Varnish 503: Workspace Overflow Diagnosis

Challenge

The employee/B2E store was consistently returning 503 errors on requests that bypassed the Varnish cache, while cached pages served correctly. The pattern pointed at the Varnish-to-backend communication rather than the application itself.

What Was Built

Primary cause: the cookie consent manager (Civic) sets a large number of cookies, making request headers significantly larger than average. When Varnish processed these large-header requests on the cache-bypass path, the internal workspace_backend buffer overflowed and Varnish could not assemble the backend request, returning 503. VCL rules added to strip consent manager cookies from backend requests (they serve no purpose server-side). workspace_backend and workspace_client Varnish parameters increased to safe headroom above the maximum observed header size. Secondary cause: an unrelated PHP exception inside the custom category sorting module on those same requests was also fixed independently.