Section 06 (a)RDBMS 21 06 (a) Supplement RDBMS Issues 2 HSQ - DATABASES & SQL And Franchise Colleges By MANSHA NAWAZ

Section 06 (a) RDBMS 2 1

06 (a) Supplement RDBMS Issues 2

HSQ - DATABASES & SQL

And Franchise Colleges

By MANSHA NAWAZ


Database Recovery

This lecture is concerned with advanced server based RDBMS needs.

RDBMS Issues 2: Database Recovery & Concurrency

Recovery is Needed

• When: – Hardware errors occur– Software errors occur– External circumstances occur (e.g., flood, fire, etc.)

• Redundancy – is key to recovery, but this redundancy is hidden from the user and is different to

data redundancy as seen previously


Transaction

A transaction is a unit of work, – that is either done completely– or not done at all

Transaction– is the unit of recovery and – the unit of concurrency

• (see later on concurrency)


Transaction Process

Assume an SQL statement is atomic, then we are concerned with situations requiring more than one SQL statements (e.g., crediting a bank account and debiting another)

BEGIN TRANSACTION

Do lots of things …..

ABORT TRANSACTION (ROLLBACK)

END TRANSACTION (COMMIT)


Transaction Process

Q: How do we know how to rollback – what were the previous operations since the Begin Transaction statement?

A: keep a LOG – A System File

• keeps details of transactions begun• operations performed (before and after states)• transactions aborted• transactions committed• and any other information of importance to recovery (see later)


Transaction Process

• Log enables an undo, as it keeps details of new and old values of attributes (i.e., update Attribute A from 3 to 5)– Old values are restored on an undo

• Failure occurs during undo, then undo – undoing many times = undo once

• Failure between operation & writing log – Write Ahead Log protocol


Possible Recovery Situations

• Particular types of software and hardware situations where recovery may be needed:

– Planned situations by programmer (in software)– Unplanned by programmer (in software)– Systems failures (e.g., CPU)– Media Failures (e.g., disk head crash)



• Planned by programmer – force an abort of a transaction by writing the code explicitly

IF Balance < 0 THEN

Message (“Can’t transfer – aborting transaction’);

Abort;

END;



• Unplanned by programmer

– Temp:= count / 0

– Divide by zero causes a software fault and results in an abort of the current transaction.



• System failures (e.g., CPU failure)

– When a COMMIT is performed, the changes are considered to be permanent by the user

– However, internally the changes may not have been transmitted from primary storage (buffers) to secondary storage (disk) at the time of systems failure, and CPU failure affects the buffers (but not the disk).



Q: How can we find out which transactions are affected?

A: LOG?– insufficient on its own

• whilst a transaction commit may have been logged on the log, via write ahead protocol, either the actual update may not have taken place

• and/or the page associated with the last operation(s) of the transaction may not have been written to disk.


Checkpoints

• System controlled points

• (snapshot)– flush all data out of the buffers to guarantee changes

– issue a checkpoint record to the log

• what transactions are active• address of the most recent log record for each transaction

– on abort, the most recent checkpoint ensures that we know which transactions are of interest to us


Using Checkpoints

T1

T4

T5

T3

T2

TfTc

Checkpoint -------------LOG---------->


Using Checkpoints

Process for determining Undo and Redo list of transactions at Tf

•Redo – completely undone, then done again– using log details of past and present states

•Redo several times = redo once

•At Checkpoint Tc: T2 & T3 are active, so put on Undo List– Redo list is initially empty

•Work through log from checkpoint to Tf

•Find BEGIN transaction– put on Undo list

•Find END transaction– change from Undo List to Redo List


Using Checkpoints

• For our transactions diagram:

– Undo {initial} = {T2, T3}– Redo {initial} = {}

– Undo {Tf} = {T3, T5}– Redo {Tf} = {T2, T4}


Media Failures:

• These affect the database

• periodic dumping at appropriate times (e.g., end of the week)

• page updating/incremental dumping (e.g., end of day)

• cold start - use archived dump information and the log if intact


Summary: Database Recovery

• What database recovery is about– Transaction– Log & Write Ahead Log Protocol

• Types of failures:– Software failures, planned and unplanned– Systems failures (e.g., CPU) where only recent transactions may be affected

• Checkpoints– Media Failures, where database may be affected


Database Concurrency

This is about ensuring that all concurrent transactions leave the database in the same state as if the transactions were executed one after the other

– Multi-user problem ONLY– Look at the major types of problems and some common solutions– Note: work in terms of reads and writes

• 1 SQL statement may involve both reads and writes• e.g., UPDATE


The Lost Update Problem

Transaction 1 Time Period Transaction 2

Read R 12 Read R

Write R = R + 1 34 Write R = R + 156

With R = 5, R’ = 6 rather than 7i.e., lost one of the updates, as it was written over!


Uncommitted Dependency Problem


1 Write R = some_valueRead R 2

3 Abort456

First transaction has a read value of R which no longer exists


Inconsistent Analysis Problem


Read R 12 Read R3 Write R = R + 104 Read P5 Write P = P + 106 Commit

Read P 7

First transaction has an inconsistent picture of R and P


Solutions

• Locking (most common):

– Lock an object so that other transactions cannot interfere with it

– Unlock it when finished

– Lock + Unlock = 2 Phase Locking (Protocol)

– Most common lock types• Exclusive / write / X Lock• Shared / read / S Lock

X Lock S LockX Lock N NS Lock N Y


Solutions

• Transaction can promote its S lock into an X lock, as long as there are no other S locks on it

• N --- Transaction goes into a wait state

• Locking can occur when a statement is used, or at the beginning of a transaction (greedy!)

• Unlocking occurs typically at COMMIT or ROLLBACK


Lost Update Problem


Read R 12 Read R

Write R = R + 1 34 Write R = R + 15

• Solved the Lost Update, but introduced Deadlock or Deadly Embrace• Solutions:

– Never allow transactions together that could result in deadlock– Choose a victim and abort, with error message


Uncommitted Dependency Problem


1 Write R = some_valueRead R 2

3 Abort456


Inconsistent Analysis Problem


Read R 12 Read R3 Write R = R + 10?456

Read P 78 Write R = R + 109 Read P10 Write P = P + 1011 Commit


Granularity of Locking

• What to lock ….

– Whole Database

– Whole Table

– Whole Record

– Whole Attribute

• Trade-off between simplicity of method vs. concurrency allowed


Summary: Database Concurrency

• Three common concurrency problems

• Standard Method of dealing with them

– 2 Phase Locking

• Show how they overcome concurrency problems

• Deadlock or Deadly Embrace

• Granularity of Locking


End of Lecture

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Section 06 (a)RDBMS 21 06 (a) Supplement RDBMS Issues 2 HSQ - DATABASES & SQL And Franchise Colleges By MANSHA NAWAZ