Last Covered (almost)

18 September 2008 CIS 340# 1

Last Covered (almost)• Variety of middleware mechanisms

Gain?

Enable n-tier architectures while not necessarily using a browser

and all that accompanies a and all that accompanies a distributed structuredistributed structure


Can you answer?

How are object-based MW different from message-based MW?

How is a “monitor” different from a “broker” system?


Topics

• To understand ACID as defining database needs of an operating system

• To understand the operating system• Basic responsibilities• Standard enhancements• Distributed processing support


ACIDS: ACID + Serializability

• The assurance governing concurrent execution

Goal: Several transactions occurring yield consistent results

ANSI standard specification:A transaction sequence continues from

beginning to end -- through all succeeding SQL statements –

until:– COMMIT statement is reached

– ROLLBACK statement is reached– End of program is reached

– Program is abnormally terminated

Def:Def:

Data management (concept)• Commit -making a set of tentative changes permanent• Rollback - an operation which returns the database to some

previous state. – Rollbacks are important for database integrity, because they

mean that the database can be restored to a clean copy even after erroneous operations are performed

• Integrity– Entity integrity concerns the concept of a primary key. Entity

integrity is an integrity rule which states that every table must have a primary key and that the column or columns chosen to be the primary key should be unique and not null.

– Referential integrity concerns the concept of a foreign key. • no child record can exist without a parent



Classic Problems for Shared DBs

Data integrity and consistency problemsData integrity and consistency problems

– Lost updates

– Uncommitted data

– Inconsistent retrievals


Lost Updates

Normal Execution: T1: PRO_QOH=PRO_QOH+100, T2: PRO_QOH=PRO_QOH-30

Problematic Execution


Uncommitted DataCorrect Execution of Two Transactions T1: PRO_QOH=PRO_QOH+100 (Rollback), T2: PRO_QOH=PRO_QOH-30

Data Never Reached Commit, But Still Appears


Inconsistent RetrievalsValid Retrieval During Update: T2: correction of typing error 10 units

Data Entry Correction

Inconsistent RetrievalsCan READS ever get wrong values? Inconsistent with circumstances?


The “Scheduler”“Scheduler”

• Utility of DBMS• Establishes order of operations

among concurrent transaction

How?• Interleaves execution

Uses concurrency control algorithms

• Ensures serializability • Ensures isolation of transactions

Goal: So two transactions do not update same data element at same time


The “Scheduler”“Scheduler”: Read/Write Conflict Matrix

No problem if T1 and T2 access unrelated data Possible confliction:1)Access same data2)At least one of them is a Write operation


Concurrency Control: LockingLockingLock

– Excludes use of a data item

– Required – Prevent reading

inconsistent data

Lock Manager– Assigns & polices

locks

Lock Granularity– Level of lock use

• Database• Table• Page• Row• Field

– Enables different degrees of speed to the concurrency

Def:Def:

Highlights

Transaction log Keeps track of all transactions that modify database

Concurrency control Coordinates simultaneous execution of transactions

Scheduler Responsible for sequencing concurrent operations

Lock Guarantees unique access to a data item

Serializability Guaranteed through the use of two-phase locking

Deadlock When two or more transactions wait indefinitely for each other to release lock

Prevention;

Detection;

Avoidance

Deadlock control techniques


Transactional Properties:

ACID Test

A A tomicity: If several components must come together for a process, then each

atomicatomic process operates correctly or entire combination

aborts

C C onsistency: A service is correct when

executed in its entirety and false false or incorrect data is not or incorrect data is not

introducedintroduced into the component databases

I I solation: If several clients request the

same service at the same time and access the same data, the

overall result will be as if they were alone – isolated --– isolated -- in the

system

D D urability: Completed transactions are complete are complete

and durable (have duration?)and durable (have duration?)If necessary, redoing and undoing of changes in case of failures can

occur

TP-Monitor

transactional coordination

CLIENT

services

Application 1 Application 3Application 2

user program

CLIENT

SERVER


Simple Batch Systems“Monitors”

• Software • Controlled sequence of events• Program branches back to monitor when

finished

Job Control Language (JCL)• Special type of programming language• Provides instruction to the monitor

– What compiler to use?What compiler to use?– What data to use?What data to use?


Time Sharing

• Uses multiprogramming to handle multiple interactive jobs

• Processor’s time is shared among multiple users

• Multiple users simultaneously access the system through terminals


EX: Multiprogramming and Multiprocessing

Documents

Last Covered (almost)