barcode add in for excel 2007 Implementation of Locking in Software

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1614 Implementation of Locking
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A lock manager can be implemented as a process that receives messages from transactions and sends messages in reply The lock-manager process replies to lock-request messages with lock-grant messages, or with messages requesting rollback of the transaction (in case of deadlocks) Unlock messages require only an acknowledgment in response, but may result in a grant message to another waiting transaction The lock manager uses this data structure: For each data item that is currently locked, it maintains a linked list of records, one for each request, in the order in which the requests arrived It uses a hash table, indexed on the name of a data item, to nd the linked list (if any) for a data item; this table is called the lock table Each record of the linked list for a data item notes which transaction made the request, and what lock mode it requested The record also notes if the request has currently been granted
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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V Transaction Management
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The McGraw Hill Companies, 2001
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Figure 1610 shows an example of a lock table The table contains locks for ve different data items, I4, I7, I23, I44, and I912 The lock table uses over ow chaining, so there is a linked list of data items for each entry in the lock table There is also a list of transactions that have been granted locks, or are waiting for locks, for each of the data items Granted locks are the lled-in (black) rectangles, while waiting requests are the empty rectangles We have omitted the lock mode to keep the gure simple It can be seen, for example, that T23 has been granted locks on I912 and I7, and is waiting for a lock on I4 Although the gure does not show it, the lock table should also maintain an index on transaction identi ers, so that it is possible to determine ef ciently the set of locks held by a given transaction The lock manager processes requests this way: When a lock request message arrives, it adds a record to the end of the linked list for the data item, if the linked list is present Otherwise it creates a new linked list, containing only the record for the request It always grants the rst lock request on a data item But if the transaction requests a lock on an item on which a lock has already been granted, the lock manager grants the request only if it is compatible with all earlier requests, and all earlier requests have been granted already Otherwise the request has to wait
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
V Transaction Management
16 Concurrency Control
The McGraw Hill Companies, 2001
Lock-Based Protocols
When the lock manager receives an unlock message from a transaction, it deletes the record for that data item in the linked list corresponding to that transaction It tests the record that follows, if any, as described in the previous paragraph, to see if that request can now be granted If it can, the lock manager grants that request, and processes the record following it, if any, similarly, and so on If a transaction aborts, the lock manager deletes any waiting request made by the transaction Once the database system has taken appropriate actions to undo the transaction (see Section 173), it releases all locks held by the aborted transaction This algorithm guarantees freedom from starvation for lock requests, since a request can never be granted while a request received earlier is waiting to be granted We study how to detect and handle deadlocks later, in Section 1663 Section 1821 describes an alternative implementation one that uses shared memory instead of message passing for lock request/grant
1615 Graph-Based Protocols
As noted in Section 1613, the two-phase locking protocol is both necessary and suf cient for ensuring serializability in the absence of information concerning the manner in which data items are accessed But, if we wish to develop protocols that are not two phase, we need additional information on how each transaction will access the database There are various models that can give us the additional information, each differing in the amount of information provided The simplest model requires that we have prior knowledge about the order in which the database items will be accessed Given such information, it is possible to construct locking protocols that are not two phase, but that, nevertheless, ensure con ict serializability To acquire such prior knowledge, we impose a partial ordering on the set D = {d1 , d2 , , dh } of all data items If di dj , then any transaction accessing both di and dj must access di before accessing dj This partial ordering may be the result of either the logical or the physical organization of the data, or it may be imposed solely for the purpose of concurrency control The partial ordering implies that the set D may now be viewed as a directed acyclic graph, called a database graph In this section, for the sake of simplicity, we will restrict our attention to only those graphs that are rooted trees We will present a simple protocol, called the tree protocol, which is restricted to employ only exclusive locks References to other, more complex, graph-based locking protocols are in the bibliographical notes In the tree protocol, the only lock instruction allowed is lock-X Each transaction Ti can lock a data item at most once, and must observe the following rules: 1 The rst lock by Ti may be on any data item 2 Subsequently, a data item Q can be locked by Ti only if the parent of Q is currently locked by Ti
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