isClosed() considered to be true invariant or not?

[blindvirtuoso]

IDDD_Samples/iddd_collaboration/src/main/java/com/saasovation/collaboration/domain/model/forum/Forum.java

Line 159 in 9b27b11

if (this.isClosed()) {

Hi Vaughn.

A discussion can not be started on closed forum.
Isn't it the "true invariant" and then mustn't Discussion be a part of Forum aggregate?
And if it's really not, can you please clarify?

Thanks a lot.

[VaughnVernon]

No. Read my red book.

[blindvirtuoso]

No. Read my red book.

The thing is that I've read it, but could't grasp the point

[VaughnVernon]

First, consider the practicality of composing potentially hundreds, thousands, or even millions of discussions under one parent forum. Bad idea. The same goes for posts within a discussion.

You are thinking too technically, as if forum owners constantly toggle forums closed, open, closed, open, ... When a forum is closed, it's closed and rarely reopened. Eventual consistency handles the rest.

But what if a new discussion is started just before it's parent forum is closed? True answer: Who cares? It's not as if an insane dictator presses The Red Button (tm).

[blindvirtuoso]

First, consider the practicality of composing potentially hundreds, thousands, or even millions of discussions under one parent forum. Bad idea. The same goes for posts within a discussion.

You are thinking too technically, as if forum owners constantly toggle forums closed, open, closed, open, ... When a forum is closed, it's closed and rarely reopened. Eventual consistency handles the rest.

But what if a new discussion is started just before it's parent forum is closed? True answer: Who cares? It's not as if an insane dictator presses The Red Button (tm).

Vaughn, thank you for the reply.

And just for a silly case if that was indeed important to start a discussion only on open forum.
Could that be a business invariant? Or a business invariant implies multiple states?
In the red book, there is an example c = a + b where a, b and c are different states.
In my example above there's just one state forum.isClosed.

I mean there are no multiple states on forum and discussion which need to be consistent for "a discussion is only allowed to be started on open forum" business rule.

[VaughnVernon]

If I understand your question, yes I think it would be an invariant because a modified forum.discussions collection (one new discussion) requires forum.open==true.

To other readers, this answer is based on a hypothetical question and does not influence on modeling a new aggregate boundary. In other words, the preexisting business rules and consistency constraints have not changed.

[blindvirtuoso]

That's much clear now.
Thanks, Vaughn!

[redaLaanait]

Backlink: https://x.com/VaughnVernon/status/1797375961782010003

…

On Mon, 3 Jun 2024 at 00:16, IAlkhasov ***@***.***> wrote: That's much clear now. Thanks, Vaughn! — Reply to this email directly, view it on GitHub <#55 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ABWB552HCW2LKFUKOD7ET6TZFORURAVCNFSM6AAAAABISTGATKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDCNBUGA2TGNBTGE> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>