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Predicate locks

In the preceding description of locks, we glossed over a subtle but important detail. In “Phantoms causing write skew” on page 250 we discussed the problem of phantoms—that is, one transaction changing the results of another transaction’s search query. A database with serializable isolation must prevent phantoms.

In the meeting room booking example this means that if one transaction has searched for existing bookings for a room within a certain time window (see Example 7-2), another transaction is not allowed to concurrently insert or update another booking for the same room and time range. (It’s okay to concurrently insert bookings for other rooms, or for the same room at a different time that doesn’t affect the proposed booking.)

How do we implement this? Conceptually, we need a predicate lock [3]. It works similarly to the shared/exclusive lock described earlier, but rather than belonging to a particular object (e.g., one row in a table), it belongs to all objects that match some search condition, such as:

SELECT * FROM bookings WHERE room_id = 123 AND

end_time > '2018-01-01 12:00' AND start_time < '2018-01-01 13:00';

A predicate lock restricts access as follows:

• If transaction A wants to read objects matching some condition, like in that SELECT query, it must acquire a shared-mode predicate lock on the conditions of the query. If another transaction B currently has an exclusive lock on any object matching those conditions, A must wait until B releases its lock before it is allowed to make its query.

• If transaction A wants to insert, update, or delete any object, it must first check whether either the old or the new value matches any existing predicate lock. If there is a matching predicate lock held by transaction B, then A must wait until B has committed or aborted before it can continue.

The key idea here is that a predicate lock applies even to objects that do not yet exist in the database, but which might be added in the future (phantoms). If two-phase locking includes predicate locks, the database prevents all forms of write skew and other race conditions, and so its isolation becomes serializable.

 
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