UML Class diagram for Chat

I'm designing a UML class diagram for a messaging app.

Each Chat must have exactly 2 Users, and each User can be part of multiple Chats. I originally modeled this with a composition from User to Chat.

However, I'm not sure if this is correct.

My goal:

• A Chat is between exactly two Users.
• Users are not owned by the chat and can exist independently.
• If both users are deleted, their chat is also deleted.

Should I be using a normal association instead of composition here? What’s the best way to represent this in UML?

Thanks in advance!

each User can be part of multiple Chats

so you have to replace the multiplicity 1 by * (or 0..* if you prefer) on the composition User / Chat

Should I be using a normal association instead of composition here

pro of a composition : the composition indicates that If both users are deleted, their chat is also deleted

cons of a composition : the main is a composite aggregation is a strong form of aggregation that requires a part object be included in at most one composite (c.f. formal/2017-12-05 §9.5.3 Semantics page 112), and also independentely of the multiplicity a composition is first an agregation and a Chat is not a part of a User

But notice the multiplicity 2 on the association User / Chat implies a Chat needs 2 associated Users and cannot exist without them, restricting its life.

So for me you must use a normal/simple association :

Out of that, are you sure a Message must know in which Chat it is ? If no Chat <*>-1----*-> Message is enough :