ETags are not cache invalidation. ETags are a way for a client to validate the cache in the client, while cache invalidation for a http proxy would involve telling the proxy that it should discard a cached file.
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Basically:
cache validation: Check if a file matches a cached version
cache invalidation: Preemptively tell a cache to discard a cached file
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For example:
Client A requests file X. Proxy caches file X.
File X changes in the upstream.
Client B requests file X. Proxy does not know the file changed upstream, so it sends a stale version.
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In this case either the proxy needs to revalidate each request with upstream (which is expensive) or we need some way to tell the proxy that it should discard it's cache for file X since it has changed.
I think I'll have to look into this to see if we can get away with document/blob cache in one of our solutions actually. I've resisted add-in Varnish in the mix, but it might be time to reconsider.
