@Override
                        public void completed(final Collection<CacheHit> variants) {
                            if (variants != null && !variants.isEmpty()) {
                                negotiateResponseFromVariants(target, request, entityProducer, scope, chain, asyncExecCallback, variants);
                            } else {
                                callBackend(target, request, entityProducer, scope, chain, asyncExecCallback);
                            }
                        }
Request executor in the request execution chain that is responsible for
transparent client-side caching.
<p>
The current implementation is conditionally
compliant with HTTP/1.1 (meaning all the MUST and MUST NOTs are obeyed),
although quite a lot, though not all, of the SHOULDs and SHOULD NOTs
are obeyed too.

@since 5.0
httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/AsyncCachingExec.java