Grace and misbehaving servers

[J X]

Hi,

I'm currently setting up Varnish for a project, and the grace feature together with health checks/probes seems to be a great savior when working with servers that might misbehave. But I'm not really sure I understand how to actually achive that, since the example doesn't really make sense:

https://varnish-cache.org/docs/trunk/users-guide/vcl-grace.html

See the section "Misbehaving servers". There the example does "set beresp.grace = 24h" in vcl_backend_response, and "set req.grace = 10s" in vcl_recv, if the backend is healthy. But since vcl_recv is run before vcl_backend_response, doesn't that mean that the 10s grace value of vcl_recv is overwritten by the 24h value in vcl_backend_response?


Also... There is always a risk of some URL's suddenly giving 500-error (or a timeout) all while the probe still returns 200. Is it possible to have Varnish behave more or less as if the backend is sick, but just for those URL? Basically I would like this logic:

If a healthy content exists in the cache:
1. Return the cached (and potentially stale) content to the client
2. Increase the ttl and/or grace, to keep the healthy content longer
3. Only do a bg-fetch if a specified time has past since the last attempt (lets say 5s), to avoid hammering the backend

If a non-health (ie 500-error) exists in the cache:
1. Return the cached 500-content to the client
2. Only do a bg-fetch if a specified time has past since the last attempt (lets say 5s), to avoid hammering the backend

If no content doesn't exists in the cache:
1. Perform a synchronous fetch
2. If the result is a 500-error, cache it with lets say ttl = 5s
3. Otherwise, cache it with a longer ttl
4. Return the result to the client

Is this possible with the community edition of Varnish?
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