Hiding internal backend requests

[Hugo Cisneiros (Eitch)]

On Wed, Jun 13, 2012 at 7:26 PM, Darvin  Denmian
<darvin.denmian at gmail.com> wrote:
> That is my scenario:
>
> 1) Varnish (172.16.217.131:80), receives a request from a client, i.e:
> http://172.16.217.131:80/a.png
> 2) Request is forwarded to the Default Backend (127.0.0.1:8000)
> 3) Default backend receive the request and process it
> 4) That processing results in a new URL, i.e:
> http://172.16.217.132:80/a.png (**As you can see the IP has changed)
> 5) 172.16.217.132:80 is another backend in Varnish's config file
> 6) The new URL points to a resource that should be provided by Varnish
> (that resource generally is an image)
>
> My problem is: The client needs to execute 2 GETs to obtain the image.
>
> My question:  How can I configure varnish to internally receive the
> response from the first backend(127.0.0.1:8000), and fetch data from
> the second backend (172.16.217.132:80), and after that, send the data
> to the client?

It's not simple, but it's possible. I use this approach with much success here.

You will need something in the first backend response to differentiate
the internal redirect from the successful request. For example, if the
first backend needs to check another backend, you must write a HTTP
header ("req.http.X-Internal-Redirect = True", for example).

Then, on vcl_fetch, you use something like this:

if (beresp.http.X-Internal-Redirect == "True") {
   set req.http.X-Internal-Redirect = "True");
   return (restart);
}

With "return (restart)", your request will be restarted from scratch
(entering vcl_recv), but with the additional header. Now you can set
your other backend on vcl_recv based on this new header, like this on
vcl_recv:

if (req.http.X-Internal-Redirect == "True") {
   set req.backend = another_backend;
}

This new backend can't reply the additional header since this could
create a restart loop.

Good luck! :)

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