Selecting an unused network and updating it (extended attributes)

[#1]

I would like to understand which is the best way to implement the following scenario through REST APIs:

I have a set of networks already defined and an Extended Attribute is used to track if the network is in use or not.

I would like to search for a not used network and then update the Extended Attribute to flag the network as in use.

I'm having difficulties identifying a mechanism that would allow me to implement the search and update operation in an atomic way: I do not want that two concurrently running queries identify the same network as free and then try to update and use the same network "twice".

Thanks in advance for the help.