@PollableBean for Reactive Suppliers in Spring Cloud Stream

Supplier beans, or functions that only publish messages in Spring Cloud Stream, are a bit special in that they aren't triggered by the receiving of events like Function or Consumer beans. This means that you often need a way to trigger them to be executed periodically.

For imperative functions the framework by default "polls" a Supplier function every 1 second, but that duration is configurable using the spring.cloud.stream.poller.fixed-delay property.

However, for reactive functions supplying a Flux it is only triggered once by default. This is because a Flux itself is potentially an infinite stream of events so in many cases it will only need to be triggered once. But don't worry, if you want to periodically trigger a reactive Supplier because you are producing a finite stream of events you can still do so using @PollableBean. This annotation then allows you to configure how often the function is triggered using the same spring.cloud.stream.poller.fixed-delay property!

One example use case here could be periodically querying a data store and publishing each entry/row as an event. The number of rows in your data store is a finite number at any given time.

Example code:

@PollableBean
public Supplier<Flux<String>> stringSupplier() {
   return () -> Flux.just("foo","bar","baz");
}