Download Lecture 1 : Introduction

CENG 352
Database Management Systems
Nihan Kesim Çiçekli
email: [email protected]
URL: http://www.ceng.metu.edu.tr/courses/ceng352
CENG 352
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•
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Instructor: Nihan Kesim Çiçekli
Office: A308
Email: [email protected]
Lecture Hours: Tue. 10:40,11:40 (BMB1);
Thu. 9:40 (BMB3)
• Course Web page:
http://www.ceng.metu.edu.tr/courses/ceng352
• Teaching Assistant:
Ömer Önder Tola
[email protected]
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Text Books and References
1. Raghu Ramakrishnan, Database Management
Systems, McGraw Hill, 3rd edition, 2003 (text
book).
2. R. Elmasri, S.B. Navathe, Fundamentals of
Database Systems, 4th edition, Addison-Wesley,
2004.
3. A. Silberschatz, H.F. Korth, S. Sudarshan,
Database System Concepts, McGraw Hill, 4th
edition, 2002.
4. H. Garcia-Molina, J. D. Ullman, J. Widom,
Database Systems The Complete Book, Prentice
Hall, 2002.
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Grading
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Midterm
Assignments and Quizzes
Project
Final Exam
25 %
20 %
20 %
35 %
Exam Date:
Midterm Exam: 2nd week of April.
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Grading Policies
• Policy on missed midterm:
– no make-up exam
• Lateness policy:
– Late assignments are penalized up to 10% per
day.
• All assignments are to be your own work.
Projects in groups of two.
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Course Outline
• The Relational Data Model, Relational Algebra
and Calculus, SQL
• Query Evaluation and Optimization
• Relational Database Design and Tuning
• Transaction Management, Concurrency Control
and Crash Recovery
• Database Security and Authorization
• Parallel and Distributed Databases
• Object-Database Systems
• Information Retrieval and XML Data
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Basic Definitions
• Database: A collection of related data.
• Data: Known facts that can be recorded and have an implicit
meaning.
• Mini-world: Some part of the real world about which data is
stored in a database. For example, student grades and
transcripts at a university.
• Database Management System (DBMS): A software
package/ system to facilitate the creation and maintenance of a
computerized database.
• Database System: The DBMS software together with the data
itself. Sometimes, the applications are also included.
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Typical DBMS Functionality
• Define a database : in terms of data types,
structures and constraints
• Construct or Load the Database on a secondary
storage medium
• Manipulating the database : querying, generating
reports, insertions, deletions and modifications to
its content
• Concurrent Processing and Sharing by a set of
users and programs – yet, keeping all data valid and
consistent
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Typical DBMS Functionality
Other features:
– Protection or Security measures to prevent
unauthorized access
– “Active” processing to take internal actions on
data
– Presentation and Visualization of data
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Example of a Database
(with a Conceptual Data Model)
• Mini-world for the example: Part of a
UNIVERSITY environment.
• Some mini-world entities:
–
–
–
–
–
STUDENTs
COURSEs
SECTIONs (of COURSEs)
(academic) DEPARTMENTs
INSTRUCTORs
Note: The above could be expressed in the ENTITYRELATIONSHIP data model.
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Example of a Database
(with a Conceptual Data Model)
• Some mini-world relationships:
–
–
–
–
–
–
SECTIONs are of specific COURSEs
STUDENTs take SECTIONs
COURSEs have prerequisite COURSEs
INSTRUCTORs teach SECTIONs
COURSEs are offered by DEPARTMENTs
STUDENTs major in DEPARTMENTs
Note: The above could be expressed in the ENTITYRELATIONSHIP data model.
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Example: Online Bookseller
• Data = information on books (including categories,
bestsellers, etc.), customers, pending orders, order
histories, trends and preferences, etc.
– Massive: many gigabytes at a minimum for mediumsize bookseller, more if keep all order histories over all
time, even more if keep images of book covers and
sample pages
=> Far too big for memory
– Persistent: data outlives programs that operate on it
– Multi-user: many people/programs accessing same
database, or even same data, simultaneously
=> Need careful controls
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Files vs. DBMS
• Application must stage large datasets between
main memory and secondary storage (e.g.,
buffering, page-oriented access, etc.)
• Special code for different queries
• Must protect data from inconsistency due to
multiple concurrent users
• Crash recovery
• Security and access control
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What is a Relational Database?
• Based on the relational model (tables):
acct #
12345
34567
…
name
Sally
Sue
…
balance
1000.21
285.48
…
• Today used in most DBMS's.
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Relational Model
Relational model is good for:
• Large amounts of data —> simple operations
• Navigate among small number of relations
Difficult Applications for relational model:
• VLSI Design (CAD in general)
• CASE
• Graphical Data
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Data Models
60’s
Hierarchical
Network
70's
80's
Relational
Choice for most new
applications
90’s
Object Bases
Knowledge Bases
now
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The DBMS Marketplace
• Relational DBMS companies – Oracle, Sybase – are among the
largest software companies in the world.
• IBM offers its relational DB2 system.
• Microsoft offers SQL-Server, plus Microsoft Access for the cheap
DBMS on the desktop, answered by “lite” systems from other
competitors.
• Relational companies also challenged by “object-oriented DB”
companies.
• But countered with “object-relational” systems, which retain the
relational core while allowing type extension as in OO systems.
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Three Aspects to Studying DBMS's
1. Modeling and design of databases.
– Allows exploration of issues before committing to an
implementation.
2. Programming: queries and DB operations like
update.
– SQL = “intergalactic dataspeak.”
3. DBMS implementation.
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Database Schema vs. Database State
• Database State (Instance): Refers to the content of a
database at a moment in time.
