Database Management Systems Session 1 CSCI 585 Instructor Cyrus Shahabi shahabi usc edu Database Management Systems 3ed R Ramakrishnan and J Gehrke 1 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 Database Management Systems 3ed R Ramakrishnan and J Gehrke 2 Files vs DBMS Application must stage 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 Database Management Systems 3ed R Ramakrishnan and J Gehrke 3 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 Database Management Systems 3ed R Ramakrishnan and J Gehrke 4 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 Database Management Systems 3ed R Ramakrishnan and J Gehrke 5 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 a 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 Database Management Systems 3ed R Ramakrishnan and J Gehrke 6 Levels of Abstraction Many views single View 1 View 2 View 3 conceptual logical schema and physical schema Conceptual Schema Views describe how users see the data Conceptual schema defines logical structure Physical schema describes the files and indexes used Physical Schema Schemas are defined using DDL data is modified queried using DML Database Management Systems 3ed R Ramakrishnan and J Gehrke 7 Example University Database Conceptual schema Physical 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 Relations stored as unordered files Index on first column of Students External Schema View Course info cid string enrollment integer Database Management Systems 3ed R Ramakrishnan and J Gehrke 8 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 Database Management Systems 3ed R Ramakrishnan and J Gehrke 9 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 Database Management Systems 3ed R Ramakrishnan and J Gehrke 10 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 Database Management Systems 3ed R Ramakrishnan and J Gehrke 11 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 Deadlock Ti or Tj is aborted and restarted Database Management Systems 3ed R Ramakrishnan and J Gehrke 12 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 Database Management Systems 3ed R Ramakrishnan and J Gehrke 13 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 Database Management Systems 3ed R Ramakrishnan and J Gehrke 14 Databases make these folks happy End users and DBMS vendors DB application programmers E g smart 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 Database Management