7. e OPTIMISTIC METHOD OF CONCURRENCY CONTROL IN

IJRIME Volume2, Issue12 (December-2012) ISSN: 2249-1619 OPTIMISTIC METHOD OF CONCURRENCY CONTROL IN DISTRIBUTED DATABASE Vaibhav Bansal Associate Professor, Tecnia Institute of Advanced Studies, Tecnia ABSTRACT Optimistic method of concurrency control is the subpart of the concurrency control Basically concurrency control ensures that correct results for concurrent operations are generated, while getting those results as quickly as possible Concurrency control in Database management systems, other transactional objects, and related distributed applications (e.g., Grid computing and Cloud computing) ensures that database transactions are performed concurrently without violating the data integrity of the respective databases Thus concurrency control is an essential element for correctness in any system where two database transactions or more, executed with time overlap, can access the same data The general area of concurrency control provides rules, methods, design methodologies, and theories to maintain the consistency of components operating concurrently while interacting, and thus the consistency and correctness of the whole system Operation consistency and correctness should be achieved with as good as possible efficiency, without reducing performance below reasonable Database Concurrency Control: Methods, Performance and Analysis is a review of developments in concurrency control methods for centralized database systems, with a quick digression into distributed databases and multi-computers, the emphasis being on performance The main goals of Database Concurrency Control: Methods, Performance and Analysis are to succinctly specify various concurrency control methods INTRODUCTION Optimistic concurrency control (OCC) is a concurrency control method that assumes that multiple transactions can complete without affecting each other and that therefore transactions can proceed without locking the data resources that they affect International Journal of Research in IT, Management and Engineering www.gjmr.org 35 IJRIME Volume2, Issue12 (December-2012) ISSN: 2249-1619 OPTIMISTIC 1.1 Reading a value/pointer can never cause loss of integrity; reads are completely unrestricted, although returning a value from a query is considered equivalent to a write 1.2 Writes are severely restricted Have read phase, validation phase, then write phase During read, all writes are on local copies Validation ensures consistency across active transactions Write phase is when local copies get written to global DB Some database management systems offer OCC natively - without requiring special application code For others, the application can implement an OCC layer outside of the database, and avoid waiting or silently overwriting records In such cases, the form includes a hidden field with the record's original content, a timestamp, a sequence number, or an opaque token On submit, this is compared against the database If it differs, the conflict resolution algorithm is invoked OPTIMISTIC APPROACH  Based on the assumption that the majority of database operations not conflict  Does not require locking or time stamping techniques  Transaction is executed without restrictions until it is committed  Phases are read, validation, and write Pessimistic concurrency control (or pessimistic locking) is called "pessimistic" because the system assumes the worst — it assumes that two or more users will want to update the same record at the same time, and then prevents that possibility by locking the record, no matter how unlikely conflicts actually are The locks are placed as soon as any piece of the row is accessed, making it impossible for two or more users to update the row at the same time Depending on the lock mode (shared, exclusive, or update), other users might be able to read the data even though a lock has been placed For more details on the lock modes, see Lock modes: shared, exclusive, and update Optimistic concurrency control (or optimistic locking) assumes that although conflicts are possible, they will be very rare Instead of locking every record every time that it is used, the system merely looks for indications that two users actually did try to update the same record at International Journal of Research in IT, Management and Engineering www.gjmr.org 36 IJRIME Volume2, Issue12 (December-2012) ISSN: 2249-1619 the same time If that evidence is found, then one user's updates are discarded and the user is informed For example, if User1 updates a record and User2 only wants to read it, then User2 simply reads whatever data is on the disk and then proceeds, without checking whether the data is locked User2 might see slightly out-of-date information if User1 has read the data and updated it, but has not yet committed the transaction Optimistic locking is available on disk-based tables (D-tables) only The solidDB implementation of optimistic concurrency control uses multi-versioning Each time that the server reads a record to try to update it, the server makes a copy of the version number of the record and stores that copy for later reference When it is time to commit the transaction, the server compares the original version number that it read against the version number of the currently committed data If the version numbers are the same, then no one else changed the record and the system can write the updated value If the originally read value and the current value on the disk are not the same, then someone has changed the data since it was read, and the current operation is probably out-of-date Thus the system discards the version of the data, aborts the transaction, and returns an error message The step of checking the version numbers is called validation The validation can be performed at the commit time (normal validation) or at the time of writing each statement (early validation) In solidDB, early validation is the default method When you use optimistic locking, you not find out that there is a conflict until just before you write the updated data In pessimistic locking, you find out there is a conflict as soon as you try to read the data To use an analogy with banks, pessimistic locking is like having a guard at the bank door who checks your account number when you try to enter; if someone else (a spouse, or a merchant