axon-springcloud module is available on the classpath. The easiest way is to include the Spring Cloud starter (axon-springcloud-spring-boot-starter) from this extension to your project.SpringCloudCommandRouter and a SpringHttpCommandBusConnector. The former is the CommandRouter and latter the CommandBusConnector, both used by the DistributedCommandBus to enable command distribution.SpringCloudCommandRouter uses Spring Cloud's discovery mechanism to find the other nodes in the cluster. To that end it uses the DiscoveryClient and Registration from Spring Cloud. These are respectively used to gather remote command routing information and maintain local information. The most straightforward way to retrieve both is by annotating your application with @EnableDiscoveryClient.ServiceInstances. A Registration is just the local ServiceInstance, whereas the DiscoveryClient provides the API to find remote ServiceInstances. Furthermore, it is the ServiceInstance which provides us with the required information (e.g. the URI) to retrieve a node's capabilities.Spring Cloud's Heartbeat RequirementWhen using theSpringCloudCommandRouter, make sure your Spring application has heartbeat events enabled. The heartbeat events published by a Spring Cloud application are the trigger to check if the set ofServiceInstances from theDiscoveryClienthas been changed. Additionally, it is used to validate whether the command routing capabilities for known nodes has been altered.Thus, if heartbeat events are disabled, your instance will no longer be updated with the current command routing capabilities. If so, this will cause issues during command routing.
ServiceInstance is maintained in the CapabilityDiscoveryMode. It is thus the CapabilityDiscoveryMode which provides us the means to actually retrieve a ServiceInstance's set of commands it can handle (if any). The sole full implementation provided of the CapabilityDiscoveryMode, is the RestCapabilityDiscoveryMode, using a RestTemplate and the ServiceInstance URI to invoke a configurable endpoint. This endpoint leads to the MemberCapabilitiesController which in turn exposes the MemberCapabilities on the RestCapabilityDiscoveryMode of that instance.CapabilityDiscoveryMode, providing two additional features:IgnoreListingDiscoveryMode - a CapabilityDiscoveryMode decorator which on failure of retrieving the MemberCapabilities will place the given ServiceInstance on a list to be ignored for future validation. It thus effectively removes discoverable ServiceInstances from the set.AcceptAllCommandsDiscoveryMode - a CapabilityDiscoveryMode decorator which regardless of what this instance can handle as commands, state it can handle anything. This decorator comes in handy if the nodes in the system are homogeneous (aka, everybody can handle the same set of commands).Registration, DiscoveryClient and CapabilityDiscoveryMode are arguably the heart of the SpringCloudCommandRouter. There are, however, a couple of additional things you can configure for this router, which are the following:RoutingStrategy - The component in charge of deciding which of the nodes receives the commands consistently. By default, a AnnotationRoutingStrategy is used (see Distributing the Command Bus for more).ServiceInstance filter - This Predicate is used to filter out ServiceInstances retrieved through the DiscoveryClient. For example, it allows the removal of instances which are known to not handle any command messages. This might be useful if you have several services within the Spring Cloud Discovery Service set up, which you do not ever want to take into account for command handling.ConsistentHashChangeListener - Adding a consistent hash change listener provides you with the opportunity to perform a specific task if new nodes have been added to the known command handlers set.Differing Command Capabilities per NodeIt is not required for all nodes to have the same set of command handlers. You may use different segments for different command types altogether. The Distributed Command Bus will always choose a node to dispatch a command to the one that has support for that specific type of command.
CommandBusConnector is in charge of sending commands, based on a given route, from one node to another. This extension to that end provides the SpringHttpCommandBusConnector, which uses plain REST for sending commands.CommandBus - This "local segment" is the command bus which dispatches commands into the local JVM. It is thus invoked when the SpringHttpCommandBusConnector receives a command from the outside, or if it receives a command which is meant for itself.RestOperations - The service used to POST a command message to another instance. In most situations the RestTemplate is used for this.Serializer - The serializer is used to serialize the command messages before they are sent over and deserialize when they are received.Executor (optional) - The Executor is used to handle incoming commands and to dispatch commands. Defaults to a DirectExecutor instance.axon-springcloud-spring-boot-starter dependency to automatically retrieve all required beans. In either case, your application should be marked to enable it as a discoverable service through Spring Cloud. This can, for example, be done by annotating the main class with @EnableDiscoveryClient.