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16 Cards in this Set
- Front
- Back
intent lock
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a mechinism used by transactions to declare their intent of acquiring a lock
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schema lock
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used to maintain structural integrity of tables - i.e. cant drop an index while it is being used
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exclusive lock
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needed for data modification
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update locks
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intermediary lock between schared and exclusive. Indicating that a resource wants to obtain an exclusive lock
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SELECT: holdlocks
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shared locks held for duration of transactions
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shared locks
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used for read operations. Data modification is blocked till locks released - multiple shared locks are possible
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transaction isolation level-- 4 settings
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a users isolation level is the level of ability a user has to control a portion of data. The 4 levels are:
1. Read Uncommitted 2. Read Committed 3. Repeatable Read 4. Serializable |
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Transaction Isolation Level: read committed
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avoids dirty reads but is suceptable to non-repeatable reads and phantom data. Shared locks held when data is read
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Transaction Isolation Level: read uncommitted
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lowest locking level-- suceptable to dirty reads, non-repeatable reads, phantom data. no chared locks held, no exclusive locks
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Transaction Isolation Level: repeatable read
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prevents dirty reads and non-repeatable reads but not phantom data. locks placed on all rows used in a query
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Transaction Isolation Level: serializeable
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places a range lock on the data-- no inserts or updates. this is the highest locking setting
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dirty reads
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reading data that is later scrolled back
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non-repeatable read
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previously read data is changed
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phantom data
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new data appears in a previously read data set
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2 ways to avoid deadlocks
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1. always access objects in the same order
2. attempt to keep transactions as short as possible |
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what is optimistic locking?
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leaving the data unlocked until it is actually time to write the changes in the optimistic hope that nobody will change it behind your back
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