Sarat_Dasika_114_Section_1_18.6_Concurrency Control

Managing Hierarchies of Database Elements
Presented by
Sarat Dasika
February 16, 2012
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
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
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
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
◦ 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
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
The rules for managing locks on hierarchy of
database elements constitute the warning
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
◦ 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
• The above matrix applies only to locks held by
other transactions
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
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
Consider a transaction T1 as follows
Consider another transaction T2
◦ 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
◦ 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
This arises when transactions create new sub
elements of lockable elements
Since we can lock only existing elements the
new elements fail to be locked
The problem created in this situation is
explained in the following example
Consider a transaction T3
◦ Select sum(length) from table where attribute1 =
◦ This calculates the total length of all tuples having
◦ Thus, T3 acquires IS for relation and S for targeted
◦ Now, if another transaction T4 inserts a new
tuple having attribute1 = ‘abc’, the result of
T3 becomes incorrect
This is not a concurrency problem since the serial
order (T3, T4) is maintained
But if both T3 and T4 were to write an element X,
it could lead to unserializable behavior
◦ r3(t1);r3(t2);w4(t3);w4(X);w3(L);w3(X)
◦ r3 and w3 are read and write operations by T3 and w4
are the write operations by T4 and L is the total length
calculated by T3 (t1 + t2)
◦ At the end, we have result of T3 as sum of lengths of
t1 and t2 and X has value written by T3
◦ This is not right; if value of X is considered to be that
written by T3 then for the schedule to be serializable,
the sum of lengths of t1, t2 and t3 should be
◦ Else if the sum is total length of t1 and t2 then
for the schedule to be serializable, X should have
value written by T4
This problem arises since the relation has a
phantom tuple (the new inserted tuple), which
should have been locked but wasn’t since it
didn’t exist at the time locks were taken
The occurrence of phantoms can be avoided
if all insertion and deletion transactions are
treated as write operations on the whole
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