5.17 What is intention locking?
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Key points
- Intention locks reduce the number of locks that must be examined when a
new lock is allocated. Intention locks allow transactions to quickly
determine if rows (or pages) in a given table have been locked by other
transactions. Without the use of intention locks, ALLBASE/SQL would have to
search all row (or page) locks on a table to determine whether or not a new
lock request could be granted. Such searching would be very inefficient,
especially on large tables.
- When a lock is acquired at a smaller granularity, an intention lock is
first acquired at a larger granularity.
- When a row lock is acquired on a PUBLICROW table, the following occurs:
- An intention lock is acquired on the table.
- After the table lock has been granted, an intention lock is acquired on the
page.
- After the page lock has been granted, the row lock is acquired.
- When a page lock is acquired on a PUBLIC table, the following occurs:
- An intention lock is acquired on the table.
- After the table lock has been granted, the page lock is acquired.
- When two locks are compatible, both access requests are allowed to
the data object (that is, the table, page or row) at the
same time. When compatible locks exist on a data object, ALLBASE/SQL computes
which lock is strongest and stores this information. When locks are not
compatible, the second access request must wait until the lock acquired by the
first access request is released.
- Any request for a lock at the table level is compared to the strongest lock on
the table. If the table has already been locked in an incompatible mode by
another transaction, the transaction that is requesting the lock will wait
until the lock can be granted (or a timeout occurs). Next, a request for a
page lock is made. If this page has been locked in an incompatible mode by
another transaction, the requesting transaction will wait until the lock can
be granted (or a timeout occurs). Finally, if row level locking is used, the
request for a row lock is made.
- A transaction timeout will only occur if the wait for a single database
resource consumes an amount of time equal to the timeout limit. If a
transaction's timeout limit is set to 10 seconds, and it takes a 5 second wait
to obtain an intention lock on the table and a 6 second wait to obtain a lock
on a page, the transaction will not timeout. If either wait consumed 10
seconds, then a timeout would occur.
- Intention locks are only acquired on PUBLICROW and PUBLIC tables.
Remember that PUBLICROW tables use row level locking, and PUBLIC tables use
page level locking.
- PRIVATE and PUBLICREAD tables use table level locking, so intention locks are
not needed by ALLBASE/SQL.
- The three types of intention locks are INTENT SHARE (IS), INTENT
EXCLUSIVE (IX), and SHARE and INTENT EXCLUSIVE (SIX).
Page last updated on November 29, 1995
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