@Override
    public void execute(
            final HttpRequest request,
            final AsyncEntityProducer entityProducer,
            final AsyncExecChain.Scope scope,
            final AsyncExecChain chain,
            final AsyncExecCallback asyncExecCallback) throws HttpException, IOException {
        final String exchangeId = scope.exchangeId;
        final HttpRoute route = scope.route;
        final CancellableDependency operation = scope.cancellableDependency;
        final HttpClientContext clientContext = scope.clientContext;
        final AsyncExecRuntime execRuntime = scope.execRuntime;

        if (LOG.isDebugEnabled()) {
            LOG.debug("{} executing {} {}", exchangeId, request.getMethod(), request.getRequestUri());
        }

        final AtomicInteger messageCountDown = new AtomicInteger(2);
        final AsyncClientExchangeHandler internalExchangeHandler = new AsyncClientExchangeHandler() {

            private final AtomicReference<AsyncDataConsumer> entityConsumerRef = new AtomicReference<>();

            @Override
            public void releaseResources() {
                final AsyncDataConsumer entityConsumer = entityConsumerRef.getAndSet(null);
                if (entityConsumer != null) {
                    entityConsumer.releaseResources();
                }
            }

            @Override
            public void failed(final Exception cause) {
                final AsyncDataConsumer entityConsumer = entityConsumerRef.getAndSet(null);
                if (entityConsumer != null) {
                    entityConsumer.releaseResources();
                }
                execRuntime.markConnectionNonReusable();
                asyncExecCallback.failed(cause);
            }

            @Override
            public void cancel() {
                if (messageCountDown.get() > 0) {
                    failed(new InterruptedIOException());
                }
            }

            @Override
            public void produceRequest(
                    final RequestChannel channel,
                    final HttpContext context) throws HttpException, IOException {

                clientContext.setRoute(route);
                clientContext.setRequest(request);
                httpProcessor.process(request, entityProducer, clientContext);

                channel.sendRequest(request, entityProducer, context);
                if (entityProducer == null) {
                    messageCountDown.decrementAndGet();
                }
            }

            @Override
            public int available() {
                return entityProducer.available();
            }

            @Override
            public void produce(final DataStreamChannel channel) throws IOException {
                entityProducer.produce(new DataStreamChannel() {

                    @Override
                    public void requestOutput() {
                        channel.requestOutput();
                    }

                    @Override
                    public int write(final ByteBuffer src) throws IOException {
                        return channel.write(src);
                    }

                    @Override
                    public void endStream(final List<? extends Header> trailers) throws IOException {
                        channel.endStream(trailers);
                        if (messageCountDown.decrementAndGet() <= 0) {
                            asyncExecCallback.completed();
                        }
                    }

                    @Override
                    public void endStream() throws IOException {
                        channel.endStream();
                        if (messageCountDown.decrementAndGet() <= 0) {
                            asyncExecCallback.completed();
                        }
                    }

                });
            }

            @Override
            public void consumeInformation(
                    final HttpResponse response,
                    final HttpContext context) throws HttpException, IOException {
                if (response.getCode() == HttpStatus.SC_SWITCHING_PROTOCOLS) {
                    final ProtocolVersion upgradeProtocol = protocolSwitchStrategy.switchProtocol(response);
                    if (upgradeProtocol == null || !upgradeProtocol.getProtocol().equals("TLS")) {
                        throw new ProtocolException("Failure switching protocols");
                    }
                    if (LOG.isDebugEnabled()) {
                        LOG.debug("Switching to {}", upgradeProtocol);
                    }
                    execRuntime.upgradeTls(clientContext, new FutureCallback<AsyncExecRuntime>() {

                        @Override
                        public void completed(final AsyncExecRuntime result) {
                            LOG.debug("Successfully switched to {}", upgradeProtocol);
                        }

                        @Override
                        public void failed(final Exception ex) {
                            asyncExecCallback.failed(ex);
                        }

                        @Override
                        public void cancelled() {
                            asyncExecCallback.failed(new InterruptedIOException());
                        }

                    });
                } else {
                    asyncExecCallback.handleInformationResponse(response);
                }
            }

            @Override
            public void consumeResponse(
                    final HttpResponse response,
                    final EntityDetails entityDetails,
                    final HttpContext context) throws HttpException, IOException {

                clientContext.setResponse(response);
                httpProcessor.process(response, entityDetails, clientContext);

                entityConsumerRef.set(asyncExecCallback.handleResponse(response, entityDetails));
                if (response.getCode() >= HttpStatus.SC_CLIENT_ERROR) {
                    messageCountDown.decrementAndGet();
                }
                final TimeValue keepAliveDuration = keepAliveStrategy.getKeepAliveDuration(response, clientContext);
                Object userToken = clientContext.getUserToken();
                if (userToken == null) {
                    userToken = userTokenHandler.getUserToken(route, request, clientContext);
                    clientContext.setUserToken(userToken);
                }
                execRuntime.markConnectionReusable(userToken, keepAliveDuration);
                if (entityDetails == null) {
                    execRuntime.validateConnection();
                    if (messageCountDown.decrementAndGet() <= 0) {
                        asyncExecCallback.completed();
                    }
                }
            }

            @Override
            public void updateCapacity(final CapacityChannel capacityChannel) throws IOException {
                final AsyncDataConsumer entityConsumer = entityConsumerRef.get();
                if (entityConsumer != null) {
                    entityConsumer.updateCapacity(capacityChannel);
                } else {
                    capacityChannel.update(Integer.MAX_VALUE);
                }
            }

            @Override
            public void consume(final ByteBuffer src) throws IOException {
                final AsyncDataConsumer entityConsumer = entityConsumerRef.get();
                if (entityConsumer != null) {
                    entityConsumer.consume(src);
                }
            }

            @Override
            public void streamEnd(final List<? extends Header> trailers) throws HttpException, IOException {
                final AsyncDataConsumer entityConsumer = entityConsumerRef.getAndSet(null);
                if (entityConsumer != null) {
                    entityConsumer.streamEnd(trailers);
                } else {
                    execRuntime.validateConnection();
                }
                if (messageCountDown.decrementAndGet() <= 0) {
                    asyncExecCallback.completed();
                }
            }

        };

        if (LOG.isDebugEnabled()) {
            operation.setDependency(execRuntime.execute(
                    exchangeId,
                    new LoggingAsyncClientExchangeHandler(LOG, exchangeId, internalExchangeHandler),
                    clientContext));
        } else {
            operation.setDependency(execRuntime.execute(exchangeId, internalExchangeHandler, clientContext));
        }
    }
Usually the last HTTP/1.1 request execution handler in the asynchronous
request execution chain that is responsible for execution of
request/response exchanges with the opposite endpoint.

@since 5.0
httpclient5/src/main/java/org/apache/hc/client5/http/impl/async/HttpAsyncMainClientExec.java