Concurrency Control in Database Management System
- 1. Subject : Database Management System Topic : Concurrency Control
- 2. CONTENTS • What is Concurrency Control? • Lock Based Protocol • Two Phase Locking Protocol • Deadlock
- 3. What is Concurrency Control? The technique is used to protect data when multiple users are accessing same data concurrently (same time) is called concurrency control
- 5. Lock Based Protocol • Lock is a mechanism to control concurrent access to 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 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
- 6. Lock Based Protocol • Lock-compatibility Matrix : S X S TRUE FALSE X FALSE FALSE • 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 transaction • 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
- 7. Lock Based Protocol • If a lock cannot be granted, the requesting transaction is made to wait till all incompatible locks held by other transaction have been released. The lock is then granted. • Example: T1: lock-S(A); // Grant-S(A,T1) read (A); unlock(A); lock-S(B); // Grant-S(B,T1) read (B); unlock(B); display(A+B)
- 8. Pitfalls of Lock Based Protocol • 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.
- 9. Pitfalls of Lock Based Protocol • 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.
- 10. Pitfalls of Lock Based Protocol • 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.
- 12. Two Phase Locking Protocol • This is a protocol which ensures conflict-serializable schedules. • Phase 1: Growing Phase – 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).
- 13. Two Phase Locking Protocol • Two-phase locking does not ensure freedom from deadlocks • Cascading roll-back is possible under two-phase locking. To avoid 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.
- 15. Deadlock •A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause. Ti Tj
- 16. Four Conditions for Deadlock A. Mutual exclusion condition – each resource assigned to 1 process or is available B. Hold and wait condition – process holding resources can request additional C. No preemption condition – previously granted resources cannot forcibly taken away D. Circular wait condition – must be a circular chain of 2 or more processes – each is waiting for resource held by next member of the chain
- 17. Strategies of Deadlock Handling •Deadlock prevention. Prevents deadlocks by restraining requests made to ensure that at least one of the four deadlock conditions cannot occur. •Deadlock avoidance. Dynamically grants a resource to a process if the resulting state is safe. A state is safe if there is at least one execution sequence that allows all processes to run to completion. •Deadlock detection and recovery. That Allows deadlocks to form; then finds and breaks them.
- 18. Deadlock Avoidance WAIT-DIE Rule : If Ti requests a lock on a data item which is already locked by Tj , then Ti is permitted to wait if ts (Ti ) < ts (Tj ). If ts (Ti ) >ts (Tj ) , then Ti is aborted and restarted with the sa me timestamp. – if ts (Ti ) <ts (Tj ) then Ti waits else Ti dies – non-preemptive: Ti never preempts Tj – prefers younger transactions
- 19. Deadlock Avoidance WOUND-WAIT Rule : If Ti requests a lock on a data item which is already locked by Tj , then Ti is permitted to wait iff ts(Ti)>ts(Tj). If ts(Ti)<ts(Tj), then Tj is aborted and the lock is granted to Ti. – if ts(Ti)<ts(Tj) then Tj is wounded else Ti waits – preemptive: Ti preempts Tj if it is younger – prefers older transactions
- 20. Deadlock Detection • For deadlock detection, the system must provide – An algorithm that examines the state of the system to detect whether a deadlock has occurred – And an algorithm to recover from the deadlock • A detection-and-recovery scheme requires various kinds of overhead – Run-time costs of maintaining necessary information and executing the detection algorithm