiple frontend and or multiple backend servers  an upgrade is anything but instantaneous  You need to be able to serve both the old and the new version of your assets at the same time  See for example how this affects you when using Google App Engine

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lt script gt      var storedSrcElements               js exampleFile js             js sampleFile js             css style css                    var head  document getElementsByTagName  head   0       var script      var link      var versionNumberNew   4 6       for i 0 i lt storedSrcElements length i          script  document createElement  script         script type   text javascript        script src  storedSrcElements i          versionNumberNew       head appendChild script                      lt  script gt                Change the version number   versionNumberNew  when you want the new files to be loaded

User · Answer

It will bust the cache once  after the client has downloaded the resource every other response will be served from client cache unless    the v parameter is updated  the client clears their cache

User · Answer

It is safer to put the version number in the actual filename   This allows multiple versions to exist at once so you can roll out a new version and if any cached HTML pages still exist that are requesting the older version  they will get the version that works with their HTML   Note  in one of the largest versioned deployments anywhere on the internet  jQuery uses version numbers in the actual filename and it safely allows multiple versions to co-exist without any special server-side logic  each version is just a different file    This busts the cache once when you deploy new pages and new linked files  which is what you want  and from then on those versions can be effectively cached  which you also want

User · Answer

It very much depends on quite how robust you want your caching to be  For example  the squid proxy server  and possibly others  defaults to not caching URLs served with a querystring - at least  it did when that article was written  If you don t mind certain use cases causing unnecessary cache misses  then go ahead with query params  But it s very easy to set up a filename-based cache-busting scheme which avoids this problem

User · Answer

Another similar approach is to use htaccess mod rewrite to ignore part of the path when serving the files  Your never-cached index page references the latest path to the files  From a development perspective it s as easy as using params for the version number  but it s as robust as the filename approach  Use the ignored part of the path for the version number  and the server just ignores it and serves the uncached file  1 2 3 css styles css serves the same file as css styles css since the first directory is stripped and ignored by the htaccess file Including versioned files  lt  php    version    quot 1 2 3 quot     gt    lt html gt     lt head gt       lt meta http-equiv  quot cache-control quot  content  quot max-age 0 quot    gt       lt meta http-equiv  quot cache-control quot  content  quot no-cache quot    gt       lt meta http-equiv  quot expires quot  content  quot 0 quot    gt       lt meta http-equiv  quot expires quot  content  quot Tue  01 Jan 1980 1 00 00 GMT quot    gt       lt meta http-equiv  quot pragma quot  content  quot no-cache quot    gt       lt link rel  quot stylesheet quot  type  quot text css quot  href  quot  lt  php echo  version   gt  css styles css quot  gt     lt  head gt     lt body gt       lt script src  quot  lt  php echo  version   gt  js main js quot  gt  lt  script gt     lt  body gt   lt  html gt   Note that this approach means you need to disable caching of your index page - Using  lt meta gt  tags to turn off caching in all browsers   htaccess file RewriteEngine On    if you re requesting a file that exists  do nothing RewriteCond   REQUEST FILENAME   -f    likewise if a directory that exists  do nothing RewriteCond   REQUEST FILENAME   -d     otherwise  rewrite foo bar baz to bar baz - ignore the first directory RewriteRule               1  L    You could take the same approach on any server platform that allows url rewriting  rewrite condition adapted from mod rewrite - rewrite directory to query string except           and if you need cache busting for your index page   site entry point  you could always use JavaSript to refresh it

[javascript] Cache busting via params

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