Chapter 9

DAVID M. KROENKE’S DATABASE PROCESSING, 10th Edition © 2006 Pearson Prentice Hall

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David M. Kroenke

Database Processing Chapter 9

Managing Multi-User Databases


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Multi-User Databases

• Serving the needs of multiple users and multiple applications adds complexity in…– design, – development, and – migration (future updates)


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Multi-User Database Issues

• Interdependency– Changes required by one user may impact

others

• Concurrency– People or applications may try to update

the same information at the same time


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Multi-User Database Issues

• Record Retention– When information should be discarded

• Backup/Recovery– How to protect yourself from losing critical

information


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Role of the Database Administrator

• Organizations typically hire a database administrator (DBA) to handle the issues and complexities associated with multi-user databases.

• A DBA facilitates the development and use of one or more databases.


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Data Administrator versus Database Administrator

Data Administrator– Handle the database functions and responsibilities

for the entire organization.

Database Administrator (DBA)– Handle the functions associated with a specific

database, including those applications served by the database.


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The Characteristics of a DBA

• Technical– The DBA is responsible for the performance and

maintenance of one or more databases.

• Diplomatic– The DBA must coordinate the efforts, requirements,

and sometimes conflicting goals of various user groups to develop community-wide solutions.


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Technical Skills of the DBA

• Managing the database structure• Controlling concurrent processing• Managing processing rights and responsibilities• Developing database security• Providing database recovery• Managing the database management system

(DBMS)• Maintaining the data repository


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Managing the Database Structure

• Managing the database structure includes configuration control and documentation regarding:– The allocation of space– Table creation– Indices creation– Storage procedures– Trigger creation


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Configuration Control

• Post-implementation changes

• Procedures & Policies

• Address unanticipated problems


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The Need for Documentation

• Crucial when trying to understand problems

• Backups and Restores

• Documentation provides the “paper” trail for changes


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Documentation

• All structural changes must be carefully documented with the following:– Reason for change– Who made the changes – Specifically what was changed– How and when the changes were implemented– How were the changes tested and what were the

results


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Controlling Concurrency Processing

• Concurrency control ensures that one user’s actions do not adversely impact another user’s actions

• At the core of concurrency is accessibility.


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Aspects of Concurrency Control

• Rollback/Commit: Ensuring all actions are successful before posting to the database

• Multitasking: Simultaneously serving multiple users

• Lost Updates: When one user’s action overwrites another user’s request


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Rollback/Commit

• Involve logical units of work (LUW).

• Commit when entire LUW successful.

• Rollback if any part of LUW is unsuccessful


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Lost Update Problem

• When two or more users are attempting to update the same piece of data at the same time.

• Resource locking scenarios are designed to address this problem


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Resource Locking

• A resource lock prevents a user from reading and/or writing to a piece of data

• The size of the piece of data (e.g., database, table, row) is termed the lock granularity


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Types of Resource Locks

• Implicit versus Explicit

• Exclusive versus Shared


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Two-Phased Resource Locking

• Two-phased locking, whereby locks are obtained as they are needed– A growing phase, whereby the transaction

continues to request additional locks– A shrinking phase, whereby the transaction

begins to release the locks


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Deadlocks

• When two transactions are waiting on one another to release resources.


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Avoiding Deadlocks

• Strategy 1:– Wait until all resources are available, then lock them

all before beginning

• Strategy 2:– Establish and use clear locking orders/sequences

• Strategy 3:– Once detected, the DBMS will rollback one

transaction


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Resource Locking Strategies

Optimistic Locking

– Read data– Process transaction– Issue update– Look for conflict– If conflict occurred,

rollback and repeat or else commit

Pessimistic Locking – Lock required

resources– Read data– Process transaction– Issue update– Release locks


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Providing Database Recovery

• Common causes of database failures…– Hardware failures– Programming bugs– Human errors/mistakes– Malicious actions

• Since these issues are impossible to completely avoid, recovery procedures are essential


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Database Recovery Characteristics

• Continuing business operations (Fall-back procedures/Continuity planning)

• Restore from backup

• Replay database activities since backup was originally made


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Fall-back Procedures/Continuity Planning

• The business will continue to operate even when the database is inaccessible

• The fall-back procedure defines how the organization will continue operations

• Careful attention must be paid to…– saving essential data– continuing to provide quality service


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Restoring from Backup

• In the event that the system must be rebuilt or reloaded, the database is restored from the last full backup.

• Since it is inevitable that activities occurred since the last full backup was made, subsequent activities must be replayed/restored.


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Recovery via Reprocessing

• This is a brute-force technique.

• Simply re-type all activities since the backup was performed.– Costly (extra time)– Risky (human error)


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Recovery via Rollback/Rollforward

• Most database management systems provide a mechanism to record activities into a log file.


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Rollforward

• Activities recorded in the log files may be replayed. In doing so, all activities are re-applied to the database.

• Use to resynchronize restored database data.


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Rollback

• Since log files save activities in sequence order, can undo activities in reverse order than when they were originally executed.

• Used to correct/undo erroneous or malicious transaction(s).


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Database Security

• Database security strives to ensure…– Only authorized users perform authorized

activities at authorized times


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Managing Processing Rights and Responsibilities

• Processing rights define who is permitted to do what, when

• The individuals performing these activities have full responsibility for the implications of their actions

• Individuals are identified by a username and a password


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Granting of Processing Rights

• Database users are known as an individual and as a member of one or more role

• Access and processing rights/privileges may be granted to an individual and/or a role

• Users possess the compilation of rights granted to the individual and all the roles for which they are members


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Granting Privileges


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Managing the Database Management System (DBMS)

• In addition to controlling and maintaining the users and the data, the DBA must also maintain and monitor the DBMS itself.– Performance statistics (performance tuning /

optimizing)– System and data integrity – Establishing, configuring, and maintaining database

features and utilities


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Maintaining the Data Repository

• The data repository contains metadata. Metadata is data about data.

• The data repository specifies the name, type, size, format, structure, definitions, and relationships among the data. They also contain the details about applications, users, add-on products, etc.


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Types of Data Repositories

• Active data repository– The development and

management tools automatically maintain and upkeep the metadata.

• Passive data repository– People manually

maintain and upkeep the metadata


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Consistent Transactions

• Consistent transactions are often referred to by the acronym ACID

– Atomic– Consistent– Isolated– Durable


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ACID: Atomic

• A transaction consists of a series of steps. Each step must be successful for the transaction to be saved.

• This ensures that the transaction completes everything it intended to do before saving the changes.


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ACID: Consistent

• No other transactions are permitted on the records until the current statement finishes

• This ensures that the transaction integrity has statement level consistence among all records


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ACID: Isolation

• Within multi-user environments, different transactions may be operating on the same data. As such, the sequencing of uncommitted updates, rollbacks, and commits continuously change the data content.

• The 1992 ANSI SQL standards define four isolation levels and specify respective issues.


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Summary of Isolation Levels


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ACID: Durable

• Once committed, durable transactions are saved to the data permanently


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Set-at-a-Time Versus Row-at-a-Time

• SQL statements act as filters for the entire data set.

• A cursor may be defined within a SQL statement to point to a particular record.

• Several types of cursors have been defined. The cursor type defines how the cursor behaves.


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Types of Cursors

Chapter 9

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