Can I use a spring cloud stream function definition for multiple tenants (kafka binder)?

We found a solution to this problem by manually registering the function beans. Sadly this was not quite as easy as we thought it would be. FunctionDetectorCondition (https://github.com/spring-cloud/spring-cloud-stream-binder-kafka/blob/main/spring-cloud-stream-binder-kafka-streams/src/main/java/org/springframework/cloud/stream/binder/kafka/streams/function/FunctionDetectorCondition.java) requires an AnnotatedBeanDefinition that used as a template for the actual Stream Processing bean. This could be taken as a proposal to spring cloud streams for registering a function defintion template that can be used multiple times.

To reach this goal we initialise a factory bean instead of the stream processor function itself:

@Configuration
public class StreamProcessorConfig {

    @Bean
    public MyFunctionDefinitionFactory myFunctionDefinitionFactory() {
        return new MyFunctionDefinitionFactory();
    }
}

The factory creates the stream processor function:

public class MyFunctionDefinitionFactory {

    public Function<KStream<String, InputType>, 
               KStream<String, OutputType>> myfunctiondefinition() {
        return inputTypeStream -> inputTypeStream.map((String k, InputType v) -> {
            return KeyValue.pair(k, OutputType.createFrom(v));
        });
    }
}

Now we need a Dummy Bean Interface that is Required for Spring Cloud Streams to apply its logic to create the stream processor:

// Behaves as dummy bean for spring cloud stream
// Has to be the same name as the original streaming function in the factory.
// In this case we named the method "myfunctiondefinition",
// so the dummy-bean has to get the name "Myfunctiondefinition".
public class Myfunctiondefinition implements Function<KStream<String, InputType>, 
                KStream<String, OutputType>> {

    // !!! It could be that changes are needed if spring cloud streams changes the logic
    // Method myfunctiondefinition() is needed, because spring cloud streams searches for 
    // a method with the same name as the class in 
    // FunctionDetectorCondition:pruneFunctionBeansForKafkaStreams
    public Function<KStream<String, InputType>, 
               KStream<String, OutputType>> myfunctiondefinition() {
        return null;
    }

    // Needed for the interface implementation. Spring cloud streams needs
    // the class Function to identify a stream processor candidate.
    @Override
    public KStream<String, OutputType> apply(KStream<String, InputType> input) {
        return null;
    }
}

Now that we have all things in place we can register a bean per tenant. We do this within an ApplicationContextInitializer that creates a bean definition with a factory method and iterate over the functions that we will define in the configuration file application.yaml.

public class StreamProcessorInitializer 
    implements ApplicationContextInitializer<GenericWebApplicationContext> {

    @Override
    public void initialize(GenericWebApplicationContext context) {
        String functionDefinitions = context.getEnvironment()
            .getProperty("spring.cloud.function.definition");
        String splitter = context.getEnvironment()
            .getProperty("spring.cloud.function.definition.splitter");
        String factoryName = CaseFormat.UPPER_CAMEL.
            .to(CaseFormat.LOWER_CAMEL, MyFunctionDefinitionFactory.class.getSimpleName());
        String factoryMethodName =
             MyFunctionDefinitionFactory.class.getMethods()[0].getName();

        AnnotatedGenericBeanDefinition def = 
            new AnnotatedGenericBeanDefinition(Myfunctiondefinition.class);
        def.setFactoryBeanName(factoryName);
        def.setFactoryMethodName(factoryMethodName);

        Arrays.stream(functionDefinitions.split(splitter))
           .forEach(function -> context.registerBeanDefinition(function, def));
    }
}

Finally we can dynamically define functions within the application.yaml. This can be done by helm oder kustomize to configure the specific tenant environment:

#--------------------------------------------------------------------------------------------------------------------------------------
# streaming processor functions (going to be filled by helm)
#--------------------------------------------------------------------------------------------------------------------------------------
spring.cloud.function.definition: <name1>,<name2>,...
#--Note-- required as spring cloud streams has changed the splitter in the past
spring.cloud.function.definition.splitter: ;

# Properties per function (<name>)
spring.cloud.stream.kafka.streams.binder.functions.<name>.applicationId: ${tenant}-${spring.application.name}-<name>
# configuring dlq (if you have one)
spring.cloud.stream.kafka.streams.bindings.<name>-in-0.consumer.deserializationExceptionHandler: sendToDlq
spring.cloud.stream.kafka.streams.bindings.<name>-in-0.consumer.dlqName: ${tenant}-<name>-dlq
# configuring in- and output topics
spring.cloud.stream.bindings.<name>-in-0.destination: ${tenant}-<inputname>
spring.cloud.stream.bindings.<name>-out-0.destination: ${tenant}-<outputname>