Implement nested commands

[cer]

Currently

• a command handler returns a reply message.
• The command handler cannot be implemented using a command

Proposed:

Support command handlers that

• Return void
• Have a parameter - CommandReplyToken - that contains the data needed to construct a reply, e.g. message headers etc.
• CommandReplyToken can be serialized/stored in a database and used to later to reply

[cer]

• More overloaded CommandHandlersBuilder methods
• Nested reactive command handlers

[cer]

Example command handler:

Example command handler:

   public void handleComplexCommand(CommandMessage<TestComplexCommand> cm, CommandReplyToken replyToken) {
               // save replyToken for later
    }

Example of sending a reply later

Use the CommandReplyProducer @Bean

@Autowired
private CommandReplyProducer commandReplyProducer;

....
CommandReplyToken replyToken = .... ; // from earlier
List<Message> replies = commandReplyProducer.sendReplies(replyToken, Collections.singletonList(withSuccess()));

[cer]

eventuate-tram/eventuate-tram-sagas@0bfe1df reflects changes made by 66d8bb7