Concurrency control deals with the isolation and consistency properties of transactions. The distributed concurrency control mechanism of a distributed DBMS ensures that the consistency of the database.
If transactions are internally consistent, the simple way of achieving objective is to execute each transaction alone, one after another. It is obvious that such an alternative is only of theoretical interest and cannot be implemented in any practical system, since it minimizes the system throughput.
Some of the major problems are:
1) Site Failure: There are situation when one or more sites in a DDBMS fails. In such situations, consistency and integrity of the database must be restored.
2) Network Problems: When communication network fails, causing one or more sites to be cut-off from the rest of the sites in the DDBMS environment.
3) Data Duplication: Multiple copies of the database must be monitor carefully for-maintaining consistency.
4) Distributed Transaction: A problem arise when a transaction distributed across various sites. Some of the sites are successfully committing/rolling, while the others may not be successfully done.
5) Distributed Deadlocks: In DDBMS, a deadlock may occur in any one or many sites. So, careful handling is necessary.
The various locking protocols can be used in a distributed environment. The only change that needs to be incorporated is in the way the lock manager deals with replicated data.
Mechanism Concurrency Control
The various mechanisms used are as follows:Mechanism in Concurrency Control
- Single Lock-Manager Approach
- Primary Copy
- Biased Protocol
- Distributed Lock Manager
- Majority Protocol
- Quorum Consensus Protocol
