18 September 2008CIS 340 # 1 Last Covered (almost)(almost) Variety of middleware mechanisms Gain?...

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 transactionsGoal: 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

18 September 2008CIS 340 # 1 Last Covered (almost)(almost) Variety of middleware mechanisms Gain?...

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