Slideshow:
System failure during
recovery...
Question:
How do we recover from a failed recovery ???
Idempotent operations
How to recovery from a failed recovery
- Problem:
- What happens when the system crashes during a recovery procedure ???
- Idempotent operation:
- Idempotent operation = an operation that produce the same result when the operation is applied any number of times.
- Example of an
idempotent operation:
assignment (=)
x = 4; x = 4; x = 4; // Produce the same result
- Fact:
- Operations used
by a database recovery procedure
are only
idempotent operations
- We restore the
old value
back into the
database element
- This operation is idempotent
- We restore the
old value
back into the
database element
- Operations used
by a database recovery procedure
are only
idempotent operations
- Consequently:
- If there is a system failure
during a
recovery procedure, we
simply:
- Re-apply the log to the database again
- If there is a system failure
during a
recovery procedure, we
simply: