Log based recovery, Database Management System

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:

  • A transaction start marker
  • The transaction identification: The transaction id, user id or terminal id etc.
  • The operations being performed by the transaction such as delete, update, insert.
  • The objects or data items that are affected by the transaction as well as name of the table, row number and column number.
  • The previous or before values (also known as UNDO values) and changed or after values (also called REDO values) of the data items that have been updated.
  • A pointer to the next transaction log record, if required.
  • The COMMIT marker of the 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

Y

T2

Add on A

1000

1200

N

Y

T3

Sub on Z

900

400

Y

                                 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

500

400 (assuming update has been done in physical database also)

UNDO

500

Y

800

800

UNDO

800

A

1000

1000 (assuming update has not been done in physical database)

UNDO

1000

Z

900

900 (assuming update has not been done in physical database)

REDO

400

 

Posted Date: 3/12/2013 5:18:06 AM | Location : United States







Related Discussions:- Log based recovery, Assignment Help, Ask Question on Log based recovery, Get Answer, Expert's Help, Log based recovery Discussions

Write discussion on Log based recovery
Your posts are moderated
Related Questions
What disadvantage of ODBC does OLE DB overcome? By breaking the features and the methods of a DBMS into COM objects, OLE DB characteristic overcomes a main disadvantage of ODBC

Need for Client Server Computing- Client/Server (C/S) architecture includes running the application on multiple machines in which every machine with its component software tackles

Drawbacks of Data Distribution: The primary drawbacks of distributed database systems are the added complexity needed to ensure proper coordination between the sites. This increas


The traffic data mgmt. systems infrastructure plan describes the software, hardware, data network, and other elements that will support the TDMS. The infrastructure plan is based o

Describe Database state in details? Database state: The actual data within the database at a particular moment in time is known as a database state.

Describe in brief about the Sequence Diagrams Sequence diagrams are interaction diagrams which emphasize time ordering of messages. In UML, it is shown as a table which shows

Describe the development process of Object oriented methods Object oriented methods have roots which are strongly anchored back in the 60s, functional and structured methods we

How does the system cope up with a record crash when recovery is going on after the first crash? Ans:  In a system the undo and redo operations are needed to be idempotent to c

Define query optimization. Query optimization refers to the process of search the lowest -cost method of evaluating a given query.