| The example of previous and following foil, illustrates why concurrency control is needed in a parallel DB server environment. |
| It shows how contention for shared resources(data,disk access) is being handled under a multi-node parallel system sharing a single database. |
| 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. |
| Concurrency control needs to be implemented to ensure a consistant database, |
| i.e. the same block from the disk must be reflected as exactly the same copy in different buffers. |
| This is achieved by PCM locks. |