Spring Cloud Bus works by adding Spring Boot autconfiguration if it detects itself on the classpath. All you need to do to enable the bus is to add spring-cloud-starter-bus-amqp or spring-cloud-starter-bus-kafka to your dependency management and Spring Cloud takes care of the rest. Make sure the broker (RabbitMQ or Kafka) is available and configured: running on localhost you shouldn’t have to do anything, but if you are running remotely use Spring Cloud Connectors, or Spring Boot conventions to define the broker credentials, e.g. for Rabbit

application.yml. 

spring:
  rabbitmq:
    host: mybroker.com
    port: 5672
    username: user
    password: secret

The bus currently supports sending messages to all nodes listening or all nodes for a particular service (as defined by Eureka). More selector criteria may be added in the future (ie. only service X nodes in data center Y, etc…​). There are also some http endpoints under the /bus/* actuator namespace. There are currently two implemented. The first, /bus/env, sends key/value pairs to update each node’s Spring Environment. The second, /bus/refresh, will reload each application’s configuration, just as if they had all been pinged on their /refresh endpoint.

	Note
	The Bus starters cover Rabbit and Kafka, because those are the two most common implementations, but Spring Cloud Stream is quite flexible and binder will work combined with spring-cloud-bus.

The HTTP endpoints accept a "destination" parameter, e.g. "/bus/refresh?destination=customers:9000", where the destination is an ApplicationContext ID. If the ID is owned by an instance on the Bus then it will process the message and all other instances will ignore it. Spring Boot sets the ID for you in the ContextIdApplicationContextInitializer to a combination of the spring.application.name, active profiles and server.port by default.

The "destination" parameter is used in a Spring PathMatcher (with the path separator as a colon :) to determine if an instance will process the message. Using the example from above, "/bus/refresh?destination=customers:**" will target all instances of the "customers" service regardless of the profiles and ports set as the ApplicationContext ID.

The bus tries to eliminate processing an event twice, once from the original ApplicationEvent and once from the queue. To do this, it checks the sending application context id againts the current application context id. If multiple instances of a service have the same application context id, events will not be processed. Running on a local machine, each service will be on a different port and that will be part of the application context id. Cloud Foundry supplies an index to differentiate. To ensure that the application context id is the unique, set spring.application.index to something unique for each instance of a service. For example, in lattice, set spring.application.index=${INSTANCE_INDEX} in application.properties (or bootstrap.properties if using configserver).

Spring Cloud Bus uses Spring Cloud Stream to broadcast the messages so to get messages to flow you only need to include the binder implementation of your choice in the classpath. There are convenient starters specifically for the bus with AMQP (RabbitMQ) and Kafka (spring-cloud-starter-bus-[amqp,kafka]). Generally speaking Spring Cloud Stream relies on Spring Boot autoconfiguration conventions for configuring middleware, so for instance the AMQP broker address can be changed with spring.rabbitmq.* configuration properties. Spring Cloud Bus has a handful of native configuration properties in spring.cloud.bus.* (e.g. spring.cloud.bus.destination is the name of the topic to use the the externall middleware). Normally the defaults will suffice.

To lean more about how to customize the message broker settings consult the Spring Cloud Stream documentation.

Bus events (subclasses of RemoteApplicationEvent) can be traced by setting spring.cloud.bus.trace.enabled=true. If you do this then the Spring Boot TraceRepository (if it is present) will show each event sent and all the acks from each service instance. Example (from the /trace endpoint):

{
  "timestamp": "2015-11-26T10:24:44.411+0000",
  "info": {
    "signal": "spring.cloud.bus.ack",
    "type": "RefreshRemoteApplicationEvent",
    "id": "c4d374b7-58ea-4928-a312-31984def293b",
    "origin": "stores:8081",
    "destination": "*:**"
  }
  },
  {
  "timestamp": "2015-11-26T10:24:41.864+0000",
  "info": {
    "signal": "spring.cloud.bus.sent",
    "type": "RefreshRemoteApplicationEvent",
    "id": "c4d374b7-58ea-4928-a312-31984def293b",
    "origin": "customers:9000",
    "destination": "*:**"
  }
  },
  {
  "timestamp": "2015-11-26T10:24:41.862+0000",
  "info": {
    "signal": "spring.cloud.bus.ack",
    "type": "RefreshRemoteApplicationEvent",
    "id": "c4d374b7-58ea-4928-a312-31984def293b",
    "origin": "customers:9000",
    "destination": "*:**"
  }
}

This trace shows that a RefreshRemoteApplicationEvent was sent from customers:9000, broadcast to all services, and it was received (acked) by customers:9000 and stores:8081.

To handle the ack signals yourself you could add an @EventListener for the AckRemoteApplicationEvent and SentApplicationEvent types to your app (and enable tracing). Or you could tap into the TraceRepository and mine the data from there.

	Note
	Any Bus application can trace acks, but sometimes it will be useful to do this in a central service that can do more complex queries on the data. Or forward it to a specialized tracing service.

The Bus can carry any event of type RemoteApplicationEvent, but the default transport is JSON and the deserializer needs to know which types are going to be used ahead of time. To register a new type it needs to be in a subpackage of org.springframework.cloud.bus.event.

To customise the event name you can use @JsonTypeName on your custom class or rely on the default strategy which is to use the simple name of the class. Note that both the producer and the consumer will need access to the class definition.

package com.acme;

public class FooEvent extends RemoteApplicationEvent {
    ...
}

package com.acme;

@Configuration
@RemoteApplicationEventScan
public class BusConfiguration {
    ...
}

package com.acme;

@Configuration
//@RemoteApplicationEventScan({"com.acme", "foo.bar"})
//@RemoteApplicationEventScan(basePackages = {"com.acme", "foo.bar", "fizz.buzz"})
@RemoteApplicationEventScan(basePackageClasses = BusConfiguration.class)
public class BusConfiguration {
    ...
}