how the drupal set css/js query string for cache buster

Css/js can be cached by browser, CDN or Varnish cache. If a new version css/js released, we want browser/CDN/Varnish request drupal to get new version.

Drupal will append a random string to css/jss url to work as cache buster.

[How to generate query string?]
This random string is stored in DB variable table by default, named as ‘css_js_query_string’. When common.inc drupal_flush_all_caches() is called, it will call _drupal_flush_css_js() then generate a random string with 6 characters.

We can also hard code the value in settings.php.

[How to use the query string?]
In common.inc drupal_pre_render_styles() function, it will call variable_get(‘css_js_query_string’,0) to get the value and append on each css.