Database Management Systems Chapter 1 Overview of DBMSs

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Database Management SystemsChapter 1Overview
of DBMSs

• Professor Iluju Kiringakiringa_at_site.uottawa.ca
• http//www.site.uottawa.ca/kiringa
• SITE 5072

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Objectives

• Definition of a DBMS
• What is a DBMS? -- Why use and study DBMSs?
• Why is a DBMS different than a file system?
• Data models
• Definition -- Data abstraction -- Data
  independence
• Main topics of study
• File management -- Query and transaction
  processing
• Data independence --Architecture of DBMSs
• Miscellaneous
• Actors -- History

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What Is a DBMS?

• Definition
• A database is a very large, integrated collection
  of data.
• A Database Management System (DBMS) is a software
  package designed to store and manage databases.
• Models real-world organization / enterprise.
• Entities (e.g., students, courses, faculty, and
  classrooms)
• Relationships (e.g., Sue is enrolled in CSI3317
  Iluju teaches CSI3317, CSI3317 is taught in
  TBT070)

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Drawbacks of Files Systems

• Each application must move large datasets between
  main memory and secondary storage (must deal
  with, e.g., buffering, page-oriented access,
  etc).
• Each application must deal with some method of
  identifying all data items in case the available
  addressing mode is not sufficient (e.g., 32-bit
  addressing cannot directly access more than 4GB).

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Drawbacks of Files Systems (Contd)

• Need special code for different queries (i.e. all
  query codes are ad hoc).
• Must protect data from inconsistency due to
  multiple concurrent users changing it.
• Must ensure consistent crash recovery.
• Must provide more security and access control
  than the password mechanism offered by operating
  systems.

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Why Use a DBMS?

• Data independence application dont see details
  of data representation and storage.
• Efficient access use of very sophisticated data
  storage and access methods.
• Reduced application development time
  functionalities of a DBMS need not be duplicated.
• Data integrity and security enforcing integrity
  constraints and access control.
• Uniform data administration experienced users
  administer data that is used by inexperienced
  ones.
• Concurrent access, recovery from crashes users
  access data without thinking of whoever else uses
  it.

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Why Study Databases??

• Datasets increasing in diversity and volume
• Todays HUGE amounts of data need to be
  efficiently managed Some applications involve
  PBs of data
• Digital libraries, interactive video, Human
  Genome project, EOS project, Mars exploration
  projects,
• ... need for DBMS exploding
• Efficient management of such a diverse amount of
  data involves research in many fundamental issues
  
• DBMS encompasses most of CS
• OS, languages, theory, AI, multimedia, logic,
  etc.

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Data Models

• A data model is a collection of high-level
  constructs for describing data.
• A schema is a description of a particular
  collection of data, using a given data model.
• An instance of a schema is a sample set of data
  organized using a given schema.
• The relational model is the most widely used
  model today.
• Main concept relation instance (relation), i.e.
  a table with rows and columns.
• Every relation has a relation schema (schema),
  which describes the name and columns, or fields.

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Levels of Abstraction

• Data schemas are given at 3 levels of
  abstractions.
• Many views, single conceptual (logical) schema
  and physical schema.
• Views (or external schemas) describe how users
  see the data.
  
• Conceptual schema defines logical structure
• Physical schema describes the files and indexes
  used.

View 1
View 2
View 3
Conceptual Schema
Physical Schema

• Schemas are defined using a data definition
  language (DDL).

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Example Enterprise Database

• Conceptual schema
• Emp(eid integer, ename string, age integer,
  sal real)
• Works_in(eid integer, did integer, rating
  real)
• Dept(did integer, budget real, mgr integer)
• Physical schema
• Relations stored as ordered files.
• Index on first column of Emp, and Dept index on
  all columns of Works_in, etc.
• External Schema (View)
• Dept_count(did integer, count integer)

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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
• Relations stored as unordered files.
• Index on first column of Students, and Courses
  index on two first columns of Enrolled, etc.
• External Schema (View)
• Course_info(cid string, enrollment integer)

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Data Independence

• Applications insulated from how data is
  structured and stored.
• Each level of abstraction protected from changes
  in the structure of the level below it.
• Logical data independence Protection from
  changes in logical structure of data.
• Physical data independence Protection from
  changes in physical structure of data.

