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Architecture of a lock scheduler

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• Overview
  • Software organization:

    Expanation:

    Transactions submit requests for database elements to the Transaction Manager The transaction manager will write log records and then forward the requests to the Scheduler The Scheduler can grant or delay the requests based on the currently granted requests The scheduler maintain a list of lock grants in the lock table (Structure of the lock table will be discussed next) If the read/write request is safe, the scheduer will grant the request The scheduer will also make a record in the lock table of this fact that can affect future requests !!! If the read/write request is UNsafe, the scheduer will delay the request

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• Inserting of lock request and lock release operations into a transaction
  • How to insert an lock request operation into a transaction:

    Transactions themselves do not make requests for locks Transactions will only make read/write requests: (using Read(X)/Write(X) (Inc(X,c)) operations) When the read/write requests are received by the scheduler, the scheduler will insert the appropriate lock request actions:

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  • How to insert an unlock operation into a transaction:

    Transactions themselves do not release locks Transactions will only execute a COMMIT/ABORT operation: When the COMMIT/ABORT is received by the scheduler, the scheduler will release the locks held by the transaction:

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• Architecture of a "lock-inserting" scheduler (how to process a lock request)
  • Overview of the architecture:

    Explanation:

    Part 1 of the scheduler: Takes the stream of READ/WRITE operations and: insert the appropriate lock/unlock actions ahead of the READ/WRITE operations (COMMIT/ABORT will be transformed into a list of unlock actions) The resulting stream is sent to Part 2 of the scheduler Part 2 of the scheduler: Takes the stream of lock + DB access requests and use the lock table to: determine if the request can be granted or denied If the lock request is granted: scheduler must update the state of the lock Pass the DB access operation to the buffer manager If action is unlock: Update the lock table Determine if some (lock) request can now be granted