@Override
            public AsyncDataConsumer handleResponse(
                    final HttpResponse backendResponse,
                    final EntityDetails entityDetails) throws HttpException, IOException {
                final Instant responseDate = getCurrentDate();
                final AsyncExecCallback callback;
                if (backendResponse.getCode() != HttpStatus.SC_NOT_MODIFIED) {
                    callback = new BackendResponseHandler(target, request, requestDate, responseDate, scope, asyncExecCallback);
                } else {
                    final ETag resultEtag = ETag.get(backendResponse);
                    if (resultEtag == null) {
                        if (LOG.isDebugEnabled()) {
                            LOG.debug("{} 304 response did not contain ETag", exchangeId);
                        }
                        callback = new AsyncExecCallbackWrapper(() -> callBackend(target, request, entityProducer, scope, chain, asyncExecCallback), asyncExecCallback::failed);
                    } else {
                        final CacheHit match = variantMap.get(resultEtag);
                        if (match == null) {
                            if (LOG.isDebugEnabled()) {
                                LOG.debug("{} 304 response did not contain ETag matching one sent in If-None-Match", exchangeId);
                            }
                            callback = new AsyncExecCallbackWrapper(() -> callBackend(target, request, entityProducer, scope, chain, asyncExecCallback), asyncExecCallback::failed);
                        } else {
                            if (HttpCacheEntry.isNewer(match.entry, backendResponse)) {
                                final HttpRequest unconditional = conditionalRequestBuilder.buildUnconditionalRequest(
                                        BasicRequestBuilder.copy(request).build());
                                callback = new AsyncExecCallbackWrapper(() -> callBackend(target, unconditional, entityProducer, scope, chain, asyncExecCallback), asyncExecCallback::failed);
                            } else {
                                callback = new AsyncExecCallbackWrapper(() -> updateVariantCacheEntry(backendResponse, responseDate, match), asyncExecCallback::failed);
                            }
                        }
                    }
                }
                callbackRef.set(callback);
                return callback.handleResponse(backendResponse, entityDetails);
            }
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