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Thursday, 24 March 2016

List of sources of unpredictability of dead-line in real-time database


The list of sources of unpredictability in real-time database that makes the prediction of a transaction’s deadline difficult, Why it is difficult to predict the deadline in a real-time database transaction?


List of sources of unpredictability:


The following are the list of sources of unpredictability in real-time database that makes the prediction of a transaction’s deadline difficult;

The nature of any database transactions or the environment in which a transaction has to execute has one or more of the following. The uncertain nature in terms of time of the following points makes it difficult to identify the deadline exactly.

Dependency of the execution sequence of any transaction on the data values
-         The order of execution (execution sequence) of a transaction is highly depend on the values of the data items being accessed. For example, data items may be held by some other transactions, or the instructions that are part of the given transaction may be in a sequence.

Data and resource conflicts
-         A transaction may have to wait until it is allocated with the required resources. This may happen because of conflicting nature of concurrent transactions. Sometimes, a transaction may be forced to wait for resources such as CPU and I/O devices.

Dynamic paging and I/O
-         Unpredictability of disc access time involved in bringing required data page into memory and page faults

Transaction aborts and the resulting rollbacks and restarts
-         Transactions may be aborted due to many reasons such as, not able to complete the transaction because of failures (power failure, software failure, or system crash), deadlocks, or forced aborts. This will definitely lead a transaction to restart from scratch or rollback the changes made.

Because of these problems, it is hard to identify whether the transaction is able to meet its deadline or not. These are the points that have to be considered in terms of transaction in a conventional database systems. The necessary modification on the concurrency control mechanisms may alter this.


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