Hello just a quick question. Is it possible to check in a Precondition of WriteRelationshipRequest if there would be any relationship left if i would delete it ?

What's the use-case? Unless you need to write and delete at the same time, and you need the delete to really happen in a single transaction, I'd suggest you look into

DeleteRelationships

, which will tell you in the response if there are any more relationships to delete when you specify a limit. Othewise you could check after the transaction takes place:

WriteRelationships

will give you a zedtoken, and so you could then issue

ReadRelationships

at the specified token, which would give you the state of the system when the write took place.

Thank you. I think I just have to do the check afterwards and roll back if necessary.

I think the solution is simple in this case. You read one supervisor that is not the one about to be deleted. Then you proceed with a

WriteRelationships

that has a precondition "this supervisor must exist before proceeding". If the precondition fails, then you know some other transaction deleted a supervisor, so all you have to do is retry again.

Genius anwer and thread safe. Thank you