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  Hierarchical locks and Lock management

Hierarchical locks:

We will first presume that the set of resources to be locked is organized in a hierarchy. Note that this hierarchy is utilized in the context of a collection of resources and has nothing to do with the data model used in a data base system. The hierarchy of the following figure perhaps suggestive. We accept the notation that each level of the hierarchy is given a node type that is a generic name for all the node instances of that type. For illustration the database has nodes of type area as its immediate descendants each area in turn has nodes of type file as its immediate descendants as well as each file has nodes of type record as its immediate descendants in the hierarchy. Since it is a hierarchy every node has a unique parent.

DATA BASE
      |
   AREAS
      |
   FILES
      |
  RECORDS

Figure: A sample lock hierarchy.

Every node of the hierarchy can be locked if one requests exclusive access (X) to a particular node then when the request is granted, the requestor has exclusive access to that node as well as implicitly to each of its descendants. If one requests pooled access (S) to a particular node, afterwards when the request is granted the requestor has shared access to that node as well as implicitly to each descendant of that node. These two access modes lock an whole sub-tree rooted at the requested node.

Our goal is to discover some technique for implicitly locking an entire sub-tree. So as to lock a sub-tree rooted at node R in share or exclusive mode it is important to prevent locks on the ancestors of R that might implicitly lock R as well as its descendants in an incompatible mode. Therefore a new access mode intention mode (I) is introduced. Intention mode is utilized to ‘tag’ (lock) all ancestors of a node to be locked in share or exclusive mode. These tags signal the actuality that locking is being done at a ‘finer’ level as well as thereby prevents implicit or explicit exclusive or share locks on the ancestors.

The protocol to lock a sub-tree rooted at node R in exclusive or else shares the protocol to lock a sub-tree rooted at node R in exclusive or share mode is to first lock all ancestors of R in intention mode and then to lock node R in exclusive or share made. For illustration using the figure above to lock a particular file one must obtain intention access to the database to the area containing the file and then request exclusive (or share) access to the file itself. This absolutely locks all records of the file in exclusive (or share) mode.

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