Locking base concurrency control
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This document summarizes research on lock-based concurrency control for distributed database management systems (DDBMS). It defines lock-based algorithms and protocols like two-phase locking (2PL) that ensure serializable access to shared data. The 2PL protocol is discussed in centralized, primary copy, and distributed implementations for DDBMS. The communication structure of distributed 2PL is also outlined, with lock managers coordinating access across database sites. In conclusion, lock-based concurrency control using 2PL is commonly used to achieve consistency while allowing maximum concurrency in transaction processing.
Locking base concurrency control
- 1. A Research On “Lock base concurrency control” For Partial Fulfillment of the Requirements for the Degree of Master of Computer Information System Awarded by Pokhara University Date: 24 July 2019
- 2. Table of Contents 1. Introduction.............................................................................................................................. 1 2. Locking Based Algorithms ...................................................................................................... 2 3. Lock-Based Protocols Example............................................................................................... 4 4. Lock Based protocols .............................................................................................................. 5 5. 2PL for DDBMS...................................................................................................................... 6 5.1 Centralized 2PL..................................................................................................................... 6 5.2 Primary copy 2PL ................................................................................................................. 6 5.3 Distributed 2PL..................................................................................................................... 7 6. Communication structure of the distributed 2PL..................................................................... 8 7. Conclusion............................................................................................................................... 9 8. References.............................................................................................................................. 10
- 3. Abstract The lock base concurrency control is a well-accepted method to handle database transaction at execution time. When the multiple users try to access data at exactly the same time lock-based concurrency control manipulation and lock the database execution and provide effective data to the user. Without the concurrency control, there might be a problem with performance in a database transaction. Multiple frameworks based database concurrency control is the most important element for proper functioning
- 4. 1 1. Introduction A lock is a variable related with an information thing that depicts a status of information thing as for conceivable activity that can be connected to it. They synchronize the entrance by simultaneous exchanges to the database things. It is required in this convention that every one of the information things must be gotten to in a fundamentally unrelated way. Give me a chance to acquaint you with two regular locks which are utilized and some wording followed in this convention. A lock is a data variable which is associated with a data item. This lock signifies that operations that can be performed on the data item. Locks help synchronize access to the database items by concurrent transactions. All lock requests are made to the concurrency-control manager. Transactions proceed only once the lock request is granted.
- 5. 2 2. Locking Based Algorithms Locking-based concurrency algorithms ensure that data items shared by conflicting operations are accessed in a mutually exclusive way. This is accomplished by associating a “lock” with each such data item. Terminology Use in this protocol Shared Lock (S): also known as Read-only lock. As the name suggests it can be shared between transactions because while holding this lock the transaction does not have the permission to update data on the data item. S-lock is requested using lock-S instruction. Exclusive Lock (X): Data item can be both read as well as written. This is Exclusive and cannot be held simultaneously on the same data item. X-lock is requested using lock-X instruction. Two types of locks (lock modes) o Read lock (rl) – also called shared lock o Write lock (wl) – also called exclusive lock Compatibility matrix of locks rl i (x) wl i (x) rl j (x) Compatible not compatible wl j (x) Not compatible not compatible General locking algorithm 1. Before using a data item x, transaction requests lock for x from the lock manager
- 6. 3 2. If x is already locked and the existing lock is incompatible with the requested lock, the Transaction is delayed 3. Otherwise, the lock is granted.
- 7. 4 3. Lock-Based Protocols Example Transaction performing locking: T2: lock-S (A); Read (A); Unlock (A); Lock-S (B); 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
- 8. 5 4. Lock Based protocols 4.1 Two-phase locking protocol (2PL) Each transaction is executed in two phases Growing phase: -Transaction may obtain locks -Transaction may not release locks Shrinking phase: the transaction releases locks - Transaction may release locks - Transaction may not obtain locks 4.1.1 Properties of the 2PL protocol Generates conflict-serializable schedules – But schedules may cause cascading aborts If a transaction aborts after it releases a lock, it may cause other transactions that have accessed the unlocked data item to abort as well. Strict 2PL locking protocol Holds the locks till the end of the transaction – Cascading aborts are avoided
- 9. 6 5. 2PL for DDBMS Various extensions of the 2PL to DDBMS 5.1 Centralized 2PL A single site is responsible for the lock management, i.e., one lock manager for the whole DDBMS – Lock requests are issued to the lock manager – Coordinating transaction manager (TM at site where the transaction is initiated) can make all locking requests on behalf of local transaction managers. • Advantage: Easy to implement • Disadvantages: Bottlenecks and lower reliability Replica control protocol is additionally needed if data are replicated. 5.2 Primary copy 2PL – Several lock managers are distributed to a number of sites – Each lock manager is responsible for managing the locks for a set of data items – For replicated data items, one copy is chosen as primary copy, others are slave copies – Only the primary copy of a data item that is updated needs to be write-locked – Once primary copy has been updated, the change is propagated to the slaves • Advantages – Lower communication costs and better performance than the centralized 2PL • Disadvantages – Deadlock handling is more complex
- 10. 7 5.3 Distributed 2PL – Lock managers are distributed to all sites – Each lock manager responsible for locks for data at that site – If data is not replicated, it is equivalent to primary copy 2PL – If data is replicated, the Read-One-Write-All (ROWA) replica control protocol is Implemented ∗ Read(x): Any copy of a replicated item x can be read by obtaining a read lock on The copy ∗ Write(x): All copies of x must be write-locked before x can be updated • Disadvantages – Deadlock handling more complex – Communication costs higher than primary copy 2PL
- 11. 8 6. Communication structure of the distributed 2PL The coordinating TM sends the lock request to the lock managers of all participating Sites. – The LMs pass the operations to the data processors – The end of the operation is signaled to the coordinating TM Fig: Communication structure of the distributed 2PL
- 12. 9 7. Conclusion Concurrency orders the operations of transactions such that two properties are achieved: the database is always in a consistent state and the maximum concurrencies of operations are achieved. Concurrency Control is a problem that arises when multiple processes are involved in any part of the system. In most commercial systems, the most popular mechanism for concurrency control is two-phase locking.
- 13. 10 8. References Principles of distributed Database System M. Tamer Ozsu (Book) www.codex.cs.yale.edu