Any way to invoke code from VCL after a request has been serviced?

[Ryan Burn]

> When it comes to header manipulations (including method, status etc)
> they are already logged so a VUT can already pick that up and save
> some work from the VMOD.

It doesn't need to just view the headers; it needs to add headers to encode the
active span context. See
http://opentracing.io/documentation/pages/api/cross-process-tracing.html
And the active span needs to be started before those headers can be added.

> I'm not familiar with OT but what I described is how Zipnish does its
> tracing. Except that Zipnish relies on the X-Varnish header to get a
> unique-ish id, so no blocking call needs to be made. So maybe they are
> highly different systems, I chimed in because I saw a Zipkin example
> while briefly skimming through the docs.

Zipkin is a distributed tracing system that provides OpenTracing
implementations, but Zipnish is just using it as a monitor for
varnish. It's not doing context propagation. If you're only interested
in monitoring varnish, that's fine; but if you want to see how a
request is processed in an entire distributed system (i.e. not just
varnish, but the backend servers it's sitting in front of or any other
service that might be in front of varnish), then you need to do
context propagation.