Transaction in a 3-tier application

Is it possible for application server to start a transaction through a method called by the client? Can someone tell me how to do it?
I want to control the stock for each item of a sale, and for every item in the clientdataset before the applyupdates I want to manage the stock for every user. If the client apllication is closed by any reason I need to do a rollback otherwise I execute commit.

Thanks for your atention.