Concurrency of Issues of Distributed Advance Transaction
- 1. CONCURRENCY Concurrency of Issues of Distributed Advance Transaction Presented by : ABDEL-HAFIZ AHMAD KHOUDOUR Email : [email protected]
- 2. Concurrency Control when several transaction execute concurrently in the database The fundamental properties of the transaction is isolation. The system must control the interaction among the concurrent transaction.
- 3. The control achieved through one of a variety of mechanism called concurrency control schemes . This concurrency – control schemes based on the serializability property .
- 4. Lock based Protocol A locking protocol is a set of rules that state when a transaction may lock and unlock . To ensure serializability is to require that data items be accessed in mutually exclusive manner.
- 5. While one transaction is accessing a data item, no other transaction can modify that data. The most common method used to implement requirements is to allow a transaction to access data item only if it is currently holding a lock on that item.
- 6. Two modes of data item may be locked Shared-mode (s) lock. Executive-mode (X) lock Lock compatibility matrix S X S true false X false false
- 7. • Shared mode is compatible with shared mode, several shared mode lock can be held at the same time on a particular data item . • Every transaction request a lock depending on the types of operations.
- 8. • A transaction will make by the concurrency – control manager. • Both of the transaction and the operation continue after the concurrency-manager grants the lock to the transaction.
- 9. Granting of Locks The concurrency-control manager grants the lock provided : • If there is no other transaction holding a lock in a conflicting mode. • Also if there is no other transaction that is waiting for a lock . • A lock request made later will never get blocked a lock request.
- 10. Implementation of locking • Receive messages from transaction and sends messages in reply. • Replay to lock-request messages with lock-grants messages. • Unlock-messages require only and acknowledgment In response, but may result in a grant messages to another waiting transaction.
- 11. Graph-Based Protocols • It impose restrictions on the order in which items are accessed. • in that way we can ensure serializability without requiring of two-phase locking and can also ensure deadlock freedom.
- 12. Timestamp – Based Protocols • A method that to select an ordering among transaction in advance. • The most common method for doing so is to use a timestamp-ordering scheme
- 13. Timestamps The timestamp of the transactions determine the serializability order. There are two simple methods for implementing this scheme: 1. Use the value of the system clock as the timestamp, a transaction’s timestamp is equal to the value of the clock when the transaction enters the system.
- 14. Use a logical counter : is incremented when after a new timestamp has been assigned We can associate with each data items two values W-timestamp R-timestamp
- 15. Weak level of consistency Serializability is a useful concept because it allows programmers to ignore issues related to concurrency when they code transaction. For example SQL allows a transaction to specify that it may executed in a way that becomes nonserializable with respect to other transaction .