Lock Based Protocols A lock is a mechanism to control concurrent access to a data item Data items can be locked in two modes 1 exclusive X mode Data item can be both read as well as written X lock is requested using lock X instruction Concurrency Control 2 shared S mode Data item can only be read S lock is requested using lock S instruction Lock requests are made to concurrency control manager Transaction can proceed only after request is granted Lock Based Protocols Lock compatibility matrix Lock Based Protocols Example of a transaction performing locking T2 lock S A read A unlock A lock S B A transaction may be granted a lock on an item if the requested lock is compatible with locks already held on the item by other transactions Any number of transactions can hold shared locks on an item but if any transaction holds an exclusive on the item no other transaction may hold any lock on the item If a lock cannot be granted the requesting transaction is made to wait till all incompatible locks held by other transactions have been released The lock is then granted read B unlock B display A B Locking as above is not sufficient to guarantee serializability if A and B get updated in between the read of A and B the displayed sum would be wrong A locking protocol is a set of rules followed by all transactions while requesting and releasing locks Locking protocols restrict the set of possible schedules Pitfalls of Lock Based Protocols Pitfalls of Lock Based Protocols Consider the partial schedule The potential for deadlock exists in most locking protocols Deadlocks are a necessary evil Starvation is also possible if concurrency control manager is badly designed For example A transaction may be waiting for an X lock on an item while a sequence of other transactions request and are granted an S lock on the same item The same transaction is repeatedly rolled back due to deadlocks Concurrency control manager can be designed to prevent starvation Neither T3 nor T4 can make progress executing lock S B causes T4 to wait for T3 to release its lock on B while executing lock X A causes T3 to wait for T4 to release its lock on A Such a situation is called a deadlock To handle a deadlock one of T3 or T4 must be rolled back and its locks released The Two Phase Locking Protocol The Two Phase Locking Protocol This is a protocol which ensures conflict serializable schedules Two phase locking does not ensure freedom from deadlocks Phase 1 Growing Phase Cascading roll back is possible under two phase locking To avoid transaction may obtain locks transaction may not release locks Phase 2 Shrinking Phase transaction may release locks transaction may not obtain locks The protocol assures serializability It can be proved that the transactions can be serialized in the order of their lock points i e the point where a transaction acquired its final lock this follow a modified protocol called strict two phase locking Here a transaction must hold all its exclusive locks till it commits aborts Rigorous two phase locking is even stricter here all locks are held till commit abort In this protocol transactions can be serialized in the order in which they commit Lock Conversions The Two Phase Locking Protocol There can be conflict serializable schedules that cannot be obtained if two phase locking is used However in the absence of extra information e g ordering of access to data two phase locking is needed for conflict serializability in the following sense Given a transaction Ti that does not follow two phase locking we can find a transaction Tj that uses two phase locking and a schedule for Ti and Tj that is not conflict serializable Two phase locking with lock conversions First Phase can acquire a lock S on item can acquire a lock X on item can convert a lock S to a lock X upgrade Second Phase can release a lock S can release a lock X can convert a lock X to a lock S downgrade This protocol assures serializability But still relies on the programmer to insert the various locking instructions Automatic Acquisition of Locks A transaction Ti issues the standard read write instruction without explicit locking calls The operation read D is processed as if Ti has a lock on D then read D else begin if necessary wait until no other transaction has a lock X on D grant Ti a lock S on D read D end Automatic Acquisition of Locks write D is processed as if Ti has a lock X on D then write D else begin if necessary wait until no other trans has any lock on D if Ti has a lock S on D then upgrade lock on D to lock X else grant Ti a lock X on D write D end All locks are released after commit or abort Implementation of Locking Lock Table A lock manager can be implemented as a separate process to which Black rectangles indicate granted locks white ones indicate waiting requests Lock table also records the type of lock granted or requested New request is added to the end of the queue of requests for the data item and granted if it is compatible with all earlier locks Unlock requests result in the request being deleted and later requests are checked to see if they can now be granted If transaction aborts all waiting or granted requests of the transaction are deleted transactions send lock and unlock requests The lock manager replies to a lock request by sending a lock grant messages or a message asking the transaction to roll back in case of a deadlock The requesting transaction waits until its request is answered The lock manager maintains a data structure called a lock table to record granted locks and pending requests The lock table is usually implemented as an in memory hash table indexed on the name of the data item being locked Granted Waiting Graph Based Protocols lock manager may keep a list of locks held by each transaction to implement this efficiently Tree Protocol Graph based protocols are an alternative to two phase locking Impose a partial ordering on the set D d1 d2 dh of all data items If di dj then any transaction accessing both di and dj must access di before accessing dj Implies that the set D may now be viewed as a directed acyclic graph called a database graph The tree protocol is a simple kind of graph protocol 1 Only exclusive locks are allowed 2 The first lock by Ti may be on any data item Subsequently a data Q can be locked by Ti only if the parent of Q is currently locked by Ti 3 Data items may be unlocked at any time 4 A data item that has been locked and unlocked by Ti cannot subsequently be relocked by Ti Graph