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The example of previous and following foil, illustrates why concurrency control is needed in a parallel DB server environment.
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It shows how contention for shared resources(data,disk access) is being handled under a multi-node parallel system sharing a single database.
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The blackened boxes in the figure are the same blocks in each node (not rows !) requested by different nodes(instances) and located in different SGA bufferes.
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Concurrency control needs to be implemented to ensure a consistant database,
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i.e. the same block from the disk must be reflected as exactly the same copy in different buffers.
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This is achieved by PCM locks.
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