In a distributed database, each transaction performs operations at several sites. The sequence of operations performed by transactions at a site is a local schedule.

An execution of n distributed transactions T1, T2,…..T„ at m sites is modeled by a set of local schedules Si, S2, , Sm.

If we apply at each node a local concurrency control mechanism, we can ensure that all local schedules are serializable. However, the serializability of local schedules is not sufficient to ensure the correctness of the execution of a set of distributed transactions. Consider, for example, the following two schedules

Both local schedules are serial; however, there is no global serial sequence of execution of both transactions because Ti < Tj, in Serial(81) and Tj < Ti, in Serial(S2). In order to guarantee the serializability of distributed transactions, a stronger condition than the serializability of local schedules is required

The execution of transactions T1…Tn is correct if:

i. Each local schedule Sk is serializable

ii. There exists a total ordering of T1... , T„ such that, if Ti < Tj, in the total ordering, then there is a serial schedule Sk' such that Sk is equivalent to Sk’ and Ti < Tj, in Serial(Sk'), for each site k where both transactions have executed some action