I have a desktop application that runs on a network and every instance connects to the same database.

So, in this situation, how can I implement a mutex that works across all running instances that are connected to the same database? 

In other words, I don't wan't that two+ instances to run the same function at the same time. If one is already running the function, the other instances shouldn't have access to it.

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PS: Database transaction won't solve, because the function I wan't to mutex doesn't use the database. I've mentioned the database just because it can be used to exchange information across the running instances.

PS2: The function takes about ~30 minutes to complete, so if a second instance tries to run the same function I would like to display a nice message that it can't be performed right now because computer 'X' is already running that function.

PS3: The function has to be processed on the client machine, so I can't use stored procedures.