Transcript Introduction to Database Systems

COP4710: Three parts
•
Database Development
•
Application Development
•
DBMS Internal
Database Management Systems, R. Ramakrishnan and J. Gehrke
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Chapter 1
Introduction to Database Systems
Database Management Systems, R. Ramakrishnan and J. Gehrke
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What Is a DBMS?

A database is a very large, integrated collection of
data. It models real-world enterprise.
–
–

Entities (e.g., students, courses)
Relationships (e.g., Tim Ashley is taking COP4710)
A Database Management System (DBMS) is a
software package designed to store and manage
databases.
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Why Use a DBMS?

Data independence
– Applications are insulated from changes in
the way the data are structured and stored

Efficient data access
– Sophisticated techniques to store and
retrieve data efficiently

Reduced application development time
– Supports functions common to many
applications
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Why Use a DBMS?

Data integrity and security
– Enforces integrity constraints (e.g., the department
budget is not exceeded when a new salary is inserted)
– Enforces access controls for different classes of users

Uniform data administration
– Database administrators have the
knowledge to minimize data
redundancy and fine tune the
database to make retrieval efficient
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Why Use a DBMS?

Concurrent access
– Multiple users can access
the database concurrently

Recovery from crashes
– Protect the users from the
effects of system failures
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Data Models

A data model is a collection of concepts for
describing data.

A schema is a description of a particular collection
of data (e.g., students), using a given data model.

The relational model of data is the most widely
used model today.
–
Main concept: relation, basically a table with rows and
columns.
–
Every relation has a schema, which describes the
columns, or fields.
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Levels of Abstraction
Many views, single conceptual
(logical) schema and physical
schema.
–
View 1
Views describe how users see
the data.
–
Conceptual schema defines
logical structure
–
Physical schema describes the
files and indexes used.
View 2
View 3
Conceptual Schema
Physical Schema
Database
 Schemas are defined using DDL (Data Definition Language)
 Data is modified/queried using DML (Data Manipulation Language)
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Example: University Database

Conceptual schema:
–
–
–

Students(sid: string, name: string, login: string,
age: integer, gpa:real)
Courses(cid: string, cname:string, credits:integer)
Enrolled(sid:string, cid:string, grade:string)
Physical schema:
Example:
– Relations stored as unordered files (data records not sorted)
– Index on first column of Students

External Schema (View):
–
Course_info(cid:string, enrollment:integer)
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Data Independence
Applications insulated from how data are
structured and stored.
Physical data independence: Protection from
changes in physical structure of data.
 Logical data independence: Protection from changes
in logical structure of data (more on next slide).

These are some of the most important benefits of
using a DBMS!
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Logical Data Independence
Application
View: Courseinfo( cid: String, fname: string, enrollment: integer)
Faculty( fid: string, fname: string, sal: real)
Courses( cid: string, cname: string, credits: integer)
Enrolled( sid: string, cid: string, grade: string)
Teaches( fid: string, cid: string)
Reorganize
data
Faculty_public( fid: string, fname: string, office: integer)
Faculty_private( fid: string, sal: real)
Changes in the
.
conceptual schema do
.
.
not affect the application
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Concurrency Control

Concurrent execution of user programs is essential
for good DBMS performance.
–
Since disk accesses are frequent and relatively slow, it is
important to keep the cpu humming by working on
several user programs concurrently.

Interleaving actions of different user programs can
lead to inconsistency: e.g., check is cleared while
account balance is being computed.

DBMS ensures such problems don’t arise: users can
pretend they are using a single-user system.
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Transaction: An Execution of a DB Program

Key concept is transaction, which is an atomic sequence of
database actions (reads/writes).

Each transaction, executed completely, must leave the DB
in a consistent state if DB is consistent when the transaction
begins.
–
–
–
Users can specify some simple integrity constraints on the data, and
the DBMS will enforce these constraints.
Beyond this, the DBMS does not really understand the semantics of
the data. (e.g., it does not understand how the interest on a bank
account is computed).
Thus, ensuring that a transaction (run alone) preserves consistency
is ultimately the user’s responsibility!
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Scheduling Concurrent Transactions
DBMS ensures that execution of {T1, ... , Tn} is
equivalent to some serial execution T1, ... Tn.
–
–
Before reading/writing an object, a transaction requests a
lock on the object, and waits till the DBMS gives it the lock.
All locks are released at the end of the transaction. (Strict
2PL locking protocol.)
I have
the lock
I wait
T2
W
X
R
T1
Database Management Systems, R. Ramakrishnan and J. Gehrke
I can lock
now
T2
W
I am
done
X
T1
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2PL Locking Protocol
2PL Locking Protocol is sufficient and
more efficient
Number of locks acquired


2PL
Strict 2PL
What if I need
the lock again
before commit ?
Time
2PL offers more concurrency; but it
is difficult to implement
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Deadlock
I wait
for X
X
R2
W3
T2
I have the lock
on Y
I have the lock
on X
W1
T1
Y
W4
I wait
for Y
A solution: T1 or T2 is aborted and restarted
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Ensuring Atomicity

DBMS ensures atomicity (all-or-nothing property) even
if system crashes in the middle of a Xact.

Idea: Keep a log (history) of all actions carried out by
the DBMS while executing a set of Xacts:
–
Before a change is made to the database, the corresponding
log entry is forced to a safe location. (WAL protocol; OS
support for this is often inadequate.)
–
After a crash, the effects of partially executed transactions
are undone using the log. (Thanks to WAL, if log entry wasn’t
saved before the crash, corresponding change was not
applied to database!)
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The Log

The following actions are recorded in the log:
–
Ti writes an object: the old value and the new value.

–
Log record must go to disk before the changed page!
Ti commits/aborts: a log record indicating this action.

Log records chained together by Xact id, so it’s easy to
undo a specific Xact (e.g., to resolve a deadlock).

Log is often duplexed and archived on “stable” storage.

All log related activities (and in fact, all CC related
activities such as lock/unlock, dealing with deadlocks
etc.) are handled transparently by the DBMS.
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ACID Properties
A transaction is a collection of actions
with the following ACID properties:

Atomicity: A transaction’s changes to the state are

Consistency: A transaction is a correct transformation

Isolation: Even though transaction execute

Durability: Once a transaction completes successfully,
atomic – either all happen or non happen.
of the state.
concurrently, it appears to each transaction, T, that
other executed either before or after T, but not both.
its changes to the state survive failures (transaction’s
effects are durable).
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Databases make these folks happy ...

End users and DBMS vendors

DB application programmers
–

e.g., smart webmasters
Database administrator (DBA)
–
–
–
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Designs logical /physical schemas
Handles security and authorization
Data availability, crash recovery
Database tuning as needs evolve
Must understand how a DBMS works!
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These layers
must consider
concurrency
control and
recovery
Structure of a DBMS


A typical DBMS has a
layered architecture.
The figure does not
show the concurrency
control and recovery
components.
Query Optimization
and Execution
Relational Operators
Files and Access Methods
Buffer Management
Disk Space Management
Database
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Summary
DBMS is used to maintain and query large datasets.
 Levels of abstraction give data independence.
 Benefits include recovery from system crashes,
concurrent access, quick application development,
data integrity and security.
 A DBMS typically has a layered architecture.
 DBAs hold responsible jobs and are well-paid!
 DBMS R&D is one of the broadest,
most exciting areas in CS.

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