A
transaction is part of
program execution that accesses and
update various data items.The transaction must like a consistent database.During
transaction, execution the database may not be consistent.When a transaction is committed, the database must not be inconsistent.
Two main issues to deal with:
Failures of various kinds, such as hardware failures and system crashes.
Concurrent execution of multiple transactions.
ACID property:
Two main issues to deal with:
Failures of various kinds, such as hardware failures and system crashes.
Concurrent execution of multiple transactions.
ACID property:
To preserve integrity of data, the database system must ensure:
Atomicity.
Either all operations of the transaction are
properly reflected in the database or one are.Consistency. Execution of a transaction in isolation preserves the consistency of the database.
Isolation. Although multiple transactions may execute concurrently, each transaction must be unaware of other concurrently executing transactions.Intermediate transaction results must be hidden from other concurrently executed transactions.
That is, for every pair of transactions Ti and Tj, it appears to Ti that either Tj, finished execution before Ti started, or Tj started execution after Ti finished.
Durability. After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures.
Example:-
Transaction to transfer $50 from account A to account B:
Consistency requirement – the sum of A and B is unchanged by the execution of the transaction.
Atomicity requirement — if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency will result.
Consistency requirement – the sum of A and B is unchanged by the execution of the transaction.
Atomicity requirement — if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency will result.
1. read(A)
2. A := A – 50
3. write(A)
4. read(B)
5. B := B + 50
6. write(B)
Durability
requirement — once the user has been notified that the transaction has
completed (i.e., the transfer of the $50 has taken place), the updates to the
database by the transaction must persist despite failures.
Isolation
requirement — if between steps 3 and 6, another transaction is allowed to
access the partially updated database, it will see an inconsistent database
(the sum A + B will be less than it should be).
(the sum A + B will be less than it should be).
Can be ensured trivially by running transactions serially, that is one after the other. However, executing multiple transactions
concurrently has significant benefits, as we will see.
Translation State:
Active,
the initial state; the transaction stays
in this state while it is executing
Partially
committed, after the final statement has been
executed.
Failed,
after the discovery that normal execution
can no longer proceed.
Aborted,
after the transaction has been rolled back and the database restored to its
state prior to the start of the transaction.
Two options after it has been aborted:
restart
the transaction – only if no internal logical error
kill
the transaction
Committed,
after successful completion
