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VI Database System Architecture
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19 Distributed Databases
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The McGraw Hill Companies, 2001
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Figure 195 False cycles in the global wait-for graph
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Note that the false-cycle situation could not occur under two-phase locking The likelihood of false cycles is usually suf ciently low that they do not cause a serious performance problem A deadlock has indeed occurred and a victim has been picked, while one of the transactions was aborted for reasons unrelated to the deadlock For example, suppose that site S1 in Figure 193 decides to abort T2 At the same time, the coordinator has discovered a cycle, and has picked T3 as a victim Both T2 and T3 are now rolled back, although only T2 needed to be rolled back Deadlock detection can be done in a distributed manner, with several sites taking on parts of the task, instead of being done at a single site, However, such algorithms are more complicated and more expensive See the bibliographical notes for references to such algorithms
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196 Availability
One of the goals in using distributed databases is high availability; that is, the database must function almost all the time In particular, since failures are more likely in large distributed systems, a distributed database must continue functioning even when there are various types of failures The ability to continue functioning even during failures is referred to as robustness For a distributed system to be robust, it must detect failures, recon gure the system so that computation may continue, and recover when a processor or a link is repaired The different types of failures are handled in different ways For example, message loss is handled by retransmission Repeated retransmission of a message across a link,
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
VI Database System Architecture
19 Distributed Databases
The McGraw Hill Companies, 2001
Availability
without receipt of an acknowledgment, is usually a symptom of a link failure The network usually attempts to nd an alternative route for the message Failure to nd such a route is usually a symptom of network partition It is generally not possible, however, to differentiate clearly between site failure and network partition The system can usually detect that a failure has occurred, but it may not be able to identify the type of failure For example, suppose that site S1 is not able to communicate with S2 It could be that S2 has failed However, another possibility is that the link between S1 and S2 has failed, resulting in network partition The problem is partly addressed by using multiple links between sites, so that even if one link fails the sites will remain connected However, multiple link failure can still occur, so there are situations where we cannot be sure whether a site failure or network partition has occurred Suppose that site S1 has discovered that a failure has occurred It must then initiate a procedure that will allow the system to recon gure, and to continue with the normal mode of operation If transactions were active at a failed/inaccessible site at the time of the failure, these transactions should be aborted It is desirable to abort such transactions promptly, since they may hold locks on data at sites that are still active; waiting for the failed/inaccessible site to become accessible again may impede other transactions at sites that are operational However, in some cases, when data objects are replicated it may be possible to proceed with reads and updates even though some replicas are inaccessible In this case, when a failed site recovers, if it had replicas of any data object, it must obtain the current values of these data objects, and must ensure that it receives all future updates We address this issue in Section 1961 If replicated data are stored at a failed/inaccessible site, the catalog should be updated so that queries do not reference the copy at the failed site When a site rejoins, care must be taken to ensure that data at the site is consistent, as we will see in Section 1963 If a failed site is a central server for some subsystem, an election must be held to determine the new server (see Section 1965) Examples of central servers include a name server, a concurrency coordinator, or a global deadlock detector Since it is, in general, not possible to distinguish between network link failures and site failures, any recon guration scheme must be designed to work correctly in case of a partitioning of the network In particular, these situations must be avoided: Two or more central servers are elected in distinct partitions More than one partition updates a replicated data item
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