33. Stream applications with multiple binder configurations

 In some cases, a stream can have its applications bound to multiple spring cloud stream binders when they are required to connect to different messaging
middleware configurations. In those cases, it is important to make sure the applications are configured appropriately with their binder
configurations. For example, let's consider the following stream:
http | transform --expression=payload.toUpperCase() | log

and in this stream, each application connects to messaging middleware in the following way:

Http source sends events to RabbitMQ (rabbit1)
Transform processor receives events from RabbitMQ (rabbit1) and sends the processed events into Kafka (kafka1)
Log sink receives events from Kafka (kafka1)

Here, rabbit1 and kafka1 are the binder names given in the spring cloud stream application properties. Based on this setup, the applications will have the following binder(s) in their classpath with the appropriate configuration:

Http - Rabbit binder
Transform - Both Kafka and Rabbit binders
Log - Kafka binder

The spring-cloud-stream binder configuration properties can be set within the applications themselves. If not, they can be passed via deployment properties when the stream is deployed.

For example,

dataflow:>stream create --definition "http | transform --expression=payload.toUpperCase() | log" --name mystream
dataflow:>stream deploy mystream --properties "app.http.spring.cloud.stream.bindings.output.binder=rabbit1,app.transform.spring.cloud.stream.bindings.input.binder=rabbit1,

One can override any of the binder configuration properties by specifying them via deployment properties.