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  Rules for requesting nodes and Hierarchical locks

Rules for requesting nodes:

The implied locking of nodes will not work if transactions are allowed to leap into the middle of the tree and begin locking nodes at random. The implicit locking inferred by the S and X modes depends on all transactions obeying the following protocol:

(a) Prior to requesting an S or IS lock on a node all ancestor nodes of the requested node must be held in IX or IS mode by the requestor.

(b) Prior to requesting an X, SIX or IX lock on a node all ancestor nodes of the requested node should be held in SIX or IX mode by the requestor.

(c) Locks must be released either at the end of the transaction (in any order) or in leaf to root order. In particular if locks aren’t held to end of transaction one must not hold a lock after releasing its ancestors.

To rephrase this locks are requested root to leaf and released leaf to root. Observe that leaf nodes are never requested in intention mode since they have no descendants and that once a node is acquired in S or X mode no further explicit locking is required at lower levels.

Several Examples:

To lock record R for read:

lock database                    with mode =IS
lock area containing R         with mode =IS
lock file containing R           with mode =IS
lock record R                     with mode = S

Doesn’t panic the transaction probably already has the area, database and file lock.

To lock record R for write-exclusive access:

lock database                     with mode =IX
lock area containing R          with mode =IX
lock file containing R             with mode =IX
lock record R                       with mode = X

Note that if the records of this and the earlier illustration are distinct every request can be granted simultaneously to different transactions even though both refer to the same file.

To lock a file F for read as well as write access:

lock database                    with mode =IX
lock area containing F         with mode =IX
lock file P                          with mode = X

Since these reserves exclusive access to the file if this request uses the same file as the previous two examples it or the other transactions will have to wait. Dissimilar examples 1, 2 and 4 no additional locking need be done (at the record level).

To lock a file F for complete scan as well as occasional update

lock database                       with node =IX
lock area containing F            with mode =IX
lock file F                             with mode =SIX

Thereafter particular records in F are able to be locked for update by locking the desired records in X mode. Notice that dissimilar the previous example this transaction is compatible with the first example. This is the cause for introducing SIX mode.

To quiescent the data base:

Lock data base with mode =X.

Note that this locks each one else out.

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