Sequence diagram with concurrent access

[flitjens]

I have a question of which I think I have the wrong idea since this question seems to be questioned more than once since the early 2000's and still I miss something. How do I model a blocking collection in a uml sequence diagram. Now I model it like:

• A consumer doing a blocking call item = collection.Take() to the blocking collection (Thread A)
• An another objects adds an item to the collection collection.Add(item) (Thread B)
• A consumer receives a return with item value (Thread A)

but I have the idea that it should be different, since each call is a seperate message group while I expect the consumer call and return to be a single message group because it is in a different context.

Kind regards,
Frank Litjens