copyright © 1998-2008 curt hill database function what should every database do?

Copyright © 1998-2008 Curt Hill

Database Function

What should every database do?


Functions of a Database• According to EF Codd a database

should provide the following services:

• Data storage and retrieval• User-accessible catalog• Transaction support• Concurrency support• Recovery services• Authorization services• Support for data communication• Integrity services• Data independence services


Data storage and retrieval

• While hiding the file structure, users may: – Add– Delete– Update in place

• Duh!


Queries• A mechanism where a question is asked

of the database and it provides an answer:– Not in natural language– Usually in Structured Query Language

• “Sequel”

– Usually state what is wanted rather than how to get it

• The Data Manipulation Language usually includes both query and update facilities


User-accessible catalog• The database itself knows the

shape of the data– Available records and fields– Attributes of fields– Meaning of the field– Relationships with other fields or

records– Applications that access

• The administrator is the most frequent user of this info


Transaction Support

• The unit of update is a transaction– All or nothing– No partial updates are allowed

• The DBMS must guarantee the integrity of a transaction, even in the face of:– Program errors– Incorrect input– System crashes


Entering an order• Gain access to the system or subsystem• Find the record of the item requested• If there are enough to satisfy the

request, then reserve them and decrement the number in stock

• This reservation is termed committed – no longer available

• If there are not enough mark this item as back ordered

• If the request brings the item below the reorder point generate a transaction that sets in motion an order

• Initiate a shipping order• Generate an invoice


Transactions: ACID• Atomic

– Transaction perceived to be indivisible• Consistent

– Transforms database from one consistent state to another

• Isolated– Understandable without regards to any

other agents

• Durable– Once committed permanence is

guaranteed even with system crashes


Concurrency support• A mechanism is needed to prevent

two concurrent applications from both getting the last airline seat

• Usually a locking protocol– A transaction may lock a table or

fragment of the table– Two types

• Shared for queries• Exclusive for updates

• Concurrency example


Concurrency example

• Two agents ask how many widgets there are– Both are told 20

• Both order 15– One gets 15– The other is told they are

backordered

• As soon as the 15 are committed everyone else is told there are 5


Recovery services

• Many dangers:– Power failure– Hardware failure– Software failure

• How do we get the database back to a consistent state?– Backups– Journaling


Recoveries• Forward

– Use if database is destroyed– Restore from backup– Reapply transactions from the

log/journal

• Backward– Use if database is inconsistent– Process log backward– Unroll transactions that may have

been incomplete


Recovery with checkpoints

• At some point delay new transactions until all current transactions finish

• Save the database• This is the point from which a

backup may be made


Authorization services

• Verify that those who access the data are authorized

• Encryption• Views or sub-schemas• Authorizations• User defined procedures• Privacy


Encryption

• Transparent to user• Data is encrypted before write and

decrypted before read• Securely prevents access from any

other program than DBMS• Does the file system perceive the

database as one or many files?


Views

• Restrict view to the authority of the viewer

• If user is not authorized to change or see a value, remove from the view

• If they may view but not change, mark as read only


Authorization rules• Subject

• Often identified by a login or password

• Object• Set of database entities that are subject

to access

• Action• View, update, delete, etc

• Constraint• Limitations on what can be done• Manager may only change pay of own

department workers


User Defined Procedures

• When a particular action is performed a subroutine is executed

• Subroutine is user-written• Vendor supplies exits to these

subroutines in the DBMS• Checks validity or logs information

customized for the organization


Privacy• A query should not violate the privacy

of individuals• A query may be attempting to find a

statistical value– Find the mean salary of VCSU math

professors

• It may be tailored to violate privacy– Find the mean salary of VCSU math

professors with a CS master’s degree


Support for data communication

• Access should not be limited to a single machine

• From where– Telephone– LAN– Intranet– WAN– Internet


Integrity services• Guaranteeing that an item satisfies

integrity constraints• Data type

– Integer must be numeric

• Legal values– A month of 13 is numeric but not legal– Legal values do not always have to be

contiguous

• Format• Key constraints


Format Rule Example• A product serial number must have:

– First two characters are digits– Next three digits must be alphabetic– Next must be a hyphen– Next must be six digits– Last character is an R or M

• Do not confuse with display formatting– Tele is stored as 8457103 and shown as

845-7103


Key Constraints (1 of 3)

• Candidate key– A minimal collection of one or more

fields that uniquely identify a record– Removing any makes the key not

unique – This may be one field or every field in

the record

• Multiple candidate keys may be possible



• Primary key– Any candidate key designated by the

DBMS– Inserting a record with an identical

primary key must be rejected by the DBMS

– Entity integrity• Primary key may not be duplicate or null



• Foreign key– A field in a record that matches a

primary key in another table– Referential integrity

• A record with a foreign key must have a corresponding record in the other table, where the key is primary


Key Example• Three tables:

– Students, primary key is NAID– Courses, primary key is department,

course, section– Grades, primary key is NAID,

department, course, section• NAID is foreign key as is the other three

• Must not allow a record where the foreign keys reference non-existing records

Data Independence• Users should be unaffected by

changes in the database structure• Physical data independence

– Changing the physical structure does not affect the user

– Such as inserting or removing an index

• Logical data independence– Adding/removing a field does not affect

any program that does not use the field– Changing the format of a field should

also be painless


Are there others?

• Every database offers other services• However these services do not

distinguish a database from other applications

• These may include– Accepting SQL or other query language– Report generation


copyright © 1998-2008 curt hill database function what should every database do?

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