to whom you wrote a check) is already in the bank accessing your account, then you cannot enter until that other person finishes her transaction and leaves Optimistic locking, on the other hand, allows you to walk into the bank at any time and try to your business, but at the risk that as you are walking out the door the bank guard will tell you that your transaction conflicted with someone else's and you will have to go back and the transaction again International Journal of Research in IT, Management and Engineering www.gjmr.org 37 IJRIME Volume2, Issue12 (December-2012) ISSN: 2249-1619 With pessimistic locking, the first user to request a lock, gets it Once you have the lock, no other user or connection can override your lock In solidDB, the lock lasts until the end of the transaction or in the case of long table locks, the lock lasts until you explicitly release it USER REQUIREMENT: In this the user will fetch data easily They don’t have to wait for a long period of time They will get immediate response NEED OF OPTIMISTIC METHOD OF CONCURRENCY CONTROL OCC is generally used in environments with low data contention When conflicts are rare, transactions can complete without the expense of managing locks and without having transactions wait for other transactions' locks to clear, leading to higher throughput than other concurrency control methods However, if conflicts happen often, the cost of repeatedly restarting transactions hurts performance significantly; other concurrency control methods have better performance under these conditions OBJECTIVE OF OPTIMISTIC METHOD OF CONCURRENCY CONTROL Objective of optimistic method of concurrency control is for multiple transaction It this we assume that there will be no conflict Also basically we will run different optimistic method with it’s advantage and disadvantage Optimistic concurrency control technique is a non-blocking and deadlock free approach for concurrency control In this technique, it is assumed that the conflicts between transactions are rare, and the transactions are allowed to proceed for execution without imposing delays When a transaction wants to commit, a validation checking is made to determine whether a conflict has occurred or not To ensure serializability, the transaction is aborted and restarted if any conflict has occurred In optimistic concurrency control technique, the execution of any operation of a transaction follows a sequence of phases depending on whether it is a read operation or an update operation OCC algorithms have only been examined on central database systems •No commercial or prototype databases have implemented using OCC •Most concurrency control algorithms are pessimistic •OCC is optimistic Validate –Read –Compute –Write Read –Compute –Validate –Write International Journal of Research in IT, Management and Engineering www.gjmr.org 38 IJRIME Volume2, Issue12 (December-2012) ISSN: 2249-1619 A lock is used when multiple users need to access a database concurrently This prevents data from being corrupted or invalidated when multiple users try to write to the database Any single user can only modify those database records (that is, items in the database) to which they have applied a lock that gives them exclusive access to the record until the lock is released Locking not only provides exclusivity to writes but also prevents (or controls) reading of unfinished modifications (AKA uncommitted data) There are two mechanisms for locking data in a database: pessimistic locking, and optimistic locking In pessimistic locking a record or page is locked immediately when the lock is requested, while in an optimistic lock the record or page is only locked when the changes made to that record are updated The latter situation is only appropriate when there is less chance of someone needing to access the record while it is locked; otherwise it cannot be certain that the update will succeed because the attempt to update the record will fail if another user updates the record first With pessimistic locking it is guaranteed that the record will be updated TECHNIQUE FOR OPTIMISTIC METHOD Since reading a value or a pointer from a node can never cause a loss of integrity, reads are completely unrestricted Writes are severely restricted Transactions consist of three phases: 3.1 Read Phase: All writes take place on local copies of the object to be modified 3.2 Validations Phase: The step in which it is determined that the transaction will not cause a loss of integrity 3.3 Write Phase: Copies are made global International Journal of Research in IT, Management and Engineering www.gjmr.org 39 IJRIME Volume2, Issue12 (December-2012) ISSN: 2249-1619 READ AND WRITE PHASE Read is also referred to as the “Working Phase” Each transaction has a tentative version of each of the object that it updates READ operations are performed immediately WRITE operations record the new values of the objects as tentative values Two records are kept of the objects accessed within a transaction: a read set and a write set If validation succeeds, then the transaction enters the write phase After write phase, all written values become “global” When a transaction completes, it will request its validation and write phases via transactionEnd VALIDATION PHASE Uses a particularly strong form of validation This is especially important with long-running transactions Method uses an overqualified update scheme to test whether the underlying data source has been updated by another transaction since the beginning of the current transaction Kung and Robinson employ Serial Equivalence for verifying the correctness of concurrent execution of transactions International Journal of Research in IT, Management and Engineering www.gjmr.org 40 IJRIME Volume2, Issue12 (December-2012) ISSN: 2249-1619 VALIDATION OF SERIAL EQUIVALENCE Explicitly assigns each transaction a Transaction Number , t(i), at the end of the read phase Transaction numbers are assigned in order, If the transaction is validated and completes successfully, it retains this number If it fails the validation checks and is aborted, or if the transaction is read-only, the number is released for reassignment Transaction numbers are integers assigned in ascending sequence The number of a transaction defines its position in time Tid satisfies the following property: t(i)