Clear the cache from the Rails asset pipeline

I'm assuming we're talking about the production environment.

When you change any of your javascripts or stylesheets in the production environment, you need to run rake assets:precompile; this task compiles and compresses the various .js and .css files and creates the application.js and application.css files that is loaded by your views.

It's possible that if you replaced jquery.autoresize.js with a version with an older timestamp, the precompile step might skip it, thinking the compiled version is up-to-date. You can avoid that by running rake assets:clean first, forcing it to rebuild everything in the public/assets directory from scratch.

Also try rake assets:clobber. This will totally reset everything and delete all compiled assets. In addition, I often need to set the environment before pushing to production by going: RAILS_ENV=production rake assets:precompile.

Rails automatically clears the cache for an individual file every time that the contents are edited. To clear the cache for a single file, simply open the file, edit a line of code, and re-save it. Rails will clear the cache for that file, and the browser will load the new file the next time the page is loaded.

The reason jquery.autoresize.js was using the old cached version of the file was because the old version was deleted and then the new version was copied and pasted with the same name into the same folder. Because the file itself was never edited, Rails continued to use the old file that was cached.

This is because the asset pipeline uses fingerprinting for the cache.

Fingerprinting is a technique that makes the name of a file dependent on the contents of the file. When the file contents change, the filename is also changed. For content that is static or infrequently changed, this provides an easy way to tell whether two versions of a file are identical, even across different servers or deployment dates.

When a filename is unique and based on its content, HTTP headers can be set to encourage caches everywhere (whether at CDNs, at ISPs, in networking equipment, or in web browsers) to keep their own copy of the content. When the content is updated, the fingerprint will change. This will cause the remote clients to request a new copy of the content. This is generally known as cache busting.

The technique that Rails uses for fingerprinting is to insert a hash of the content into the name, usually at the end. For example a CSS file global.css could be renamed with an MD5 digest of its contents:

global-908e25f4bf641868d8683022a5b62f54.css

So, if you delete a file you're referencing in the manifest, and then copy in a new file with the same name, the cache busting never occurs. When you edit the file, the fingerprinting kicks in, and a new hash is generated for the file name. This busts the cache for that file.

For the full story, see What is Fingerprinting and Why Should I Care?.