Communication between Varnish and the Backend

Hugues Alary hugues.alary at
Mon Mar 19 20:50:23 CET 2012

Happy Monday everybody,

Nobody, got some good advices for me? :)


On Fri, Mar 16, 2012 at 3:42 PM, Hugues Alary <hugues.alary at>wrote:

> Hi everybody,
> I'm configuring varnish to be used with my web application.
> I need my backend to be able to give instructions to Varnish depending on
> the page requested. For example, some of my pages contain ESI includes,
> some don't.
> I want my application to tell varnish wether or not do the esi
> processing. This approach allows me to keep my VCL configuration file
> abstracted from the application. I can then redistribute the VCL to other
> users and let their application control the cache.
> So far my solution is to instruct varnish via HTTP headers.
> In my current example I set a header X-Application-DoEsi to 1 or 0. When
> varnish receives the response from the backend, it looks for the value
> of X-Application-DoEsi and does or not the ESI processing.
>    - I was wondering if communicating with varnish via HTTP header is a
>    good solution?
>    - How bad can it be performance wise?
>    - Should I be worried about security (interception/modification of the
>    communication between Varnish and the backend), even if no credential will
>    never be sent through HTTP headers? (Also, currently Varnish and the
>    backend are on the same machine, but chances are that they will in the
>    future not live on the same host).
> In the future, I plan on instructing varnish not to cache certain pages
> containing user defined query strings. I want the user to be able to
> specify these un-cachable urls query strings directly in the application.
> The application will then send the un-cachable query strings in a header
> X-Application-QueryStringNoCache: "querystring1,querystring2,...".
> Is that a bad idea?
> Thank you for your help!
> --
> Hugues ALARY

Hugues ALARY
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