Lecture1.ppt

Chapter 1: Introduction Purpose of Database Systems View of Data Data Models Data Definition Language Data Manipulation Language Transaction Management Storage Management Database Administrator Database Users Overall System Structure  Database Management System (DBMS) Collect...

Chapter 1: Introduction Purpose of Database Systems View of Data Data Models Data Definition Language Data Manipulation Language Transaction Management Storage Management Database Administrator Database Users Overall System Structure  Database Management System (DBMS) Collection of interrelated data Set of programs to access the data DBMS contains information about a particular enterprise DBMS provides an environment that is both convenient and efficient to use. Database Applications:  Banking: all transactions  Airlines: reservations, schedules  Universities: registration, grades  Sales: customers, products, purchases  Manufacturing: production, inventory, orders, supply chain  Human resources: employee records, salaries, tax deductions Databases touch all aspects of our lives  Purpose of Database System In the early days, database applications were built on top of file systems Drawbacks of using file systems to store data:  Data redundancy and inconsistency  Multiple file formats, duplication of information in different files  Difficulty in accessing data  Need to write a new program to carry out each new task  Data isolation — multiple files and formats  Integrity problems  Integrity constraints (e.g. account balance > 0) become part of program code  Hard to add new constraints or change existing ones  Purpose of Database Systems (Cont.) Drawbacks of using file systems (cont.)  Atomicity of updates  Failures may leave database in an inconsistent state with partial updates carried out  E.g. transfer of funds from one account to another should either complete or not happen at all  Concurrent access by multiple users  Concurrent accessed needed for performance  Uncontrolled concurrent accesses can lead to inconsistencies – E.g. two people reading a balance and updating it at the same time  Security problems Database systems offer solutions to all the above problems  Previous Lecture Models a real-world enterprise A Database Management System (DBMS) is a software system designed to store, manage, and facilitate access to databases. Advantages Data redundancy and inconsistency Difficulty in accessing data Data isolation – multiple files and formats Integrity problems Atomicity of updates Concurrent access by multiple users Security problems Example  Is the WWW a DBMS? Fairly sophisticated search available  crawler indexes pages on the web  Keyword-based search for pages But, currently  data is mostly unstructured and untyped  search only:  can’t modify the data  can’t get summaries, complex combinations of data  few guarantees provided for freshness of data, consistency across data items, fault tolerance, …  Web sites typically have a DBMS in the background to provide these functions. XML, Semantic Web can help data modeling  Levels of Abstraction Physical level describes how a record (e.g., customer) is stored. Logical level: describes data stored in database, and the relationships among the data. type customer = record name : string; street : string; city : integer; end; View level: application programs hide details of data types. Views can also hide information (e.g., salary) for security purposes.  View of Data An architecture for a database system View 1 View 2 View 3 Conceptual Schema Physical Schema DB  Instances and Schemas Similar to types and variables in programming languages Schema – the logical structure of the database  e.g., the database consists of information about a set of customers and accounts and the relationship between them)  Analogous to type information of a variable in a program  Physical schema: database design at the physical level  Logical schema: database design at the logical level Instance – the actual content of the database at a particular point in time  Analogous to the value of a variable  Data Independence Ability to modify a schema definition in one level without affecting a schema definition in the other levels. The interfaces between the various levels and components should be well defined so that changes in some parts do not seriously influence others. Two levels of data independence  Physical data independence  Logical data independence Physical Data Independence – the ability to modify the physical schema without changing the logical schema  Applications depend on the logical schema  In general, the interfaces between the various levels and components should be well defined so that changes in some parts do not seriously influence others.  Data Models A data model is a collection of concepts and constructs for describing data. A collection of tools for describing  data  data relationships  data semantics  data constraints Object-based logical models  Entity-relationship model  Object-oriented model  Semantic model  Functional model Record-based logical models  Relational model (e.g., SQL/DS, DB2)  Network model  Hierarchical model (e.g., IMS) Other models:  semi-structured data models  Basic DBMS types Linear files Sequence of records with a fixed format usually stored on a single file Limitation: single file Example query: Salesperson='Mary' AND Price>100 Hierarchical structure Trees of records: one-to-many relationships Limitations: Requires duplicating records (e.g. many-to-many relationship) Problems when updated Retrieval requires knowing the structure (limited data