The approach that has been followed is to use application knowledge to enforce application specific correctness criteria that will permit more concurrency. ![]() There are also questions about whether MDBSs should enforce serializability since in many applications this is an unnecessary restriction. Deadlock is a state of a database system having two or more transactions, when each transaction is waiting for a data item that is being locked by some other. Using multi–level nested transaction concepts we have been able to recast many of these mechanisms into a single framework. It has been difficult to relate many of the proposed solutions with conventional concurrency control theory. To enable DBSs to remain autonomous they must be able to continue to execute operations independent of the MDBS on the shared databases. In deadlock resolution, SQL Server will choose a victim at random (more on this later) and kill the process. The problem is that the MDBS is not aware of all operations executing on the shared databases. Enforcing serializability is more difficult without these assumptions. ![]() However, this requires restrictive assumptions to be made about the capabilities of component DBSs that have been regarded as a violation of autonomy. In many MDBSs the concurrency control mechanisms developed for more conventional distributed DBSs have been used. In general, database deadlock resolution involves the following nontrivial steps 6-9: 1) Select a victim (the transaction to be aborted) for the optimal resolution of a deadlock (this step may be computationally tedious). That is the execution of all transactions must be equivalent to a serial execution of the same transactions. The scheduler maintains a wait-for-graph for detecting cycle. In a situation in which resources are used fairly heavily, continuous deadlock detection is the deadlock resolution strategy of choice. ![]() The conventional concurrency control correctness criterion for DBSs is serializability. Database Concurrency Control Dealing with Deadlock and Starvation Deadlock detection and resolution In this approach, deadlocks are allowed to happen. Our focus is on the concurrency control issues in such a system. Resolutions The first section displays the deadlock victim and time of deadlock, along with the sessions involved in the deadlock. The MDBS design goal is to create a system that enables users easily to access the data stored in the different component DBSs while still permitting them to remain autonomous. A Multidatabase System (MDBS) is a type of distributed database system that is built up from a collection of different centralized Database Systems (DBSs) located on sites in a computer network.
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