Generalized quasi serializability theory

Quasi serializability is a correctness criterion for global concurrency control in multidatabase systems (MDBSs) [1]. A global execution is quasi serializable if it is equivalent to a quasi serial execution in which global transactions are executed sequentially. The main motivation for quasi serializability is the difficulties in maintaining global serializability in MDBSs, due to local autonomy and incompatibility between serialization order and execution order of global transactions [3]. It is generally impossible for the global concurrency controller to maintain global serializability by controlling submission of global transactions only. Local transactions may introduce indirect conflicts between global transactions of which the global concurrency controller is not aware. Quasi serializability is different from serializability in that it coordinates executions of global transactions only and ignores those indirect conflicts that do not introduce interactions between global transactions. As a result of the relaxation, quasi serialization order of global transactions is compatible with their execution order. Thus, quasi serializability can be effectively maintained at the global level without violating local autonomy. The price for this is, however, the possible mutual interactions between remote local transactions. Therefore, quasi serializability is most suited to those MDBSs where local transactions at different sites do not interact with each other. It may also be used in more general MDBSs. Undesirable remote interactions among local transactions can be prevented by controlling information flow between sites through global transactions [4].