Editing pass (#108)
I edited for punctuation, grammar, corporate voice, and so on.
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Jay Bryant
@@ -1,2 +1,6 @@
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Spring Cloud Bus links nodes of a distributed system with a lightweight message broker. This can then be used to broadcast state changes (e.g. configuration changes) or other management instructions. A key idea is that the Bus is like a distributed Actuator for a Spring Boot application that is scaled out, but it can also be used as a communication channel between apps. Starters are provided for an AMQP broker as the transport or for Kafka, but the same basic feature set (and some more depending on the transport) is on the roadmap for other transports.
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Spring Cloud Bus links the nodes of a distributed system with a lightweight message
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broker. This broker can then be used to broadcast state changes (such as configuration
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changes) or other management instructions. A key idea is that the bus is like a
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distributed actuator for a Spring Boot application that is scaled out. However, it can
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also be used as a communication channel between apps. This project provides starters for
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either an AMQP broker or Kafka as the transport.
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@@ -1,4 +1,10 @@
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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
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Spring Cloud Bus works by adding Spring Boot autconfiguration if it detects itself on the
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classpath. To enable the bus, add `spring-cloud-starter-bus-amqp` or
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`spring-cloud-starter-bus-kafka` to your dependency management. Spring Cloud takes care of
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the rest. Make sure the broker (RabbitMQ or Kafka) is available and configured. When
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running on localhost, you need not do anything. If you run remotely, use Spring Cloud
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Connectors or Spring Boot conventions to define the broker credentials, as shown in the
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following example for Rabbit:
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.application.yml
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----
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@@ -10,6 +16,13 @@ spring:
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password: secret
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----
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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.
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The bus currently supports sending messages to all nodes listening or all nodes for a
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particular service (as defined by Eureka). The `/bus/*` actuator namespace has some HTTP
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endpoints. Currently, two are implemented. The first, `/bus/env`, sends key/value pairs to
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update each node's Spring Environment. The second, `/bus/refresh`, reloads each
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application's configuration, as though they had all been pinged on their `/refresh`
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endpoint.
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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`.
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NOTE: The Spring Cloud Bus starters cover Rabbit and Kafka, because those are the two most
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common implementations. However, Spring Cloud Stream is quite flexible, and the binder
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works with `spring-cloud-bus`.
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@@ -15,49 +15,62 @@ include::quickstart.adoc[]
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== Addressing an Instance
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Each instance of the application has a service ID, whose value can be set using `spring.cloud.bus.id`, and whose value is expected to be a colon-separated list of identifiers, in order of least specific to most specific. The default value is constructed from the environment as a combination of the `spring.application.name` and `server.port` (or `spring.application.index` if set). The default value of the ID is constructed in the form `app:index:id` where:
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Each instance of the application has a service ID, whose value can be set with
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`spring.cloud.bus.id` and whose value is expected to be a colon-separated list of
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identifiers, in order from least specific to most specific. The default value is
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constructed from the environment as a combination of the `spring.application.name` and
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`server.port` (or `spring.application.index`, if set). The default value of the ID is
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constructed in the form of `app:index:id`, where:
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* `app` is the `vcap.application.name` if it exists, or `spring.application.name`
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* `index` is the `vcap.application.instance_index` if it exists, or else `spring.application.index`, or else `local.server.port` (or `server.port` or `0`).
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* `id` is the `vcap.application.instance_id` if it exists, or else a random value.
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* `app` is the `vcap.application.name`, if it exists, or `spring.application.name`
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* `index` is the `vcap.application.instance_index`, if it exists,
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`spring.application.index`, `local.server.port`, `server.port`, or `0` (in that order).
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* `id` is the `vcap.application.instance_id`, if it exists, or a random value.
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The HTTP endpoints accept a "destination" parameter, e.g. "/bus/refresh?destination=customers:9000", where the destination is a service 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.
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The HTTP endpoints accept a "`destination`" parameter, such as
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`/bus/refresh?destination=customers:9000`, where `destination` is a service ID. If the ID
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is owned by an instance on the bus, it processes the message, and all other instances
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ignore it.
