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The third approach, preemption of claims granted to transactions, requires a rollback capability Either the transaction, when it is noti ed that it cannot proceed, has to restore the database and place itself in the queue for another turn, or the system has to kill the transaction, restore the database, and restart the transaction anew Since this approach depends on deadlock detection, it will be further discussed in Sec 13-2-4
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The fourth choice is to avoid circularity in request sequences There are three approaches here, monitoring for circularity, sequencing of objects to avoid circularity, and two-phase locking, treated, because of its importance, in a distinct subsection To avoid deadlock the request pattern of all the transactions can be monitored If conditions of potential circularity which can lead to deadlock are detected the candidate transactions can be blocked The bankers algorithm of Example 3-6 provided a similar example If the request cannot be satis ed because of the potential for deadlock, the request is queued As claims are released, the transactions waiting in the queue are checked to see if they can be restarted A scheduling algorithm can be used to choose among multiple candidates for processing, but at least one request should be granted to prevent scheduler-caused deadlocks [Holt72 ] In dynamic algorithms the transaction arrival order may have to be maintained Unfortunately the computation becomes quite complex when there are many resources, as is the case when we deal with data elements instead of devices A proper analysis also has to consider serializability and compatibility of interfering operations The computation takes much longer when many transaction are active; an undesirable feature during busy times References to such algorithms are found in the background section A simple algorithm, used in IBM OS, allows incremental claims only in ascending
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Presequence lexical sequence for the le name The objects being locked are les A proper pair of sequences to avoid deadlock is shown in Example 13-7 Two ascending sequences cannot create a circular sequence so that the deadlock problem seems solved This technique greatly limits exibility for incremental requests The IMS system operating under OS uses its own locking scheme, shown in Table 13-1 Example 13-7
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The ascending request rule can be relaxed by allowing requests to les lower in the lexical sequence to be made; but if such a request is denied the transaction is obliged to release all objects required up to this point and start over fresh This extension can be seen as combining approaches 3 and 4
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A simple approach to avoid circularity is to require preclaiming of all objects before granting any locks Claiming resources before promising to grant access to them means that a transaction may not be able to complete the preclaiming phase Once all claims have been made locks can be set to protect the transaction from interference by others This technique is called two-phase locking and works well in an environment using a two-phase commit protocol for transactions Two-phase locking is hence characterized by a phase where resources are acquired and a phase where they are used and released A failure to acquire resources in phase one can cause the transaction to be aborted without much pain No resources may be acquired during phase two, since a failure to obtain them would require undoing of changes made to committed resources The major problem with two-phase locking is that preclaiming can lead to having to claim more and larger objects than will actually be needed If a computation on part of the data determines what further object is needed, an entire le rather than a record may have to be preclaimed
In Example 1-1 we determined from a Department le the name of the manager who was to receive a raise and found "Joe" Preclaiming requires locking of the entire Employee le since we do not yet know which Employee will be updated Not preclaiming and nding the Employeename = "Joe" record claimed could mean that an interfering transaction is just demoting Joe to a position which does not qualify him for the raise
In order to reduce the granule size prereading may be used here, as presented in Sec 13-1-2 The noti cation mechanism used in non-blocking schemes can warn the transaction of interference potential The simpler rule is that a transaction shall claim the entire region to be locked at one, and make no incremental claims System primitives are used to lock out claims by other transactions until the availability of all objects in the region is veri ed and the entire claim is recorded To issue a comprehensive claim requires perfect foresight; the cost of failure is having to release all acquired resources in their original state, and initiating a new claim A transaction, in order to reduce its probability of failure, will claim a generous region
Summary
Deadlock avoidance methods suggest a familar theme, binding (Sec 1-6) By imposing restrictions on the choices of the objects to be accessed, multioperation is made safe This in turn allows a major improvement in the productivity of the system As stated earlier, the system may not allocate the claimed objects to the user until they are actually needed Prior to allocating an object which is the subject of another claim, a check will be made if now a deadlock potential is created Deferred allocation of claims is used in Burroughs 6500 systems
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