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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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V Transaction Management
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16 Concurrency Control
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The McGraw Hill Companies, 2001
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In the wait die scheme, if a transaction Ti dies and is rolled back because it requested a data item held by transaction Tj , then Ti may reissue the same sequence of requests when it is restarted If the data item is still held by Tj , then Ti will die again Thus, Ti may die several times before acquiring the needed data item Contrast this series of events with what happens in the wound wait scheme Transaction Ti is wounded and rolled back because Tj requested a data item that it holds When Ti is restarted and requests the data item now being held by Tj , Ti waits Thus, there may be fewer rollbacks in the wound wait scheme The major problem with both of these schemes is that unnecessary rollbacks may occur
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1662 Timeout-Based Schemes
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Another simple approach to deadlock handling is based on lock timeouts In this approach, a transaction that has requested a lock waits for at most a speci ed amount of time If the lock has not been granted within that time, the transaction is said to time out, and it rolls itself back and restarts If there was in fact a deadlock, one or more transactions involved in the deadlock will time out and roll back, allowing the others to proceed This scheme falls somewhere between deadlock prevention, where a deadlock will never occur, and deadlock detection and recovery, which Section 1663 discusses The timeout scheme is particularly easy to implement, and works well if transactions are short and if long waits are likely to be due to deadlocks However, in general it is hard to decide how long a transaction must wait before timing out Too long a wait results in unnecessary delays once a deadlock has occurred Too short a wait results in transaction rollback even when there is no deadlock, leading to wasted resources Starvation is also a possibility with this scheme Hence, the timeout-based scheme has limited applicability
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1663 Deadlock Detection and Recovery
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If a system does not employ some protocol that ensures deadlock freedom, then a detection and recovery scheme must be used An algorithm that examines the state of the system is invoked periodically to determine whether a deadlock has occurred If one has, then the system must attempt to recover from the deadlock To do so, the system must: Maintain information about the current allocation of data items to transactions, as well as any outstanding data item requests Provide an algorithm that uses this information to determine whether the system has entered a deadlock state Recover from the deadlock when the detection algorithm determines that a deadlock exists In this section, we elaborate on these issues
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
V Transaction Management
16 Concurrency Control
The McGraw Hill Companies, 2001
16
Concurrency Control
T26 T25 T27
Wait-for graph with no cycle
16631 Deadlock Detection
Deadlocks can be described precisely in terms of a directed graph called a wait-for graph This graph consists of a pair G = (V, E), where V is a set of vertices and E is a set of edges The set of vertices consists of all the transactions in the system Each element in the set E of edges is an ordered pair Ti Tj If Ti Tj is in E, then there is a directed edge from transaction Ti to Tj , implying that transaction Ti is waiting for transaction Tj to release a data item that it needs When transaction Ti requests a data item currently being held by transaction Tj , then the edge Ti Tj is inserted in the wait-for graph This edge is removed only when transaction Tj is no longer holding a data item needed by transaction Ti A deadlock exists in the system if and only if the wait-for graph contains a cycle Each transaction involved in the cycle is said to be deadlocked To detect deadlocks, the system needs to maintain the wait-for graph, and periodically to invoke an algorithm that searches for a cycle in the graph To illustrate these concepts, consider the wait-for graph in Figure 1618, which depicts the following situation: Transaction T25 is waiting for transactions T26 and T27 Transaction T27 is waiting for transaction T26 Transaction T26 is waiting for transaction T28 Since the graph has no cycle, the system is not in a deadlock state Suppose now that transaction T28 is requesting an item held by T27 The edge T28 T27 is added to the wait-for graph, resulting in the new system state in Figure 1619 This time, the graph contains the cycle T26 T28 T27 T26 implying that transactions T26 , T27 , and T28 are all deadlocked Consequently, the question arises: When should we invoke the detection algorithm The answer depends on two factors: 1 How often does a deadlock occur 2 How many transactions will be affected by the deadlock
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