A multidatabase system is a high-level confederation of a number of pre-existing, autonomous and possibly heterogeneous databases. In this environment, transaction management is performed at two levels, at a local level by the transaction managers (LTMs) of the pre-existing databases, and at a global level by a global transaction manager (GTM). There are two types of data, local data that correspodn to data that existed prior to the integration and are related with integrity constraints to data lcoated at the same database, and global data that are created after the integration and may be related with itnegrity constraints to lcoal data at the same database or to global data at the saem or different databases. We distinguish two types of transactions, local and global transactions. Local transactions access data items at a single database and are outside the control of the FTM. Local transactions correspond either to applications that existed prior to integration or to applications that were written after the integration and access data at single database. Local transactions can read but not write global data. Global transactions are submitted to the GTM, where they are parsed into a number of global subtrans-actions, each of which accesses data at a single database. These substransactions are then submitted for execution to the appropriate LTM. The GTM has no control over the global subtransactions after their submission to the LTM.
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