In distributed deadlock detection, the delay in propagating local information might cause the deadlock detection algorithms to identify deadlocks that do not really exist. Such situations are called phantom deadlocks and they lead to unnecessary aborts.Atomicity:
Either all actions are carried out or none are. Users should not have to worry about the effect of incomplete transactions. DBMS ensures this by undoing the actions of incomplete transactions.
Aggregation:
A concept which is used to model a relationship between a collection of entities and relationships. It is used when we need to express a relationship among relationships.
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The set of dispatchable processes is in a safe state if there exists at least one temporal order in which all processes can be run to completion without resulting in a deadlock.
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Tuesday, May 13, 2008
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All processes in the main memory are asleep.
All 'ready-to-run' processes are swapped out.
There is no space in the swap device for the new incoming process that are swapped out of the main memory.
There is no space in the main memory for the new incoming process.
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Tuesday, May 13, 2008
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1) Unresolved contention for the use of a resource. 2) In APPN, when two elements of a process each wait for action by or a response from the other before they resume the process.
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Saturday, May 10, 2008
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When a process requests an available resource, system must decide if
immediate allocation leaves the system in a safe state. System is in
safe state if there exists a safe sequence of all processes. Deadlock
Avoidance: ensure that a system will never enter an unsafe state.
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Monday, April 28, 2008
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Deadlock is a situation when two processes, each having a lock on one piece of
data, attempt to acquire a lock on the other's piece. Each process would wait
indefinitely for the other to release the lock, unless one of the user processes
is terminated. SQL Server detects deadlocks and terminates one user's process.
A livelock is one, where a request for an exclusive lock is repeatedly denied
because a series of overlapping shared locks keeps interfering. SQL Server detects
the situation after four denials and refuses further shared locks. A livelock also
occurs when read transactions monopolize a table or page, forcing a write
transaction to wait indefinitely.
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Monday, March 10, 2008
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