Flashcards
Transactions & ACID
Cards in this deck (15)
- 1what is ACID?Atomicity
Consistency
Isolation
Durability - 2What does Atomicity entail?if one part of the transction fails, then the entire transaction fails.
- 3Consistencyany transaction will bring the database from one valid state to another.
- 4Isolation -The isolation property ensures that the concurrent execution of transactions results in a system state that would be obtained if transactions were executed sequentially, i.e., one after the other.
- 5DurabilityThe durability property ensures that once a transaction has been committed, it will remain so, even in the event of power loss, crashes, or errors.
- 6what happens when you "rollback"?you abort the transaction
- 7Isolation levelsRead un-committed
Read committed
Repeatable read
Serializable - 8What is the "default" isolation-level?Repeatable read
- 9Read uncommittedSpecifies that statements can read rows that have been modified by other transactions but not yet committed. (i.e you can have dirty reads).
- 10Read committedRead committed guarantees that any data read was committed at the moment is read. It simply restricts the reader from seeing any intermediate, uncommitted, 'dirty' read. IT makes no promise whatsoever that if the transaction re-issues the read, will find the Same data, data is free to change after it was read. I.e if you READ twice, the second READ might return another value.
- 11Repeatable readin addition to the guarantees of the read committed level, RR also guarantees that any data read cannot change, if the transaction reads the same data again, it will find the previously read data in place.
BUT - phantom reads are possible. - 12Phantom readsA phantom read occurs when, in the course of a transaction, two identical queries are executed, and the collection of rows returned by the second query is different from the first.
- 13Example of phantom readtransaction fetched a range of records multiple times from the database and obtained different result sets (containing phantom records).
- 14SerializableHighest level of isolation. ACID compliant. Transactions are executed with locking at all levels (read, range and write locking) so they appear as if they were executed in a serialized way. (no dirty, phantom or unrepeatable reads)
- 15—
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- Databas Flashcards on Transactions & ACID, created by Emma Tysk on 26/12/2017.
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- Emma Tysk
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