• Initial Database State: Refers to the database when it
is loaded
• Valid State: A state that satisfies the structure and
constraints of the database.
• Distinction
• The database schema changes very infrequently. The
database state changes every time the database is updated.
• Schema is also called intension, whereas state is called
extension.
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Three-Schema Architecture
• Proposed to support DBMS characteristics
of:
• Program-data independence.
• Support of multiple views of the data.
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Three-Schema Architecture
• Many views (External schemas), View 1 View 2 View 3
single conceptual (logical)
schema and physical
Conceptual Schema
schema(internal schema).
–
–
–
Views describe how users see the
data.
Conceptual schema defines logical
structure
Physical schema describes the files
and indexes used.
Physical Schema
* Schemas are defined using DDL; data is modified/queried using DML.
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Three-Schema Architecture
Mappings among schema levels are needed
to transform requests and data. Programs
refer to an external schema, and are mapped
by the DBMS to the internal schema for
execution.
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Data Independence
• Logical Data Independence: The capacity
to change the conceptual schema without
having to change the external schemas and
their application programs.
• Physical Data Independence: The capacity
to change the internal schema without
having to change the conceptual schema.
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Data Independence
• When a schema at a lower level is changed,
only the mappings between this schema
and higher-level schemas need to be
changed in a DBMS that fully supports data
independence.
• The higher-level schemas themselves are
unchanged. Hence, the application
programs need not be changed since they
refer to the external schemas.
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Query Languages
Employee
Name
Department
Dept
Dept
Manager
SQL
SELECT Manager
FROM Employee, Department
WHERE Employee.name = "Clark Kent” AND Employee.Dept = Department.Dept
Query Language
Data definition language (DDL) ~ like type definitions
Data Manipulation Language (DML)
Query (SELECT)
UPDATE < relation name >
SET <attribute> = < new-value>
WHERE <condition>
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Host Languages
C, C++, Java, Lisp, COBOL
Application prog.
Calls to
DB
DBMS
Local Vars
(Memory)
(Storage)
• Host language is completely general (Turing complete)
• Query language—less general "non procedural" and
optimizable
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Concurrency Control
• Concurrent execution of user programs is essential
for good DBMS performance.
–
Because 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.
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–
–
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.)
Idea: If an action of Ti (say, writing X) affects Tj (which
perhaps reads X), one of them, say Ti, will obtain the lock on
X first and Tj is forced to wait until Ti completes; this
effectively orders the transactions.
What if Tj already has a lock on Y and Ti later requests a lock
on Y? (Deadlock!) Ti or Tj 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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Structure of a DBMS
• A typical DBMS has a
layered architecture.
• The figure does not
show the concurrency
control and recovery
components.
• This is one of several
possible architectures;
each system has its
own variations.
These layers
must consider
concurrency
control and
recovery
Query Optimization
and Execution
Relational Operators
Files and Access Methods
Buffer Management
Disk Space Management
DB
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Centralized and Client-Server
Architectures
• Centralized DBMS: combines everything
into single system including- DBMS
software, hardware, application programs
and user interface processing software.
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Basic Client-Server Architectures
• The idea is to define specialized servers with
specific functions.
•
•
•
•
File Servers
Printer Servers
Web Servers
E-mail Servers …
• The client machines provide the user with the
appropriate interfaces to utilize these servers, as
well as with local processing power to run local
applications.
• All equipment is connected via a network.
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DBMS Server
• DBMS server provides database query and
transaction services to the clients
• Sometimes called query and transaction servers
• It is common that client and server software run
on separate machines.
• Two main types of basic DBMS architectures
were created under this client/server framework:
• Two-tier
• Three-tier
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Two Tier Client-Server Architecture
• User Interface Programs and Application
Programs run on the client side
• Interface called ODBC (Open Database
Connectivity) provides an Application
program interface (API) allow client side
programs to call the DBMS. Most DBMS
vendors provide ODBC drivers.
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Two Tier Client-Server Architecture
• A client program may connect to several DBMSs.
• Other variations of clients are possible:
• e.g., in some DBMSs, more functionality is transferred
to clients including data dictionary functions,
optimization and recovery across multiple servers, etc.
• In such situations the server may be called the Data
Server (because it provides data in disk pages)
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Three Tier Client-Server Architecture
• Common for Web applications
• Intermediate Layer called Application Server or Web
Server:
• stores the web connectivity software and the rules and
business logic (constraints) part of the application used to
access the right amount of data from the database server
• acts like a conduit for sending partially processed data
between the database server and the client.
• Additional Features- Security:
• encrypt the data at the server before transmission
• decrypt data at the client
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Classification of DBMSs
• Based on the data model used:
• Traditional: Relational, Network, Hierarchical.
• Emerging: Object-oriented, Object-relational.
• Other classifications:
• Single-user (typically used with microcomputers) vs. multi-user (most DBMSs).
• Centralized (uses a single computer with one
database) vs. distributed (uses multiple
computers, multiple databases)
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Classification of DBMSs
Distributed Database Systems have now
come to be known as client server based
database systems because they do not
support a totally distributed environment,
but rather a set of database servers
supporting a set of clients.
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Variations of Distributed
Environments:
• Homogeneous DDBMS
• Heterogeneous DDBMS
• Federated or Multidatabase Systems
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Databases make these folks happy
• End users and DBMS vendors
• DB application programmers
–
e.g., smart webmasters
• Database administrator (DBA)
–
–
–
–
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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Summary
• DBMS used to maintain, query large datasets.
• Benefits include recovery from system crashes,
concurrent access, quick application development,
data integrity and security.
• Levels of abstraction give data independence.
• 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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