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Querying Data

• A query is a question involving the stored data,
  e.g.,
• What is the name of the professor who teaches
  CSI3317?
• Which percentage of students got an A in
  CSI3317?
• A query language is a special purpose language in
  which queries can be posed against databases.
• Data is modified/queried using a data
  manipulation language (DML). So a query language
  is a subset of a DML
• The relational model supports 2 query languages
  
• Relational calculus logic-based query language
• Relational algebra based on a set of operators
  for manipulating relations.

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Transactions

• A transaction is an atomic sequence of database
  actions (reads/writes) corresponding to the
  execution of a DB transaction program.
• 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. Thus, ensuring that a
  transaction preserves consistency is ultimately
  the users responsibility!

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Transactions 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
  working by processing several user programs
  concurrently.
• Interleaving actions of different user programs
  can lead to inconsistency e.g., depositing money
  while account balance is being computed may lead
  to lost of money if not done properly.
• DBMS ensures such problems dont arise users
  can pretend they are using a single-user system.

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Scheduling Concurrent Transactions

• A DBMS ensures that execution of T1, ... , Tn
  is equivalent to some serial execution of 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.
• If Tj already has a lock on Y and Ti later
  requests a lock on Y, there is Deadlock! Ti or Tj
  is aborted and restarted!

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Transactions Crash Recovery

• A DBMS ensures atomicity (all-or-nothing
  property) even if there is a crash in the middle
  of a transaction.
• Idea Keep a log (history) of all actions carried
  out by the DBMS while executing a set of
  transactions
• Before a change is made to the database, the
  corresponding log entry is forced to a safe
  location. (WAL protocol)
• After a crash, the effects of partially executed
  transactions are undone using the log. (Thanks to
  WAL, if log entry wasnt saved before the crash,
  corresponding change was not applied to database!)

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People Involved with Databases

• End users just use an interface to a DBMS!
• DBMS vendors IBM, Oracle, Informix, Microsoft,
  etc
• DBMS researchers and implementers invent new
  theories and algorithms for, and write code of
  DBMSs
• DB application programmers write C/C/Java/
  programs that interact with DBMSs
• E.g. smart webmasters, CSI3317 students, etc
• Database administrator (DBA)
• Designs logical /physical schemas
• Handles security and authorization
• Data availability, crash recovery
• Database tuning as needs evolve

DBAs must understand how a DBMS works!
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Structure of a DBMS
These layers must consider concurrency control
and recovery

• 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.

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Historical Perspective

• Early 1960s GE designed 1st general-purpose
  DBMS, based on the network data model.
• Late 1960s IBM developed IMS, based on the
  hierarchical data model.
• Early 1970s E. Codd introduced the relational
  model to solve problems related to the previous
  models.
• Late 1970s and early 1980s work on transaction
  processing, mainly by Jim Gray and P. Bernstein,
  
• All the 1980s use of relational DBs became
  standard practice in large corporations, many
  vendors entered the market, standardization
  efforts on SQL, the query language for relational
  DBs, etc.

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Historical Perspective (Contd)

• Late 1980s and early 1990s richer data models
  (object-oriented, object-relational, etc), and
  more expressive query languages (Datalog, nested
  relations, etc) are introduced.
• Late 1990s major vendors extend relational DBMSs
  towards new data types (images, text, multimedia
  content) and queries based hereon.
• All along the way
• Scientific recognition Turing Awards to DB
  researchers E. Codd and J. Gray
• Birth of a huge, multibillion industry (IBM,
  Oracle )

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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
• DB researchers, implementers, and administrators
  hold responsible jobs and are well-paid
• DBMS RD is a very broad and exciting area of CS
• Database companies form a multibillion industry