Pros and cons of concurrency:
If transactions are run one at a time then every transaction will see the consistent state left behind by its predecessor. However if several transactions are scheduled concurrently then the inputs of some transaction may be inconsistent even though every transaction in isolation is consistent.
Concurrency is introduced to get better system response as well as utilization.
a) It shouldn’t cause programs to malfunction.b) Concurrency control shouldn’t consume more resources than it “saves”.
If the database is read-only afterwards no concurrency control is needed. Nevertheless if transactions update shared data then their concurrent execution needs to be regulated so that they don’t update the same item at the same time.
If all transactions are simple as well as all data are in primary storage then there is no need for concurrency.
Nevertheless if any transaction runs for a long time or does I/O then concurrency perhaps needed to improve responsiveness and utilization of the system. If concurrency is authorized then long-running transactions will (usually) not delay short ones.
Concurrency should be regulated by some facility that regulates access to shared resources.
Data management systems usually use locks for this purpose.
The simplest lock protocol acquaintances a lock with each object. Whenever utilizing the object the transaction acquires the lock and holds it until the transaction is complete. The lock is a serialization mechanism that assures that only one transaction accesses the object at a time. It has the effect of alert others that the object is busy and of protecting the lock requestor from modifications of others.
This protocol varies from the in sequence reusable resource protocol common to most operating systems (and recently renamed monitors) in that the lock protocol holds locks to transaction commit. It will be discussed below that this is a critical difference.
Responsibility for requesting as well as releasing locks can either be assumed by the user or be delegated to the system. User controlled locking results in potentially less locks due to the user’s knowledge of the semantics of the data. Alternatively user controlled locking requires difficult and potentially unreliable application programming. Therefore the approach taken by most data base systems is to use automatic lock protocols which insure protection from inconsistency while still allowing the user to specify alternative lock protocols as an optimization.
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