Protocol for requesting locks on a DAG:
(a) Prior to requesting an S or IS lock on a node one should request at least one parent (and by induction a path to a root) in IS (or greater) mode. As a result none of the ancestors along this path can be granted to another transaction in a mode incompatible with IS.
(b) Prior to requesting IX, SIX or x mode access to a node one should request all parents of the node in IX (or greater) mode. As a result all ancestors will be held in IX (or greater mode) and cannot be held by other transactions in a mode incompatible with IX (i.e. S, SIX, X).
(c) Locks must be released either at the end of the transaction (in any order) or in leaf to root order. Particularly if locks are not held to the end of transaction one shouldn’t hold a lower lock after releasing its ancestors.
To give an illustration using the non-hierarchical lock graph in the figure above a sequential scan of all records in file F need not use an index so one can get an implicit share lock on each record in the file by:
lock data base with mode =ISlock area containing F with mode =ISlock file F with mode = S
This gives implied S mode access to all records in P. On the other hand to read a record in a file via the index I for file P one need not get an implicit or explicit lock on file F:
lock data base with mode =ISlock area containing R with mode =ISlock index I with mode = S
This once more gives implicit S mode access to all records in index I (in file F). In both these circumstances only one path was locked for reading.
However to insert, delete or update a record R in file F with index I one should get an implicit or explicit lock on all ancestors of R.
The first instance of this section showed how an explicit X lock on a record is obtained. To get an implied X lock on all records in a file one can simply lock the index and file in X mode or lock the area in X mode.
The latter illustration allow bulk load or update of a file without further locking since all records in the file are implicitly granted in X mode.
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