Unexpected Cache-Control Header Transmission in Dual-Server API Setup

[Guillaume Quintard]

Hi Uday,

That one should be quick: Varnish doesn't add cache-control headers on its
own.

So, from what I understand it can come from two places:
- either the VCL in varnish1
- something in tomcat1

It should be very easy to check with varnishlog's. Essentially, run
"varnishlog -H request -q 'ReqHeader:uday'" on both varnish nodes and send
a curl request like "curl http://varnish1/some/request/not/in/cache.html -H
"uday: true"

You should see the request going through both varnish and should be able to
pinpoint what created the header. Or at least identify whether it's a
varnish thing or not.

Kind regards

For a reminder on varnishlog:
https://docs.varnish-software.com/tutorials/vsl-query/


On Wed, Jun 28, 2023, 06:28 Uday Kumar <uday.polu at indiamart.com> wrote:

> Hello All,
>
> Our application operates on a dual-server setup, where each server is
> dedicated to running a distinct API.
>
> *Technical specifications:*
> Framework: Spring-boot v2.4 (Java 1.8)
> Runtime Environment: Tomcat
> Version: Apache Tomcat/7.0.42
> Server1 runs API-1 and Server2 runs API-2. Both servers are equipped with
> an installed Varnish application. When either API is accessed, the request
> is processed through the Varnish instance associated with the respective
> server.
>
> *Issue Description:*
> In a typical scenario, a client (browser) sends a request to API-1, which
> is handled by the Varnish instance on Server1. After initial processing,
> API-1 makes a subsequent request to API-2 on Server2.
>
> The Request Flow is as follows:
> *Browser --> Varnish on Server1 --> Tomcat on Server1 --> Varnish on
> Server2 --> Tomcat on Server2*
>
> *Assuming, the request from Browser will be a miss at Server1 Varnish so
> that the request reaches Tomcat(Backend) on server1.*
>
> In cases where the browser *does not include any cache-control headers in
> the request* (e.g., no-cache, max-age=0), the Server1 Varnish instance
> correctly *does not receive any cache-control headers*.
>
> *However, when API-1 calls API-2, we observe that a cache-control:
> no-cache and p**ragma: no-cache headers are being transmitted to the
> Varnish instance on Server2*, despite the following conditions:
>
> 1. We are not explicitly sending any cache-control header in our
> application code during the call from API-1 to API-2.
> 2. Our application does not use the Spring-security dependency, which by
> default might add such a header.
> 3. The cache-control header is not being set by the Varnish instance on
> Server2.
>
> This unexpected behavior of receiving a cache-control header at Server2's
> Varnish instance when invoking API-2 from API-1 is the crux of our issue.
>
> We kindly request your assistance in understanding the cause of this
> unexpected behavior. Additionally, we would greatly appreciate any guidance
> on how to effectively prevent this issue from occurring in the future.
>
> Thanks & Regards
> Uday Kumar
> _______________________________________________
> varnish-misc mailing list
> varnish-misc at varnish-cache.org
> https://www.varnish-cache.org/lists/mailman/listinfo/varnish-misc
>
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