Varnish 3: Accept JSON returns HTML - varnish

Question

I try to make an REST-API, but varnish returns always the first called response and I have no idea why.
If I open a page with a Browser, Varnish returns HTML -> is OK.
If I curl the same page curl -i https://example.com -H "Accept: application/json" Varnish also returns HTML -> which is False.
As I see, Varnish always returns the first cached item, If this is JSON varnish returns JSON, if this is HTML Varnish returns HTML.
Without Varnish everything works like expected.

Answer 1

If you're serving different content type on the same URL, you you might want to tell Varnish to partition cache accordingly.
In fact, Varnish doesn't do much special about it, and it behaves like other proxies would. If they see a URL without information specifying how a resource's cache should partition, then no matter if it is a JSON or a regular request, the first request will be cached and served the same irrespective of request type.
So you need to tell Varnish how to partition cache for a resource.
The "Vary" header
The most straightforward and "HTTP" compatible way for other proxies in the wild, is Vary response header.
It tells the proxy cache (Varnish in this case), to partition, vary cache for a resource based on a header value coming from a client.
E.g. client sends header X: some-value and your app sends header Vary: X is what it takes to make the cache different between different value of X.
For Varnish 3, there is an example with Accept-Encoding.
The article details an implementation challenge with Vary - different clients may be sending quite different values for varied header thus resulting in severely partitioned cache. So you typically want to normalize the varying header's value, to a set of known, expected values.
In your case you want to Vary (and normalize) the Accept header. So something along the lines of (in vcl_recv procedure):
if (req.http.Accept) {
if (req.http.Accept ~ "application/json") {
set req.http.Accept = "application/json";
} else {
set req.http.Accept = "text/html";
}
}
Next you need to have your app actually send Vary: Accept (inside your app source files). Alternatively, you can throw some Varnish VCL instead, if modiying app source files is not feasible:
sub vcl_fetch {
if (!beresp.http.Vary) { # no Vary at all
set beresp.http.Vary = "Accept";
} elseif (beresp.http.Vary !~ "Accept") { # add to existing Vary
set beresp.http.Vary = beresp.http.Vary + ", Accept";
}
}