Slideshow:
What can cause an
inconsistent database state ?
How can a system failure cause an
inconsistent database state ?
How can a system failure cause an
inconsistent database state ?
How can a system failure cause an
inconsistent database state ?
How can concurrent execution cause an
inconsistent database state ?
Consider the non-concurrent execution of these 2 transactions
A non-concurrent execution of the 2 transactions will produce (by definition) a consistent database state !
How can concurrent execution cause an
inconsistent database state ?
How can concurrent execution cause an
inconsistent database state ?
Definition of Database Consistency
- Recall the
definitions of
transaction and the
associated notion of a
consistent state:
- Transaction =
a sequence of
changes to
one or more
database elements
-
When all changes in
a transaction are
made to the
database state:
- The resulting database state is a consistent state (if the initial state is consistent)
Graphically:
- Transaction =
a sequence of
changes to
one or more
database elements
- There
are 2 causes of
inconsistent
database states:
- System failure
- Concurrent execution
- Consider the
following transaction:
- transfer $100 from A to B
Graphically:
There are 2 possible consistent states:
Consistent state 1: A B Consistent state 2: A-100 B+100
- Consider the
database state that
result from
the following
system failure
Transaction: transfer $100 from A to B read A A.balance = A.balance - 100 write A ================================== system fails hereread B B.balance = B.balance + 100 write BGraphically:
The resulting database state is:
Database state = A-100 B
Notice that:
- The database state
A-100 Bis not one of the 2 possible consistent states !!!
- The database state
- Consider the following
2 transactions:
T1 = Deposit $1 to A: read A A = A + 1 write A
T2 = Deposit $2 to A: read A A = A + 2 write A
- The possible
cosnsistent database states for
executing
T1 and
T2
are:
- Case 1: T1 before
T2
- Case 2: T2 before
T1
- Case 1: T1 before
T2
- Consider the
following
concurrent execution
of T1 and
T2:
T1 T2 ============================= read A Initially: A = 10 read A A = A + 1 (A = 11) A = A + 2 (A = 12) write A Writes 11 to A write A Writes 12 to A
Final database state: A = 12 (= A + 2)Graphically:
- Theory:
- Assuming that
the database is in a
consistent state.
Then:
- A transaction will
transform
the database
into
a (another)
consistent state
if:
- There are no
system failures
- There are no other transactions executing in the database system
- There are no
system failures
- A transaction will
transform
the database
into
a (another)
consistent state
if:
- Assuming that
the database is in a
consistent state.
- Schematically:
- Therefore:
- In the absence of:
- System failures and
- Concurrent execution of transactions
a transaction will transform a consistent DB state into another consistent DB state
- In the absence of: