Advanced Database Management System - Tutorials and Notes: Majority Based Protocol - Distributed Lock Manager Concurrency Control

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Friday, 4 April 2014

Majority Based Protocol - Distributed Lock Manager Concurrency Control



Majority Based Protocol for Concurrency Control / Variants of Distributed Lock Manager based Concurrency Control mechanism


Majority Based Protocol:

Assume that we have the data item Q which is replicated in several sites and the Majority Based protocol works as follows;

  • A transaction which needs to lock data item Q has to request and lock data item Q in half+one sites in which Q is replicated (i.e, majority of the sites in which Q is replicated).

  • The lock-managers of all the sites in which Q is replicated are responsible for handling lock and unlock requests locally individually.

  • Irrespective of the lock types (read or write, i.e, Shared or Exclusive), we need to lock half+one sites.

Example:

Let us assume that Q is replicated in 6 sites. Then, we need to lock Q in 4 sites (half+one = 6/2 + 1 = 4). When transaction T1 sends the lock request message to those 4 sites, the lock-managers of those sites have to grant the locks based on the usual lock procedure.

How does Majority Based protocol work?

Implementation:

Figure 1 show the implementation of Majority Based protocol.
In the figure,
  • Q, R, and S are the different data items.
  • Q is replicated in sites S1, S2, S3 and S6.
  • R is replicated in sites S1, S2, S3, and S4.
  • S is replicated at sites S1, S2, S4, S5, and S6.
Figure 1 - Majority Based Protocol Implementation


Let us assume that Transaction T1 needs data item Q to be locked (either read or write mode).

Step 1: Transaction T1 initiated at site S5 and requests lock on data item Q. Q is available in S1, S2, S3 and the site S6. According to the protocol, T1 has to lock Q in half+one sites, i.e, in our example, we need to lock any 3 out of 4 sites. Assume that we have chosen sites S1, S2, and S3.
Step 2: S5 requests S1, S2 and S3 for lock on Q. The lock request is represented in purple color.
Step 3: If the lock on Q can be granted, S1, S2, and S3 grant lock and send a message to S5.
On receiving lock grant, S5 executes the Transaction T1 (Executed on the data item Q which is taken from any one of the locked sites). The lock grant is represented in green color.
Step 4: On successful completion of Transaction, S5 sends unlock message to all the sites S1, S2, and S3.
The unlock message is represented in blue color.
Note: If the transaction T1 writes the data item Q, then the changes must be forward to all the sites where Q is replicated. If the transaction read the data item Q, then no problem.

Advantages:



  • Replicated data handled in decentralized manner. Hence, no single point-of-failure problem.

Disadvantages:

  • Implementation is complex. We need to send (n/2 + 1) lock request messages, (n/2 + 1) lock grant messages, and (n/2 + 1) unlock messages, irrespective of lock requested (read or write).
  • Both read and write involves same level of complexity.
  • Deadlock would occur. (To handle this deadlock, we may need to impose an order in which the locks can be requested)

Points to note:

-Transaction can execute only after successful acquisition of locks on majority of the replicas.
-Needs to send more messages, 2(n/2+1) lock messages and (n/2+1) unlock messages, when compared to Primary Copy protocol (2n+1 messages).
-Local lock-managers are responsible for granting or denying the locks on requested items.
-Not suitable for applications where read operation is frequent.


-When writing the data item, a transaction performs writes on all replicas.
-When handling with unreplicated data, both requests can be handled by requesting the site at which the data item available




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