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



Operation on ACCOUNTS table

UNDO values (assumed)



Transaction Commit Marker



Sub on X Add on Y




Not done yet




Add on A






Sub on Z




                                 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.



Just before the failure


Required for recovery

Recovered Database Values



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










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





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




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
Which file organization provides very fast access to any arbitrary record of a file? Hashed file organization provides very fast access to any arbitrary record of a file.

Question 1 Discuss on transmission impairments Question 2 What is framing? Question 3 Write a note on 10Base2 10Base- T Question 4 Explain the rev

Discuss along with examples about several types of attributes present in the ER model? Types of Attributes are: • SIMPLE attributes are attributes which are drawn from the ato

Project Description: The work is about founding the best skilled person for the job. I will give you the job description. Skills required are Data Entry, Database Administrat

Question: Data Mining is the process of extracting knowledge hidden from large volumes of raw data. (a) Using a suitable example, illustrate how Data Mining can be useful t

We want have a website; we need to test the security of our website. You need to: -Try to bypass our security by simulate a website hacking -Try some hack Techniques like

Insulation between conceptual and physical structures It can be defined as: 1. The Database Administrator should be capable to vary the storage structures without affectin

Aggregate functions Some of these functions are min, count, max, and avg. These functions help in getting consolidated information from a set of tuples. Example:

A file of employee have 10,000 blocks on a cylinder of a disk with characteristics r=8ms and btt=0.6ms I want to know the cost of read inthe file under following conditions for a q

Problem 1. Write an assembly program to find the smallest among two numbers Writing an assembly program to find the smallest among two numbers 2. Explain the intern