This partial ordering may be the result of either the logical or the physical organization of the data, or it may be imposed solely for the purpose of concurrency control. The partial ordering implies that the setDmay now be viewed as a directed acyclic graph, called a database graph.The tree protocol, which is restricted to employ only exclusive locks. In the tree protocol, the only lock instruction allowed is lock-X. Each transaction Ti can lock a data item at most once, and must observe the following rules: