From 1fba66d105702b465a1e91c59954c757d60e91eb Mon Sep 17 00:00:00 2001 From: Mark van Renswoude Date: Sun, 19 Feb 2017 10:59:34 +0100 Subject: [PATCH] Started flow documentation --- docs/flow.rst | 40 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 40 insertions(+) diff --git a/docs/flow.rst b/docs/flow.rst index 9e84e94..f587913 100644 --- a/docs/flow.rst +++ b/docs/flow.rst @@ -1,6 +1,46 @@ Flow extension ============== +*Flow* in the context of Tapeti is inspired by what is referred to as a Saga or Conversation in messaging. It enables a controller to communicate with other services, temporarily yielding it's execution while waiting for a response. When the response arrives the controller will resume, retaining the original state of it's public fields. + +This process is fully asynchronous, the service initiating the flow can be restarted and the flow will continue when the service is back up (assuming the queues are durable and a persistent flow state store is used). + + +Request - response pattern +-------------------------- +Tapeti implements the request - response pattern by allowing a message handler method to simply return the response message. Tapeti Flow expands on this idea by enforcing you to explicitly declare that pattern. This is intended to prevent bugs where a flow will forever wait on a response that never comes. + +Due to this requirement your messages need to follow a pattern where broadcasts (events) are separated from true request - response messages (where the reply is sent directly to the originator). + +This may result in a message having two versions; one where a reply is expected and one where it's not. This is not considered a design flaw but a clear contract between services. An example is given in `Request - response versus transfer of responsibility`_. + +Declaring request - response messages +------------------------------------- +A message must be annotated with the Request attribute, where the Response property declares the expected response message class. + +:: + + [Request(Response = RabbitCountResponseMessage)] + public class RabbitCountRequestMessage + { + public int? MinimumAge { get; set; } + } + + public class RabbitCountResponseMessage + { + public int Count { get; set; } + } + + +Starting a flow +--------------- + +Ending a flow +------------- + +Request - response versus transfer of responsibility +---------------------------------------------------- + .. error:: You've stumbled upon a piece of unfinished documentation. Behind you is all prior knowledge. In front of you is nothing but emptyness. What do you do?