126. Function Catalog and Flexible Function Signatures

One of the main features of Spring Cloud Function is to adapt and support a range of type signatures for user-defined functions. So users can supply a bean of type Function<String,String> , for instance, and the FunctionCatalog will wrap it into a Function<Flux<String>,Flux<String>> . Users don’t normally have to care about the FunctionCatalog at all, but it is useful to know what kind of functions are supported in user code.

Generally speaking users can expect that if they write a function for a plain old Java type (or primitive wrapper), then the function catalog will wrap it to a Flux of the same type. If the user writes a function using Message (from spring-messaging) it will receive and transmit headers from any adapter that supports key-value metadata (e.g. HTTP headers). Here are the details.

User Function Catalog Registration
Function<S,T> Function<Flux<S>, Flux<T>>
Function<Message<S>,Message<T>> Function<Flux<Message<S>>, Flux<Message<T>>>
Function<Flux<S>, Flux<T>> Function<Flux<S>, Flux<T>> (pass through)
Supplier<T> Supplier<Flux<T>>
Supplier<Flux<T>> Supplier<Flux<T>>
Consumer<T> Function<Flux<T>, Mono<Void>>
Consumer<Message<T>> Function<Flux<Message<T>>, Mono<Void>>
Consumer<Flux<T>> Consumer<Flux<T>>

Consumer is a little bit special because it has a void return type, which implies blocking, at least potentially. Most likely you will not need to write Consumer<Flux<?>> , but if you do need to do that, remember to subscribe to the input flux. If you declare a Consumer of a non publisher type (which is normal), it will be converted to a function that returns a publisher, so that it can be subscribed to in a controlled way.

A function catalog can contain a Supplier and a Function (or Consumer ) with the same name (like a GET and a POST to the same resource). It can even contain a Consumer<Flux<>> with the same name as a Function , but it cannot contain a Consumer<T> and a Function<T,S> with the same name when T is not a Publisher because the consumer would be converted to a Function and only one of them can be registered.