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[ci skip] docs asset_pipeline#CDNs

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schneems 2014-08-24 15:40:22 -05:00
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guides/source/asset_pipeline.md
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### CDNs
If your assets are being served by a CDN, ensure they don't stick around in your
cache forever. This can cause problems. If you use
`config.action_controller.perform_caching = true`, Rack::Cache will use
`Rails.cache` to store assets. This can cause your cache to fill up quickly.
CDN stands for [Content Delivery
Network](http://en.wikipedia.org/wiki/Content_delivery_network), they are
primarily designed to cache assets all over the world so that when a browser
requests the asset, a cached copy will be geographically close to that browser.
If you are serving assets directly from your Rails server in production, the
best practice is to use a CDN in front of your application.
Every cache is different, so evaluate how your CDN handles caching and make sure
that it plays nicely with the pipeline. You may find quirks related to your
specific set up, you may not. The defaults NGINX uses, for example, should give
you no problems when used as an HTTP cache.
A common pattern for using a CDN is to set your production application as the
"origin" server. This means when a browser requests an asset from the CDN and
there is a cache miss, it will grab the file from your server on the fly and
then cache it. For example if you are running a Rails application on
`example.com` and have a CDN configured at `mycdnsubdomain.fictional-cdn.com`,
then when a request is made to `mycdnsubdomain.fictional-
cdn.com/assets/smile.png`, the CDN will query your server once at
`example.com/assets/smile.png` and cache the request. The next request to the
CDN that comes in to the same URL will hit the cached copy. When the CDN can
serve an asset directly the request never touches your Rails server. Since the
assets from a CDN are geographically closer to the browser, the request is
faster, and since your server doesn't need to spend time serving assets, it can
focus on serving application code as fast as possible.
If you want to serve only some assets from your CDN, you can use custom
`:host` option of `asset_url` helper, which overwrites value set in
`config.action_controller.asset_host`.
```ruby
asset_url 'image.png', :host => 'http://cdn.example.com'