Concurrency Control Managing Hierarchies of Database Elements (18.6) Presented by Priyank (204)

Agenda Managing Hierarchies of Database Elements Locks with Multiple Granularity Warning (Intention) Locks Database Elements Organized in Hierarchy Rules of Warning Protocol Group Modes of Intention Locks

Managing Hierarchies of Database Elements Two problems that arise with locks when there is a tree structure to the data are: When the tree structure is a hierarchy of lockable elements Determine how locks are granted for both large elements (relations) and smaller elements (blocks containing tuples or individual tuples) When the data itself is organized as a tree (B-tree indexes) This will be discussed in the next section

Locks with Multiple Granularity A database element can be a relation, block or a tuple Different systems use different database elements to determine the size of the lock Thus some may require small database elements such as tuples or blocks and others may require large elements such as relations

Example of Multiple Granularity Locks Consider a database for a bank Choosing relations as database elements means we would have one lock for an entire relation If we were dealing with a relation having account balances, this kind of lock would be very inflexible and thus provide very little concurrency Why? Because balance transactions require exclusive locks and this would mean only one transaction occurs for one account at any time But as each account is independent of others we could perform transactions on different accounts simultaneously

…(contd.) Thus it makes sense to have block element for the lock so that two accounts on different blocks can be updated simultaneously Another example is that of a document With similar arguments as above, we see that it is better to have large element (a complete document) as the lock in this case

SHARED (S) Used for read operations that do not change or update data, such as a SELECT statement. Exclusive - Used for data-modification operations, such as INSERT, UPDATE, or DELETE. Ensures that multiple updates cannot be made to the same resource at the same time INTENT - Used to establish a lock hierarchy. The types of intent locks are: intent shared (IS), intent exclusive (IX)

The Database Engine uses intent locks to protect placing a shared (S) lock or exclusive (X) lock on a resource lower in the lock hierarchy. Intent locks are named intent locks because they are acquired before a lock at the lower level, and therefore signal intent to place locks at a lower level. Intent locks serve two purposes: To prevent other transactions from modifying the higher-level resource in a way that would invalidate the lock at the lower level. To improve the efficiency of the Database Engine in detecting lock conflicts at the higher level of granularity.

Warning (Intention) Locks These are required to manage locks at different granularities In the bank example, if the a shared lock is obtained for the relation while there are exclusive locks on individual tuples, unserializable behavior occurs The rules for managing locks on hierarchy of database elements constitute the warning protocol

Database Elements Organized in Hierarchy

Rules of Warning Protocol These involve both ordinary (S and X) and warning (IS and IX) locks The rules are: Begin at the root of hierarchy Request the S/X lock if we are at the desired element If the desired element id further down the hierarchy, place a warning lock (IS if S and IX if X) When the warning lock is granted, we proceed to the child node and repeat the above steps until desired node is reached

Compatibility Matrix for Shared, Exclusive and Intention Locks IS IX S X Yes No The above matrix applies only to locks held by other transactions

Group Modes of Intention Locks An element can request S and IX locks at the same time if they are in the same transaction (to read entire element and then modify sub elements) This can be considered as another lock mode, SIX, having restrictions of both the locks i.e. No for all except IS SIX serves as the group mode

Example Select * from table where attribute1 = ‘abc’ Consider a transaction T1 as follows Select * from table where attribute1 = ‘abc’ Here, IS lock is first acquired on the entire relation; then moving to individual tuples (attribute = ‘abc’), S lock in acquired on each of them Consider another transaction T2 Update table set attribute2 = ‘def’ where attribute1 = ‘ghi’ Here, it requires an IX lock on relation and since T1’s IS lock is compatible, IX is granted

On reaching the desired tuple (ghi), as there is no lock, it gets X too If T2 was updating the same tuple as T1, it would have to wait until T1 released its S lock

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