By default, Spring Cloud Stream relies on Spring Boot’s auto-configuration to configure the binding process. If a single binder implementation is found on the classpath, Spring Cloud Stream will use it automatically. So, for example, a Spring Cloud Stream project that aims to bind only to RabbitMQ can simply add the following dependency:

<dependency>
  <groupId>org.springframework.cloud</groupId>
  <artifactId>spring-cloud-stream-binder-rabbit</artifactId>
</dependency>

When multiple binders are present on the classpath, the application must indicate which binder is to be used for each channel binding. Each binder configuration contains a META-INF/spring.binders, which is a simple properties file:

rabbit:\
org.springframework.cloud.stream.binder.rabbit.config.RabbitServiceAutoConfiguration

Similar files exist for the other binder implementations (e.g. Kafka), and it is expected that custom binder implementations will provide them, too. The key represents an identifying name for the binder implementation, whereas the value is a comma-separated list of configuration classes that contain one and only one bean definition of the type org.springframework.cloud.stream.binder.Binder.

Selecting the binder can be done globally by either using the spring.cloud.stream.defaultBinder property, e.g. spring.cloud.stream.defaultBinder=rabbit, or by individually configuring them on each channel binding.

For instance, a processor app that reads from Kafka and writes to Rabbit can specify the following configuration: spring.cloud.stream.bindings.input.binder=kafka,spring.cloud.stream.bindings.output.binder=rabbit.

By default, binders share the Spring Boot auto-configuration of the application and create one instance of each binder found on the classpath. In scenarios where an application should connect to more than one broker of the same type, Spring Cloud Stream allows you to specify multiple binder configurations, with different environment settings. Please note that turning on explicit binder configuration will disable the default binder configuration process altogether, so all the binders in use must be included in the configuration.

For example, this is the typical configuration for a processor that connects to two RabbitMQ broker instances:

spring:
  cloud:
    stream:
      bindings:
        input:
          destination: foo
          binder: rabbit1
        output:
          destination: bar
          binder: rabbit2
      binders:
        rabbit1:
          type: rabbit
          environment:
            spring:
              rabbitmq:
                host: <host1>
        rabbit2:
          type: rabbit
          environment:
            spring:
              rabbitmq:
                host: <host2>