I had serious problems with a Drupal website with many Varnish optimizations. It so occurs that one of them, a return(lookup) on images and css extensions, was really the one causing the problem:
if (req.url ~ ".(png|jpg|jpeg|swf|css|ico)") {
return(lookup);
}
Now I don't remember precisely why I added this condition in the first place (lookup means you force Varnish to re-use the version it has in cache) but apparently in my case it doesn't suit my purposes.
The most common actions you can decide to ask Varnish to execute in the vcl_fetch can be found here: https://www.varnish-cache.org/docs/2.1/tutorial/vcl.html#actions
In short:
- pass
- When you call pass the request and subsequent response will be passed to and from the backend server. It won’t be cached. pass can be called in both vcl_recv and vcl_fetch.
- lookup
- When you call lookup from vcl_recv you tell Varnish to deliver content from cache even if the request othervise indicates that the request should be passed. You can’t call lookup from vcl_fetch.
- pipe
- Pipe can be called from vcl_recv as well. Pipe short circuits the client and the backend connections and Varnish will just sit there and shuffle bytes back and forth. Varnish will not look at the data being send back and forth - so your logs will be incomplete. Beware that with HTTP 1.1 a client can send several requests on the same connection and so you should instruct Varnish to add a “Connection: close” header before actually calling pipe.
- deliver
- Deliver the cached object to the client. Usually called in vcl_fetch.
- esi
- ESI-process the fetched document.