Varnish returning synthetic 500 error even though it has stale content it should serve. But only seems to happen during/after a burst of traffic

[Batanun B]

> Could be a vary issue

Ah, I remember hearing about that earlier, and made a mental note to read up on that. But I forgot all about it. Now I did just that, and boy was that a cold shower for me! I definitely need to unset that header. But why, for the love of all that is holy, does Varnish not include the vary-data in the hash? Why isn't the hash the _single_ key used when storing and looking up data in the cache? Why does Varnish hide this stuff from us?

However, when checking the Vary header from the backend, it is set to "Accept-Encoding". And since I haven't changed anything in my browser, it should send the same "Accept-Encoding" request header whenever I surf the website. And since I have visited the startpage multiple times the last 10 days, it should have a cached version of it matching my "Accept-Encoding".

> can you post the output of `varnishlog -d -g request -q 'RespStaus eq 500'?

Well, that gives me nothing that is relevant here, sadly. The last time this happened was a few days ago, and the buffer doesn't seem to be big enough to keep data that far back.

But maybe you could describe what you would look for? I would love to learn how to troubleshoot this.

> In the meantime, here's a cheat sheet to get started with varnishlog:
> https://docs.varnish-software.com/tutorials/vsl-query/

Thanks, although most of that stuff I already knew. And it doesn't really give any more advanced examples. Like the problem I mentioned earlier. I really would like to know if it is possible to find the first request where it served the 500 page for the "/" url, as well as the request just before that, for the same url. Do you know how to construct a query that gives me that?
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