Database Management Systems 1 Ramakrishnan & Gehrke Introduction to Database Systems Chpt 1 Instructor: Weichao Wang

Database Management Systems 2 Ramakrishnan & Gehrke

Database Management Systems 3 Ramakrishnan & Gehrke History v 60s C. Bachman GE network data model v Late 60s IBM IMS hierarchical data model v 70 E.Codd relational model v 80s SQL IBM R trasaction J. Gray v Late 80s-90s DB2, Oracle, informix, sybase v 90s DW, internet, distributed database v Now Big Data Turing award and Turing test?

Database Management Systems 4 Ramakrishnan & Gehrke What Is a DBMS? v A very large, integrated collection of data. v Models real-world enterprise. – Entities (e.g., students, courses) – Relationships (e.g., Madonna is taking ITCS6160) v A Database Management System (DBMS) is a software package designed to maintain and utilize databases.

Database Management Systems 5 Why not just OS file systems? v Size of the data and size of your memory/harddisk v Query processing: remember your file read/write C programs? Now think about several tera-bytes of data. You need a separate program for every query. v Consistency: multiple users access the same data v Recovery: is it on harddisk now? v All these can be implemented directly upon OS. But then you are just designing your own DB and DBMS. Ramakrishnan & Gehrke

Database Management Systems 6 Ramakrishnan & Gehrke Why Use a DBMS? v Data independence and efficient access. v Reduced application development time. v Data integrity and security. v Uniform data administration. (not sure about this now) v Concurrent access, recovery from crashes.

Database Management Systems 7 Ramakrishnan & Gehrke Why Study Databases?? v Shift from computation to information (application oriented vs data oriented) –at the “low end”: scramble to webspace –at the “high end”: scientific applications v Datasets increasing in diversity and volume. –Digital libraries, interactive video, Human Genome project, EOS project –... need for DBMS exploding v DBMS encompasses most of CS –OS, languages, theory, AI, multimedia, logic

Database Management Systems 8 Ramakrishnan & Gehrke Data Models v A data model is a collection of concepts for describing data. v A schema is a description of a particular collection of data, using the given data model. v 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.

Database Management Systems 9 Ramakrishnan & Gehrke Levels of Abstraction v 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 1View 2View 3

Database Management Systems 10 Ramakrishnan & Gehrke Example: University Database v 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) v Physical schema: –Relations stored as unordered files. –Index on first column of Students. v External Schema (View): – Course_info(cid:string,enrollment:integer)cid:string,enrollment:integer – Each data entry is stored only once. Views are created.

Database Management Systems 11 Ramakrishnan & Gehrke Data Independence v Applications insulated from how data is structured and stored. v Logical data independence : Protection from changes in logical structure of data. v Physical data independence : Protection from changes in physical structure of data. v Key is to reduce workload and overhead of end users. * One of the most important benefits of using a DBMS!

Database Management Systems 12 Ramakrishnan & Gehrke Structure of a DBMS v A typical DBMS has a layered architecture. v The figure does not show the concurrency control and recovery components. v 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

Database Management Systems 13 Ramakrishnan & Gehrke Transaction Management: ACID properties v A v A tomicity: All actions in the Xact happen, or none happen. v C v C onsistency: If each Xact is consistent, and the DB starts consistent, it ends up consistent. v I v I solation: Execution of one Xact is isolated from that of other Xacts. v D v D urability: If a Xact commits, its effects persist. v The Recovery Manager guarantees Atomicity & Durability.

Database Management Systems 14 Ramakrishnan & Gehrke Motivation of concurrency control v Consistency v Isolation v Example –Two parallel transactions T1 and T2 –Serial execution –Execution with interleaving actions –Similar situations in OS and any other resource competitions

Database Management Systems 15 Ramakrishnan & Gehrke Motivation of recovery management v Atomicity: –Transactions may abort (“Rollback”). v Durability: –What if DBMS stops running? (Causes?) crash! v Desired Behavior after system restarts: –T1, T2 & T3 should be durable. –T4 & T5 should be aborted (effects not seen). T1 T2 T3 T4 T5

Database Management Systems 16 Ramakrishnan & Gehrke Databases make these folks happy... v End users and DBMS vendors v DB application programmers –E.g. smart webmasters v 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!

Database Management Systems 17 New challenges v Application oriented to data oriented v Unstructured data v Conflict b/w data and user privacy –Data taint/trace v Challenges caused by cloud: –Storage places –Index of encrypted data files –Proof of retrievability –Mobile: compute it locally or transmit it Ramakrishnan & Gehrke

Database Management Systems 18 Ramakrishnan & Gehrke Summary v DBMS used to maintain, query large datasets. v Benefits include recovery from system crashes, concurrent access, quick application development, data integrity and security. v Levels of abstraction give data independence. v A DBMS typically has a layered architecture. v DBAs hold responsible jobs and are well-paid! v DBMS R&D is one of the broadest, most exciting areas in CS.