20-07-2012, 01:05 PM
An Extended Transaction Model for Multidatabase Systems
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Introduction
A Multidatabase System(MDBS) Is a facility that allows access to data. stored
in multiple autonomous and possibly heterogeneous database systems. Although
transaction management in such systems has been a. subject of extensive
research, many problems remain unresolved because of the complexity
caused by data. distribution, heterogeneity and the need to preserve autonomy
of the member da.tabase systems. The research has been concentra.ted
mostly on developing concurrency control and commitment protocols suit.
ble for these new environments [EH88][pu88J(AGMS87J(BST87J(BS88]. In
another direction, the basic notion of multidatabase consistency has been reexamined
,leading to the emergence of the new pa.radigms that reject serializability
as the correctness criterion for multidatabase systems [LT88][DE89].
Potential for long-lived transactions
A multidatabase transaction
is potentially long-lived since its execution time may span over a long period
of time. There are several factors which may contribute to the long
execution time of a global transaction. Frequently, a multidatabase transaction
involves sites and LDBSs with vastly different speeds and capabilities.
In order to enforce agreement protocols in such environment (such as in
the case of deciding whether a MDBS transaction can be committed), the
MDBS software must proceed at a rate determined by the slowest member
LDBS that may run on a microcomputer and communicate over serial lines.
Similarly, due to the execution autonomy, a local database may decide to
delay the execution of a subtransaction of a global transaction, which in turn
delays the completion of the global transaction. Finally, the communication
delay on the heterogeneous communication network may also contribute to
the long execution time of a global transaction. These factors may delay the
completion of a global transaction even though the global transaction is not
long-lived by itself.
Transaction Model for MDBS
The most widely accepted transaction processing model for a MDBS has
been proposed by Gligor and Popescu-Zeletin in [GPZ86]. The following are
the main assumptions made in this model:
• A MDBS consists of a set of local database management systems.
• A global transaction is a transaction which accesses more than one
local database system, while a local transaction accesses only one
database system.
• A global transaction is composed of a set of subtransactions. Each
subtransaction accesses one local database system on behalf of the
global transaction.
Execution Dependency among Subtransactions
In order to define an extended distributed transaction, we have to specify
the dependency among the subtransa.ctions. A positive dependency between
subtransactions exists if a subtransaction can not be executed until another
subtransaction is successfully completed. This occurs, for example, if a subtransaction
must wait for the results from another Bubtransactions [ED89J.
The relationship among subtra.nsactions h, t2 and t3 of the travel agent
transaction is an example. ta has to wait until either tt or t2 succeed, before
it can be executed. If two subtransactions are functionally replicated,
one of them must wait for the failure of the other one, before it can be
executed. This is called a. negative dependency.