BISQL – Laymen to SQL Developer # 39 – Recovery Techniques #2 – Deferred Update, Immediate Update

Hi Folks,

This post is part of Series Database Management Systems

Currently running topic for this series is listed as below :

Series of Database Management Systems

>>Chapter 1 : DBMS [Database Management Systems]

>>Chapter 2 : Database Core Concepts and  Applications

>>Chapter 3 : Record Storage and Primary File Organization

>>Chapter 4 : Index Structures of Files

>>Chapter 5 : Entity-Relationship Model

>>Chapter 6 : Relational Algebra

>>Chapter 7 : SQL

>>Chapter 8 : Functional Dependencies and  Normalization For Relational Databases

>>Chapter 9 : Transaction Processing

>>Chapter 10 : Concurrency Control Techniques

>>Chapter 11 : Recovery Techniques<You are Here>

Continuing from my previous post on this series.

We are going to Cover the Following Points in this article

• Deferred Update
• Immediate Update

Deferred Update

It defers or postpones any actual updates to the database until the transaction completes its execution successfully, and reaches its commit point. During transaction execution the updates are recorded only in the log and in each buffer. After the transaction reaches its commit point, the log is force written to the disk, i.e., the updates are recorded in the database. If a transaction fails before it reaches the commit point, there is no need to undo any operations, any how changes are not updated to db, so no undoing is required, and i.e. the information on the log is simply ignored. REDO is needed in case the system fails after a transaction commits, but before all the changes are recorded in the database on disk [after a failure has occurred, the recovery sub system consults the log to determine which transactions need to be redone]. Transaction Ti needs to be redone if and only if the log contains both the record <Ti Start> and the record <Ti commit>. Thus information in the log is used in restoring the system to a previous consistent state.

Hence it is also known as No-undo/redo algorithm. E.g.: consider a transaction t1 that transfers

Rs.50 from account A to account B. This is defined as follows:

T0: read (A)

log database

T0: read (A) <T0 START> A:=A-50 <t0.A.950> Write (A) <T0.B.2050> Read (B) <T0.commit> Write (B) A=950

B=2050

Let t1 be a transaction that withdraws R.100 from account C, defined as

T1:read© <T1.SAR> C:C-100 <T1.c.60> Write(C) <T1.commit> (C=100 C=600

Before execution consider the values for A.B.C were Rs.1000 Rs.2000 and

Rs. 700 respectively.

Now let us assume that system crash occurs just after the step write (C) of transaction T1,.i.e., the log at the time of the crash is shown in the fig.

<T0.START>

<T0.A.950>

<T0.B.2050>

<T0.commit>

<T1.START>

<T1.C.600>

When the system comes back up, the operation read <T0> is performed. Since the record <T0 commit>appears in the log on the disk. After this operation is executed, the values of accounts A and B are Rs950 and Rs.2050. The value of account C remains Rs.700; due to incomplete transaction T1 can be deleted from the log.

Immediate Update

In this method updates are applied to the database without waiting to reach the commit point i.e. when a transaction issues an update command, the database can be updated immediately. An updated operation must be recorded in the log before it is applied to the database. If transaction fails after recording some changes in db, but before reaching its commit point, the effect of the transaction on the database must be undone (rolled back). We have to redo the already updated operation and undo (transactions must be rolled back) the effects of uncommitted transactions. So in the case of immediate update technique, both undo and redo operations are required during recovery. Hence immediate update technique is known as UNDO/REDO algorithm.

Now we shall take up the algorithm for UNDO/REDO scheme.

Step1. Redo all transactions for which the log has both start and commit entries.

Step2. Undo all transactions for which the log has start entry but no

commit entry.

· Undo (Ti) restores the value of all data items updated by transaction Ti to the old values.

· Redo (Ti) set the value of all data items updated by transaction (Ti) to the new values.

After a failure has occurred, the recovery scheme consults the log to determine which transactions need to be undone and which need to be redone. This classification of transaction is accomplished as follows:

· Transaction Ti needs to be undone if the log contains the record <Ti start> but does not contain the record <Ti commit>

· Transaction Ti needs to be redone if the log contains both the records

<Ti start> and the record <Ti commit>. Changes made by the transactions are stored back to database.

E.g. Immediate update:

In which the actual updates take place to both the database and the log as a result of execution of T0 and T1.

Lot Database

<T0 START>

<T0.S.1000.950>

<T0.B.2000.2050>

<T0 CP, OT?>

t1.START>

<T1.C.700.600>

<T1 COMMIT>

A=950

B=2050

C=600

Here assume that the crash occurs just after the statement.

<T1.C.700.600> but before <T1 commit> (i.e. before commit of T1)

When the system comes back up, two recovery actions have to be taken. The operation undo (T1) must be performed because only <T1START> is existing in the log, but there is no <T1 commit>record.

The operation redo<T0>must be performed since the log contains both the records.

<TO START> and <TO commit>

Hope you will like Series of Database Management Systems series !

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