Multigranularity Locking with the use of Semantic Knowledge in a Layered Object Server

Object-oriented database programming languages use a data model that, by its nature, leads to a hierarchical organisation of persistent data. The Multigranularity Locking (MGL) protocol is the concurrency control framework that allows to better analyse concurrent accesses to such hierarchy of data items. Furthermore, modern Object-Oriented Database Management Systems are organised accordingly to the client-server architecture, where the server component is often an object server. The application of software engineering criteria to the design of an object server usually leads to a system structured in interpretation layers. In a layered object server, the semantic knowledge necessary to decide which is the "best" granule to lock in the MGL data item hierarchy is distributed among all the system layers, therefore a suitable technique to co-ordinate such decisions is necessary. This paper presents some guidelines on the design of the hierarchical organisation of data items that should be used from an Object-Oriented Database Management System supporting the MGL protocol, and an original concurrency control technique called Expandable MGL that provides all the system layers with the ability of locking those granules that each layer considers more appropriate on the basis of its partial knowledge of a transaction's behaviour.