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Log based recovery:Let us first describe the term transaction log in the context of DBMS. A transaction log is a record in DBMS that remains track of all the transactions of a database system that update any data values in the database. A log haves the following information about a transaction:
In a database system various transactions run simultaneously. When a transaction commits, the data buffers used by it require not be written back to the physical database stored on the secondary storage as these buffers may be used by various other transactions that have not yet committed. On the other hand, some of the data buffers that may have updates by various uncommitted transactions may be forced back to the physical database, as they is no longer being used by the database. So the transaction log helps in remembering which transaction did which alters. Therefore the system knows exactly how to divide the changes made by transactions that have already committed from those alters that is made by the transactions that did not yet commit. Any operation such as start transaction, insert /delete/update and end transaction (commit), adds information to the log having the transaction identifier and enough information to redo or undo the changes.
But how do we recover using log? Let us show this with the help of an example having three concurrent transactions that are active on ACCOUNTS table as:
Transaction T1
Transaction T2
Transaction T3
Read X
Read A
Read Z
Subtract 100
Add 200
Subtract 500
Write X
Write A
Write Z
Read Y
Add 100
Write Y
Figure: The sample transactions
Suppose that these transactions have the following log file (hypothetical) at a point:
Transaction Begin Marker
Transaction
Id
Operation on ACCOUNTS table
UNDO values (assumed)
REDO
values
Transaction Commit Marker
Y
T1
Sub on X Add on Y
500
800
400
Not done yet
N
T2
Add on A
1000
1200
T3
Sub on Z
900
Figure: A sample (hypothetical) Transaction log
Now suppose at this point of time a failure occurs, then how the recovery of the database will be completed on restart.
Values
Initial
Just before the failure
Operation
Required for recovery
Recovered Database Values
X
400 (assuming update has been done in physical database also)
UNDO
A
1000 (assuming update has not been done in physical database)
Z
900 (assuming update has not been done in physical database)
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