varnish_backend_fail

High rates of hitpass in Varnish suggest requests are bypassing cache and hitting the backend directly, potentially overloading Apache or other upstream servers. This commonly occurs when cache policies are misconfigured or when content is marked as uncacheable.

Rising varnish_backend_fail and varnish_backend_toolate metrics indicate Varnish is timing out waiting for Apache responses, often due to misaligned timeout configurations between proxy and origin.

Elevated varnish_esi_errors or varnish_esi_warnings indicate Edge Side Includes processing failures, which can trigger multiple backend requests per page load and overwhelm Apache with cascading sub-requests.

Apache prefork MPM's process-per-connection model becomes memory-inefficient when fronted by Varnish, as cache misses can trigger sudden process spawn bursts that exhaust available RAM and force swapping.