Slideshow:
Transaction is the
unit of change
in database system
How to denote transactions
The execution of a transaction can succeed or fail....
Commit = succesful completion Abort = unsuccesful completion
The 2 possible outcomes of
transaction execution
The outcome/result of the execution of a transaction can be:
The ACID properties of
transaction execution
To ensure that transaction will perserve database consistency, the transaction execution must meet the ACID properties:
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The ACID properties of
transaction execution
To ensure that transaction will perserve database consistency, the transaction execution must meet the ACID properties:
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- Transaction:
- Transaction =
the (smallest) unit of
execution of
database operations (updates)
Unit = whole thing, indivisible !!!
- A transaction is:
- executed completely or
- nothing from the transaction is executed
- Transaction =
the (smallest) unit of
execution of
database operations (updates)
- Notation for a transaction:
begin transaction .... .... operations performed by the transaction .... e.g.: read, compute, write .... commit // success
Or: begin transaction .... .... operations performed by the transaction .... e.g.: read, compute, write .... abort // failure
- Result:
- All
operations
between:
begin transaction ... ... ALL operations executed !!! ... commitwill be executed
- None of the
operations between:
begin transaction ... ... NO operations executed !!! ... aborthas be executed
- All
operations
between:
- The execution of a
transaction by
the database system
(must) have the
following
properties:
- Atomicity:
- All operations in a transaction are performed or
- None of the operations in a transaction are performed
- Consistency:
- The operations in a transaction is guaranteed to transform the database from a consistent state to another consistent state
(This property is intrinsic to a transaction)
- Isolation:
- If two transactions
are executing concurrently:
- Each transaction will see the database as if the transaction was executing sequentially (= in isolation)
- If two transactions
are executing concurrently:
- Durability :
- When a transaction is
completed successfully:
- The changes made
to the database state by the
transaction
must be permanent
(Permanant = the changes will survive no matter what happens)
- The changes made
to the database state by the
transaction
must be permanent
- When a transaction is
completed successfully:
- Atomicity: