Database Management Systems Chapter 1 Instructor: Oliver Schulte oschulte@cs.sfu.ca

Outline What is a database? Why use a database? Why study databases? Database Overview

What is a database? A database (DB) is a very large, integrated, permanent collection of data. Models real-world Entities (e.g., students, courses) Relationships (e.g., Madonna is taking CMPT354). Example databases: Customer Transactions Human Genome Online Bookstore Mondial is a sample DB for teaching purposes.

What Is a DBMS? A Database Management System (DBMS) is a software package designed to store and manage databases.

This course is important for... End users of DBS DB application programmers Database administrators (DBA) DBMS vendors Must understand how a DBMS works!

Why Use Databases?

The Inreasing Flood of Data Human Genome Customer Transactions Online Bookstore As of 2004, Walmart data-warehouse was 500terabytes in size. In 2007, it was over 1petabyte (1m gigabytes) Sources: http://www.eweek.com/c/a/Enterprise-Applications/At-WalMart-Worlds-Largest-Retail-Data-Warehouse-Gets-Even-Larger/ http://www.informationweek.com/news/storage/showArticle.jhtml?articleID=201203024 The human genome contains 3.2 billion chemical nucleotide base pairs (A, C, T, and G). Largest known human gene is dystrophin at 2.4 million base pairs. Functions are unknown for more than 50% of discovered genes. Source: http://www.ornl.gov/sci/techresources/Human_Genome/project/journals/insights.shtml Amazon has roughly a bazillion products, give or take a couple zillion.

Amazon: Website, database or application?

Data Storage Without DBMS File 1 Application program 1 File 2 Application program 2 . . . . . . Example: my telus bills storing music with Itunes Application program n File m reads / writes

Data Storage With DBMS File 1 Application program 1 File 2 . . . DBMS . . . show mysql files using finder alias “mysql data alias” Application program n File m reads / writes

Files vs. DBMS Application must move large datasets between main memory and secondary storage (e.g., buffering, page-oriented access, 32-bit addressing, etc.) Special code for different queries Must protect data from inconsistency due to multiple concurrent users Crash recovery Security and access control But note NoSQL “movement”. For direct manipulation of certain data, files can be a better choice. For example, standard DBMS do not provide good support for text manipulation and hence not for databases of texts.

Why Use a DBMS? Summary Data independence (abstract view of data) and efficient access. Reduced application development time. Data integrity (enforce constraints) and security. Uniform (central) data administration. Concurrent access, recovery from crashes. 3

Why Study Databases?? ? Shift from computation to information Datasets increasing in diversity and volume. Digital libraries, interactive video, Human Genome project ... need for DBMS exploding DBMS intersects with most of CS OS, languages, theory, AI, multimedia, logic

Database Overview

Data Models A data model is a collection of concepts for describing data. A schema is a description of a particular collection of data, using the given data model and its data definition language. 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. You can think of a schema as a kind of “template”. (put example on whiteboard) 5

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. View 1 View 2 View 3 Conceptual Schema Physical Schema Schemas are defined using DDL = data description language. Data is modified/queried using DML = data manipulation language. 6

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) Another example: ChessBase. Set up example in Access. 7

Data Independence Applications insulated from how data is structured and stored. Logical data independence: Protection from changes in logical structure of data (e.g., adding new fields). Physical data independence: Protection from changes in physical structure of data (e.g., sorting, indexing, compressing). One of the most important benefits of using a DBMS!

Database Transaction Processing ACID: Atomicity. Consistency. Isolation. Durability. Atomicity: all or nothing Consistency: consistent state, including constraints. Isolation: concurrent processing = serial processing. durability: once committed, info not lost. not in this course.

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. Example of integrity constraint: dates must be in a certain range. Transactions are treated in 454. 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! Each transaction, executed completely, must leave the DB in a consistent state if DB is consistent when the transaction begins.

Concurrency Control (Isolation) Concurrent execution of user programs is essential for good DBMS performance. Interleaving actions of different user programs can lead to inconsistency: e.g., check is cleared while account balance is being computed. DBMS ensures that users can pretend they are using a single-user system. We don’t treat concurrency control in this course. Because disk accesses are frequent, and relatively slow, it is important to keep the cpu humming by working on several user programs concurrently.

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. Think about writing papers together, keeping versions controlled. Serialization example: one program wants to credit an account, another wants to debit. The program schedules them in some order. Example of deadlock: T1 wants to write John’s SIN, and read Jane’s phone number. T2 wants to read John’s SIN and write Jane’s phone number. T2 is given an exclusive lock on Jane’s phone number, and T1 is given an exclusive lock on John’s SIN. Neither process can terminate. (Strict 2PL locking protocol.) 2PL stands for Two Phase Locking. Transaction Management is part of 454, NOT of our course.

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. (Write-ahead log) After a crash, the effects of partially executed transactions are undone using the log. Countexample to atomicity: email to multiple senders. If one bounces back, the others still go through. Creates uncertainty about what happened. (Thanks to WAL, if log entry wasn’t saved before the crash, corresponding change was not applied to database!) ,WAL protocol.)

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 by the DBMS. CC = concurrency control 20

Exercise 1.6 Scrooge McNugget wants to store information (names, addresses, descriptions of embarrassing moments, etc.) about the many ducks on his payroll. Not surprisingly, he wants to buy one with the fewest possible features, and he plants to run it as a stand-alone application on his PC clone. Scrooges does not plan to share his list with anyone. Indicate which of the following DBMS features Scrooge should pay for, and why (or why not). 1. A security facility 2. Access Control 3. Concurrency Control 4. Crash recovery 5. A query language.

Files and Access Methods Structure of a DBMS These layers must consider concurrency control and recovery A typical DBMS has a layered architecture. 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 The figure does not show the concurrency control and recovery components. 22

Exercise 1.7 Which of the following plays an important role in representing information about the real world in a database? The data definition language. The data manipulation language. The buffer manager. The data model. 1 and 4.

Summary: Function and Benefits DBMS used to maintain and query permanent large datasets. Benefits include recovery from system crashes concurrent access, quick application development data integrity security.

Summary: Database Design Levels of abstraction give data independence. Transaction processing satisfies the ACID properties. 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.