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_sess_drop signals that Varnish is rejecting connections due to worker thread exhaustion or queue overflow, often caused by slow backend responses keeping workers occupied.
Long KeepAliveTimeout values in Apache combined with high concurrent connections can starve worker threads, causing Varnish to see backends as busy or unresponsive even when Apache has available capacity.
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.
