If I interpret this sequence diagram correctly, in case of invalid input, the Error message should end the activation of Sign Up Service. That is, just because a branch is the last one it shouldn’t be the one deactivating the participant, as activation doesn’t fall through branches (well, I think it shouldn’t) since they represent alternate flows of execution. In other words, if the valid input branch went first, it would end up activation of Sign Up Service and the error message would appear to be sent out of activation.
I imagine this is all due to a limitation of Mermaid, as I tried to deactivate SUS in the error message and got an error. I would report this to Mermaid but wanted to double check first, I also imagine someone else has already reported it.
The way I see it, the activation should end on the error message, and start again on the line dividing alt branches, to represent that on this branch, activation remains on since the GET /sign_up message.
This is an interesting one, and I can’t actually find any consensus on how it should render whether it be Mermaid or otherwise. I can see why Mermaid has done what it’s done, as ending the activation after the error and starting a new block in the alt would separate the initial activation from the alt case (the rectangle would be broken up).
I’m going to ponder on this one, as the alternative is put the start of the interaction in each alt branch, and then it works, but that feels wrong to me too, as I’d rather create a separate diagram at that point than create duplication.
Here’s the Mermaid, just to show what I mean in case it’s not obvious from my description:
sequenceDiagram
actor Browser
participant Sign Up Service
participant User Service
participant Kafka
Browser->>Sign Up Service: GET /sign_up
activate Sign Up Service
Sign Up Service-->>Browser: 200 OK (HTML page)
deactivate Sign Up Service
alt invalid input
Browser->>+Sign Up Service: POST /sign_up
Sign Up Service->>Sign Up Service: Validate input
Sign Up Service-->>-Browser: Error
else valid input
Browser->>+Sign Up Service: POST /sign_up
Sign Up Service->>Sign Up Service: Validate input
Sign Up Service->>+User Service: POST /users
User Service--)Kafka: User Created Event Published
User Service-->>-Sign Up Service: 201 Created (User)
Sign Up Service-->>-Browser: 301 Redirect (Login page)
end
Yes, it was totally clear, and I agree that extracting the common messages is preferable to duplicating them, which is done only so that activation remains unbroken across branches.
I don’t think that “resuming” the activation at the start of each branch would be wrong, but it’s apparent that Mermaid’s authors don’t agree with me. I’ll raise that issue directly with them.