independence): Traversing the tree from top to bottom using a procedural language Network structure: similar to the hierarchical database with the implementation of many-to-many relationships Relational structure Object-Oriented structure Objects (collection of data items and procedures) and interactions between them. Separate implementation vs. implementation on top of a RDBMS  Entity-Relationship Model Example of schema in the entity-relationship model  Entity Relationship Model (Cont.) E-R model of real world  Entities (objects)  E.g. customers, accounts, bank branch  Relationships between entities  E.g. Account A-101 is held by customer Johnson  Relationship set depositor associates customers with accounts Widely used for database design  Database design in E-R model usually converted to design in the relational model (coming up next) which is used for storage and processing  Relational Model Attributes Example of tabular data in the relational model customer- customer- customer- account- Customer-id name street city number 192-83-7465 Johnson Alma Palo Alto A-101 019-28-3746 Smith North Rye A-215 192-83-7465 Johnson Alma Palo Alto A-201 321-12-3123 Jones Main Harrison A-217 019-28-3746 Smith North Rye A-201 A Sample Relational Database Inventory Relational Database  Data Definition Language (DDL) Specification notation for defining the database schema  E.g. create table account ( account-number char(10), balance integer) DDL compiler generates a set of tables stored in a data dictionary Data dictionary contains metadata (i.e., data about data)  database schema  Data storage and definition language  language in which the storage structure and access methods used by the database system are specified  Usually an extension of the data definition language  Data Manipulation Language (DML) Language for accessing and manipulating the data organized by the appropriate data model  DML also known as query language Two classes of languages  Procedural – user specifies what data is required and how to get those data  Nonprocedural – user specifies what data is required without specifying how to get those data SQL is the most widely used query language  SQL SQL: widely used non-procedural language  E.g. find the name of the customer with customer-id 192-83-7465 select customer.customer-name from customer where customer.customer-id = ‘192-83-7465’  E.g. find the balances of all accounts held by the customer with customer-id 192-83-7465 select account.balance from depositor, account where depositor.customer-id = ‘192-83-7465’ and depositor.account-number = account.account- number Application programs generally access databases through one of  Language extensions to allow embedded SQL  Application program interface (e.g. ODBC/JDBC) which allow SQL queries to be sent to a database  Database Users Users are differentiated by the way they expect to interact with the system Application programmers – interact with system through DML calls Sophisticated users – form requests in a database query language Specialized users – write specialized database applications that do not fit into the traditional data processing framework Naïve users – invoke one of the permanent application programs that have been written previously  E.g. people accessing database over the web, bank tellers, clerical staff  Database Administrator Coordinates all the activities of the database system; the database administrator has a good understanding of the enterprise’s information resources and needs. Database administrator's duties include:  Schema definition  Storage structure and access method definition  Schema and physical organization modification  Granting user authority to access the database  Specifying integrity constraints  Acting as liaison with users  Monitoring performance and responding to changes in requirements  Transaction Management A transaction is a collection of operations that performs a single logical function in a database application Transaction-management component ensures that the database remains in a consistent (correct) state despite system failures (e.g., power failures and operating system crashes) and transaction failures. Concurrency-control manager controls the interaction among the concurrent transactions, to ensure the consistency of the database.  ACID Properties Key concept is a transaction: a sequence of database actions (reads/writes). DBMS ensures atomicity (all-or-nothing property) even if system crashes in the middle of a Xact. Each transaction, executed completely, must take the DB between consistent states or must not run at all. DBMS ensures that concurrent transactions appear to run in isolation. DBMS ensures durability of committed Xacts even if system crashes. Note: can specify simple integrity constraints on the data. The DBMS enforces these.  Beyond this, the DBMS does not understand the semantics of the data.  Ensuring that a single transaction (run alone) preserves consistency is largely the user’s responsibility  Storage Management Storage manager is a program module that provides the interface between the low-level data stored in the database and the application programs and queries submitted to the system. The storage manager is responsible to the following tasks:  interaction with the file manager  efficient storing, retrieving and updating of data  Overall System Structure DBMS  Application Architectures Two-tier architecture: E.g. client programs using ODBC/JDBC to communicate with a database Three-tier architecture: E.g. web-based applications, and applications built using “middleware”