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Thread: Interested Transcation List(ITL).

  1. #1
    Oracle Administrator
    Join Date
    Dec 2011

    Interested Transcation List(ITL).

    When a row is locked by a transaction, that information is placed in the block header where the row is located. When another transaction wishes to acquire the lock on the same row, it has to travel to the block containing the row anyway, and upon reaching there, it can easily identify that the row is locked, from the block header. There is no need to queue up for some single resource like a lock manager. This makes applications immensely scalable.

    So what portion of the block header contains information on locking?
    It is a simple data structure called Interested Transaction List (ITL), a linked list data structure that maintains information on transaction address and rowid. ITL contains several slots for transactions. When a row in the block is locked for the first time, the transaction places a lock in one of the slots with the rowid of the row that is has locked. In other words, the Transactions is Interested in the row, hence the name Interested Transaction List. When the same transaction or another one locks another row, the information is stored in another slot and so on and so forth.

    So how many slots are available?
    During the table creation, the parameter INITRANS parameter defines how many slots are initially created in the ITL. When the transactions exhaust all the available slots and a new transaction comes in to lock a row, the ITL grows to create another slot. But to what extent can it grow? It can grow up to the number defined by the MAXTRANS parameter of the table, provided there is space in the block.
    Last edited by Hemant; 01-10-2012 at 05:17 PM.

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