Sang H. Son son@virginia.edu CS6750: Database Systems The slides for this text are organized into chapters. This lecture covers Chapter 1. Chapter 1: Introduction to Database Systems Chapter 2: The Entity-Relationship Model Chapter 3: The Relational Model Chapter 4 (Part A): Relational Algebra Chapter 4 (Part B): Relational Calculus Chapter 5: SQL: Queries, Programming, Triggers Chapter 6: Query-by-Example (QBE) Chapter 7: Storing Data: Disks and Files Chapter 8: File Organizations and Indexing Chapter 9: Tree-Structured Indexing Chapter 10: Hash-Based Indexing Chapter 11: External Sorting Chapter 12 (Part A): Evaluation of Relational Operators Chapter 12 (Part B): Evaluation of Relational Operators: Other Techniques Chapter 13: Introduction to Query Optimization Chapter 14: A Typical Relational Optimizer Chapter 15: Schema Refinement and Normal Forms Chapter 16 (Part A): Physical Database Design Chapter 16 (Part B): Database Tuning Chapter 17: Security Chapter 18: Transaction Management Overview Chapter 19: Concurrency Control Chapter 20: Crash Recovery Chapter 21: Parallel and Distributed Databases Chapter 22: Internet Databases Chapter 23: Decision Support Chapter 24: Data Mining Chapter 25: Object-Database Systems Chapter 26: Spatial Data Management Chapter 27: Deductive Databases Chapter 28: Additional Topics Sang H. Son son@virginia.edu

What is DBMS? Need for information management A very large, integrated collection of data. Models real-world enterprise. Entities (e.g., students, courses) Relationships (e.g., John is taking CS662) A Database Management System (DBMS) is a software package designed to store and manage databases.

Why Use a DBMS? Data independence and efficient access. Data integrity and security. Uniform data administration. Concurrent access, recovery from crashes. Replication control Reduced application development time. 3

Any Reason NOT to Use DBMS? DBMS is a complex piece of software, optimized for certain kinds of workload (i.e., complex queries and many concurrent requests) While requiring significant computing resources, its performance may not be adequate for certain specialized applications For instance, time-critical applications with tight hard real-time constraints Applications may need to manipulate data in ways not supported by the query language

Why Study Databases?? ? Shift from computation to information at the “low end”: access to physical world at the “high end”: scientific applications Datasets increasing in diversity and volume. Digital libraries, interactive video, bioinformatics, e-commerce, sensor networks need for DBMS/data services exploding DBMS encompasses several areas of CS OS, languages, theory, AI, multimedia, logic

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. 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. 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 is modified/queried using DML. 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) 7

Data Independence Applications insulated from how data is structured and stored. Logical data independence: Protection from changes in logical structure of data. Physical data independence: Protection from changes in physical structure of data. One of the most important benefits of using a DBMS!

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 Unit of a DB 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). Why not? Thus, ensuring that a transaction (run alone) preserves consistency is ultimately the user’s responsibility!

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. (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. What if Tj already has a lock on Y and Ti later requests a lock on Y? What is it called? What will happen?

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.) 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. 20

Databases make these folks happy ... End users and DBMS vendors DB application programmers e.g. webmasters 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! 21

Files and Access Methods 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. Query Optimization and Execution Relational Operators Files and Access Methods Buffer Management Disk Space Management DB 22

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, mature areas in CS.

What to Study in 6750? ? DBMS basics ER, relational model, SQL, DB design using FD Transaction management atomicity, consistency, isolation, durability concurrency control and recovery Databases/data services for real-time applications scheduling and CC QoS metrics and issues Data services in emerging applications real-time databases, event services, sensor networks, streaming data