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Each transaction Ti executes in two or three different phases in its lifetime, depending on whether it is a read-only or an update transaction. The phases are, in order, 1. Read phase. During this phase, the system executes transaction Ti. It reads the values of the various data items and stores them in variables local to Ti. It performs all write operations on temporary local variables, without updates of the actual database.
2. Validation phase. Transaction Ti performs a validation test to determine whether it can copy to the database the temporary local variables that hold the results of write operations without causing a violation of serializability.
3. Write phase. If transaction Ti succeeds in validation (step 2), then the system applies the actual updates to the database. Otherwise, the system rolls back
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