https://docs.spring.io/spring-cloud-stream/docs/current/reference/html/spring-cloud-stream.html#spring_cloud_function
https://docs.spring.io/spring-cloud-stream/docs/current/reference/html/spring-cloud-stream.html#_functional_binding_names
    <functionName> + -<in|out>- + <index>
https://docs.spring.io/spring-cloud-stream/docs/current/reference/html/spring-cloud-stream.html#binding-properties
    org.springframework.cloud.stream.config.BindingProperties
    org.springframework.cloud.stream.binder.ConsumerProperties
        https://docs.spring.io/spring-cloud-stream/docs/current/reference/html/spring-cloud-stream.html#_advanced_consumer_configuration
        ListenerContainerCustomizer
        MessageSourceCustomizer
    org.springframework.cloud.stream.binder.ProducerProperties
        https://docs.spring.io/spring-cloud-stream/docs/current/reference/html/spring-cloud-stream.html#_advanced_producer_configuration
        ProducerMessageHandlerCustomizer
???
https://docs.spring.io/spring-cloud-stream/docs/current/reference/html/spring-cloud-stream.html#_using_channel_interceptors_with_streambridge

https://github.com/spring-cloud/spring-cloud-stream-samples
    https://github.com/spring-cloud/spring-cloud-stream-binder-rabbit
    https://github.com/spring-cloud/spring-cloud-stream

https://stackoverflow.com/questions/65576021/enablebinding-deprecated-as-of-3-1-in-favor-of-functional-programming-model

2019
https://spring.io/blog/2019/10/14/spring-cloud-stream-demystified-and-simplified

package org.springframework.cloud.function.context.catalog;

public class SimpleFunctionRegistry implements FunctionRegistry, FunctionInspector {
    ...
		private Object enrichInvocationResultIfNecessary(Object input, Object result) {
			if (result != null && !(result instanceof Publisher) && input instanceof Message) {
				if (result instanceof Message) {
					if (functionInvocationHelper != null && CloudEventMessageUtils.isCloudEvent(((Message) input))) {
						result = functionInvocationHelper.postProcessResult(result, (Message) input);
					}
					else {
						Map<String, Object> headersMap = (Map<String, Object>) ReflectionUtils
								.getField(SimpleFunctionRegistry.this.headersField, ((Message) result).getHeaders());
						this.sanitizeHeaders(((Message) input).getHeaders()).forEach((k, v) -> headersMap.putIfAbsent(k, v));
					}
				}
				else {
					if (functionInvocationHelper != null && CloudEventMessageUtils.isCloudEvent(((Message) input))) {
						result = functionInvocationHelper.postProcessResult(result, (Message) input);
					}
					else if (!FunctionTypeUtils.isCollectionOfMessage(this.outputType)) {
						result = MessageBuilder.withPayload(result).copyHeaders(this.sanitizeHeaders(((Message) input).getHeaders())).build(); // !!!
					}
				}
			}
			return result;
		}
    ...
}

package org.springframework.cloud.stream.function;
...
public class FunctionConfiguration {
    ...
		@Override
		public void afterPropertiesSet() throws Exception {
			Map<String, BindableProxyFactory> beansOfType = applicationContext.getBeansOfType(BindableProxyFactory.class);
			this.bindableProxyFactories = beansOfType.values().toArray(new BindableProxyFactory[0]);
			for (BindableProxyFactory bindableProxyFactory : this.bindableProxyFactories) {
				String functionDefinition = bindableProxyFactory instanceof BindableFunctionProxyFactory
						? ((BindableFunctionProxyFactory) bindableProxyFactory).getFunctionDefinition()
								: this.functionProperties.getDefinition();

				boolean shouldNotProcess = false;
				if (!(bindableProxyFactory instanceof BindableFunctionProxyFactory)) {
					Set<String> outputBindingNames = bindableProxyFactory.getOutputs();
					shouldNotProcess = !CollectionUtils.isEmpty(outputBindingNames)
							&& outputBindingNames.iterator().next().equals("applicationMetrics");
				}
				if (StringUtils.hasText(functionDefinition) && !shouldNotProcess) {
					FunctionInvocationWrapper function = functionCatalog.lookup(functionDefinition);
					if (function != null && !function.isSupplier()) {
						this.bindFunctionToDestinations(bindableProxyFactory, functionDefinition);
					}
				}
			}
		}

    // !!!
		private void bindFunctionToDestinations(BindableProxyFactory bindableProxyFactory, String functionDefinition) {
			this.assertBindingIsPossible(bindableProxyFactory);


			Set<String> inputBindingNames = bindableProxyFactory.getInputs();
			Set<String> outputBindingNames = bindableProxyFactory.getOutputs();
      ...
    }
    ...
}