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== Addressing all instances of a service
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== Addressing All Instances of a Service
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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 rest of the service ID.
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The "`destination`" parameter is used in a Spring `PathMatcher` (with the path separator
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as a colon -- `:`) to determine if an instance processes the message. Using the example
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from earlier, `/bus/refresh?destination=customers:**` targets all instances of the
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"`customers`" service regardless of the rest of the service ID.
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== Service ID must be unique
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== Service ID Must Be Unique
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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 service ID againts the current service ID. If multiple instances of a service have the same 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 ID. Cloud Foundry supplies an index to differentiate. To ensure that the ID is unique outside Cloud Foundry, set `spring.application.index` to something unique for each instance of a service.
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The bus tries twice to eliminate processing an event -- once from the original
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`ApplicationEvent` and once from the queue. To do so, it checks the sending service ID
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against the current service ID. If multiple instances of a service have the same ID,
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events are not processed. When running on a local machine, each service is on a different
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port, and that port is part of the ID. Cloud Foundry supplies an index to differentiate.
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To ensure that the ID is unique outside Cloud Foundry, set `spring.application.index` to
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something unique for each instance of a service.
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== Customizing the Message Broker
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Spring Cloud Bus uses
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https://cloud.spring.io/spring-cloud-stream[Spring Cloud Stream] to
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broadcast the messages so to get messages to flow you only need to
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include the binder implementation of your choice in the
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classpath. There are convenient starters specifically for the bus with
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AMQP (RabbitMQ) and Kafka
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(`spring-cloud-starter-bus-[amqp,kafka]`). Generally speaking
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Spring Cloud Stream relies on Spring Boot autoconfiguration
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conventions for configuring middleware, so for instance the AMQP
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broker address can be changed with `spring.rabbitmq.{asterisk}`
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configuration properties. Spring Cloud Bus has a handful of native
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configuration properties in `spring.cloud.bus.{asterisk}`
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(e.g. `spring.cloud.bus.destination` is the name of the topic to use
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the the externall middleware). Normally the defaults will suffice.
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Spring Cloud Bus uses https://cloud.spring.io/spring-cloud-stream[Spring Cloud Stream] to
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broadcast the messages. So, to get messages to flow, you need only include the binder
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implementation of your choice in the classpath. There are convenient starters for the bus
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with AMQP (RabbitMQ) and Kafka (`spring-cloud-starter-bus-[amqp|kafka]`). Generally
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speaking, Spring Cloud Stream relies on Spring Boot autoconfiguration conventions for
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configuring middleware. For instance, the AMQP broker address can be changed with
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`spring.rabbitmq.{asterisk}` configuration properties. Spring Cloud Bus has a handful of
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native configuration properties in `spring.cloud.bus.{asterisk}` (for example,
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`spring.cloud.bus.destination` is the name of the topic to use as the external
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middleware). Normally, the defaults suffice.
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To lean more about how to customize the message broker settings
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consult the Spring Cloud Stream documentation.
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To learn more about how to customize the message broker settings, consult the Spring Cloud
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Stream documentation.
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== Tracing Bus Events
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Bus events (subclasses of `RemoteApplicationEvent`) can be traced by
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setting `spring.cloud.bus.trace.enabled=true`. If you do this then the
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Spring Boot `TraceRepository` (if it is present) will show each event
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sent and all the acks from each service instance. Example (from the
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`/trace` endpoint):
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Bus events (subclasses of `RemoteApplicationEvent`) can be traced by setting
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`spring.cloud.bus.trace.enabled=true`. If you do so, the Spring Boot `TraceRepository`
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(if it is present) shows each event sent and all the acks from each service instance. The
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following example comes from the `/trace` endpoint:
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[source,json]
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----
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@@ -93,50 +106,50 @@ sent and all the acks from each service instance. Example (from the
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}
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----
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This trace shows that a `RefreshRemoteApplicationEvent` was sent from
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`customers:9000`, broadcast to all services, and it was received
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(acked) by `customers:9000` and `stores:8081`.
