graph TD
  req[HTTP Request to TARDIS]-->sha1[Convert Path to hash]
  sha1-->cached{Hash Path cached?}
  cached--YES-->prx1[Get Backend value]
  prx1-->prxEnd[Proxy to Backend: Primary or Fallback]
  cached--NOT-->chk[Perform HEAD Request]
  chk-->exists{Exists in BE_PRIMARY}
  exists--YES-->cache1[Cache path hash to BE_PRIMARY]
  cache1-->prxPrm[Proxy to BE_PRIMARY]
  exists--NOT-->cache2[Cache path hash to BE_FALLBACK]
  cache2-->prxFbk[Proxy to BE_FALLBACK]
  prxEnd-->res[Return HTTP Response]
  prxPrm-->res
  prxFbk-->res

