Slideshow:
Review: (simple) protection against failures
Failures and data consistency
The data stored in a database are not "just" a bunch of numbers with no meaning !!!
Failures and data consistency
Example:
Failures and data consistency
Focus:
- Previously we have studied
simple techniques to
protect against
various types of
failures:
- Media failure (i.e.:
disk crash)
- Pretect against 1 disk crash:
RAID
----
See:
click here
- General technique: Archiving (= backup)
- Pretect against 1 disk crash:
RAID
----
See:
click here
- Power failure ---
or power outage
- Stable storage update policy ---- See: click here
- Media failure (i.e.:
disk crash)
- Important fact about the
data stored in a
database:
- The data stored in a
database system represent
facts in
reality
- There are consistency constraints on the data stored in the database !
- The data stored in a
database system represent
facts in
reality
- Example:
- Consider a bank database that
store account balances of
customers
- The database does not
simple store
data on
account balances....
- There are account rules that must be followed to keep the data consistent !!
For example, when customer A transfers $100 to customer B, we must perform the following update:
Transaction: transfer $100 from A to B read A A.balance = A.balance - 100 write A ================================== system fails here read B B.balance = B.balance + 100 write BIf the system fails in the middle of the execution, then the simple stable storage update policy will recover A.balance − 100 but will not set B.balance to B.balance + 100 !!
Then A would lose his $100 !!!
- The database does not
simple store
data on
account balances....
- Consider a bank database that
store account balances of
customers
- In this part of the class notes, we will study:
- Database consistency:
- When we update some
data stored in the
database,
when will an
update
preserve
data integrity ?
- After
defining the
notion of
database consistency, we will study
- How to implement techniques that will ensure database consistency
- When we update some
data stored in the
database,
when will an
update
preserve
data integrity ?
- Database consistency: