Chapter specific transition within a saga

[Pavneet Singh Saund]

When transitioning between chapters in a saga, there's a concept of ChapterValidation. Given a successful validation, the transitions completes, otherwise it fails: 

public SomeChapter : Chapter, ICanTranstionTo<AnotherChapter> {...}

public class SomeChapterValidator : ChapterValidator<SomeChapter> {...}

 

sagaNarrator.TransitionTo<AnotherChapter>(saga); //success or fail based on SomeChapterValidator

This falls through a bit, when implementing flow between chapters, where different validation applies to different transitions. In the following example, if SecondChapter isn't valid, it cant transition back to FirstChapter

Example:

public SecondChapter : Chapter, ICanTranstionTo<FirstChapter>, ICanTransitionTo<LastChapter> {...}

public class SecondChapterValidator : ChapterValidator<SecondChapter> {...} 

sagaNarrator.TransitionTo<FirstChapter>(saga); //fails, even though we might not want validation when going back to first step

Given that Chapters build up a state machine, it leads me to believe that transitions should be able to validate differently. Meaning that different transitions should have it's own validators. Or rather be specific when a specific transition needs validation. Something on the lines of:

public class SecondChapterValidator : ChapterValidatorBetween<SecondChapter,LastChapter> {...}  //complex rules

public class SecondChapterValidator : ChapterValidatorBetween<SecondChapter,FirstChapter> {...}  //no rules, or just non existent

Given CurrentChapter is SecondChapter:

sagaNarrator.TransitionTo<LastChapter>(saga); // fails if is unvalid

sagaNarrator.TransitionTo<FirstChapter>(saga); // fails if is unvalid 

 

 

Thoughts?

[Pavneet Singh Saund]

Ignore the obvious syntax typos :)

[Pavneet Singh Saund]

Alternatively, there could be something that requires you to specify if validation is desired between transitions. Or maybe even build in a state-machine flow, where moving back doesn't require validation, but this does seem a little naive in the long run

[Michael Smith]

We've already run into problems with ChapterValidation.  The ChapterValidation validates that the chapter you are transitioning from is in a valid state (which is reasonable enough) but not that the chapter you are transitioning to has it's pre-requisites taken care of.  If I remember right, it was we needed the Payment Credentials in the PlaceOrderChapter as we would perform the reservation in the OnTransitionedTo event.  I think I ended up letting the absence of credentials mean that the attempt to perform the reservation would throw an exception and the transition would therefore fail.

The transitioning / validation is too simplistic.  We shouldn't be validating the chapter, we should validate the transition between two chapters (a from and a to).  Though we've also spoken about making the Saga itself a proper state machine in which case it would handle state transitions internally, rather than rely on something external to say "try and transition to the next chapter".

[Pavneet Singh Saund]

Saga as a statemachine, handling transistions makes sense. The ChapterTransitionValidator is still going to have to play a part in that logic as well, methinks.

I think I'll go ahead and make an issue on GH for this, and we can continue things there.