Understanding Database Management Systems: Concepts, Benefits, and Architecture - Prof. nA, Summaries of Database Programming

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Database Management

Systems

Chapter 1

Database Management Systems 3ed, R. Ramakrishnan and J. Gefrke

What Is a DBMS?

A very large, integrated collection of

data.

Models real-world enterprise.

 (^) Entities (e.g., students, courses)  (^) Relationships (e.g., Madonna is taking CS564) 

A Database Management System

(DBMS) is a software package designed

to store and manage databases.

Why Use a DBMS?

Data independence and efficient access.

Reduced application development time.

Data integrity and security.

Uniform data administration.

Concurrent access, recovery from

crashes.

Why Study Databases??

 Shift from computation to information

 at the “low end”: scramble to webspace (a mess!)

 at the “high end”: scientific applications

 (^) Datasets increasing in diversity and volume.

 Digital libraries, interactive video, Human

Genome project, EOS project

 ... need for DBMS exploding

 (^) DBMS encompasses most of CS

 OS, languages, theory, AI, multimedia, logic

Levels of Abstraction

 (^) Many views , single conceptual (logical) schema and physical schema.  (^) Views describe how users see the data.  (^) Conceptual schema defines logical structure  (^) Physical schema describes the files and indexes used.  (^) Schemas are defined using DDL; data is modified/queried using DML. Physical Schema Conceptual Schema View 1 View 2 View 3

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.  (^) External Schema (View):  (^) Course_info(cid:string,enrollment:integer)

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.

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!

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!)

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.

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. Query Optimization and Execution Relational Operators Files and Access Methods Buffer Management Disk Space Management DB These layers must consider concurrency control and recovery

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.