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The preceding trace shows that a `RefreshRemoteApplicationEvent` was sent from
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`customers:9000`, broadcast to all services, and received (acked) by `customers:9000` and
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`stores:8081`.
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To handle the ack signals yourself you could add an `@EventListener`
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for the `AckRemoteApplicationEvent` and `SentApplicationEvent` types
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to your app (and enable tracing). Or you could tap into the
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`TraceRepository` and mine the data from there.
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To handle the ack signals yourself, you could add an `@EventListener` for the
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`AckRemoteApplicationEvent` and `SentApplicationEvent` types to your app (and enable
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tracing). Alternatively, you could tap into the `TraceRepository` and mine the data from
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there.
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NOTE: Any Bus application can trace acks, but sometimes it will be
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NOTE: Any Bus application can trace acks. However, sometimes, it is
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useful to do this in a central service that can do more complex
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queries on the data. Or forward it to a specialized tracing service.
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queries on the data or forward it to a specialized tracing service.
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== Broadcasting Your Own Events
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The Bus can carry any event of type `RemoteApplicationEvent`, but the
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default transport is JSON and the deserializer needs to know which
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types are going to be used ahead of time. To register a new type it
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needs to be in a subpackage of `org.springframework.cloud.bus.event`.
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The Bus can carry any event of type `RemoteApplicationEvent`. The default transport is
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JSON, and the deserializer needs to know which types are going to be used ahead of time.
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To register a new type, you must put it in a subpackage of
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`org.springframework.cloud.bus.event`.
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To customise the event name you can use `@JsonTypeName` on your custom class
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or rely on the default strategy which is to use the simple name of the class.
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Note that both the producer and the consumer will need access to the class
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definition.
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To customise the event name, you can use `@JsonTypeName` on your custom class or rely on
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the default strategy, which is to use the simple name of the class.
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NOTE: Both the producer and the consumer need access to the class definition.
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=== Registering events in custom packages
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If you cannot or don't want to use a subpackage of `org.springframework.cloud.bus.event`
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for your custom events, you must specify which packages to scan for events of
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type `RemoteApplicationEvent` using `@RemoteApplicationEventScan`. Packages
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If you cannot or do not want to use a subpackage of `org.springframework.cloud.bus.event`
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for your custom events, you must specify which packages to scan for events of type
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`RemoteApplicationEvent` by using the `@RemoteApplicationEventScan` annotation. Packages
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specified with `@RemoteApplicationEventScan` include subpackages.
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For example, if you have a custom event called `FooEvent`:
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For example, consider the following custom event, called `MyEvent`:
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[source,java]
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----
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package com.acme;
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public class FooEvent extends RemoteApplicationEvent {
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public class MyEvent extends RemoteApplicationEvent {
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...
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}
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----
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you can register this event with the deserializer in the following way:
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You can register that event with the deserializer in the following way:
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[source,java]
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----
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@@ -150,11 +163,12 @@ public class BusConfiguration {
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----
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Without specifying a value, the package of the class where `@RemoteApplicationEventScan`
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is used will be registered. In this example `com.acme` will be registered using the
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package of `BusConfiguration`.
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is used is registered. In this example, `com.acme` is registered by using the package of
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`BusConfiguration`.
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You can also explicitly specify the packages to scan using the `value`, `basePackages` or
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`basePackageClasses` properties on `@RemoteApplicationEventScan`. For example:
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You can also explicitly specify the packages to scan by using the `value`, `basePackages`
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or `basePackageClasses` properties on `@RemoteApplicationEventScan`, as shown in the
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following example:
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[source,java]
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----
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@@ -169,7 +183,8 @@ public class BusConfiguration {
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}
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----
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All examples of `@RemoteApplicationEventScan` above are equivalent,
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in that the `com.acme` package will be registered by explicitly specifying the
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packages on `@RemoteApplicationEventScan`. Note, you can specify multiple base
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packages to scan.
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All of the preceding examples of `@RemoteApplicationEventScan` are equivalent, in that the
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`com.acme` package is registered by explicitly specifying the packages on
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`@RemoteApplicationEventScan`.
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NOTE: You can specify multiple base packages to scan.
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