Describe log based recovery
The most usably structure for recording data base modification is the LOG the log is a sequence of log records recording all the update activities in the data base. There are several types of log records. An update log records describes a single data base write it has these fields
Transaction identifier
Data item identifier
Old value
New value
The various types log records as.
- < Ti start >. Transaction Ti has started
- < Ti , Tx v1,v2 > .Transaction Ti has performed a right on data item
- < Ti commit > Transaction Ti has committed
- < Ti about > Transaction Ti has aborted
Ø Deferred data base modification
Ø Immediate data base modification
Ø Check point
Ø Shadow paging