1. CS194-3/CS16x Introduction to Systems Lecture 7 Monitors, Readers/Writers problem, Language support for concurrency control, Relational model September 19, 2007 Prof. Anthony D. Joseph http://www.cs.berkeley.edu/~adj/cs16x

2. Lec 7.2 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Value=2Value=1Value=0 Review: Semaphores Definition: a Semaphore has a non-negative integer value and supports the following two operations: –P(): an atomic operation that waits for semaphore to become positive, then decrements it by 1 »Think of this as the wait() operation –V(): an atomic operation that increments the semaphore by 1, waking up a waiting P, if any »This of this as the signal() operation –Only time can set integer directly is at initialization time Semaphore from railway analogy –Here is a semaphore initialized to 2 for resource control: Value=1Value=0

3. Lec 7.3 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Atomicity: guarantee that either all of the tasks of a transaction are performed, or none of them are Consistency: database is in a legal state when the transaction begins and when it ends – a transaction cannot break the rules, or integrity constraints Isolation: operations inside a transaction appear isolated from all other operations – no operation outside transaction can see data in an intermediate state Durability: guarantee that once the user has been notified of success, the transaction will persist (survive system failure) Review: ACID Summary

4. Lec 7.4 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Goals for Today Continue with Synchronization Abstractions –Monitors and condition variables Readers-Writers problem and solution Java language support for synchronization Relational model Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne. Slides courtesy of Kubiatowicz, AJ Shankar, George Necula, Alex Aiken, Eric Brewer, Ras Bodik, Ion Stoica, Doug Tygar, and David Wagner.

5. Lec 7.5 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Motivation for Monitors and Condition Variables Semaphores are a huge step up, but: –They are confusing because they are dual purpose: »Both mutual exclusion and scheduling constraints »Example: the fact that flipping of P’s in bounded buffer gives deadlock is not immediately obvious –Cleaner idea: Use locks for mutual exclusion and condition variables for scheduling constraints Definition: Monitor: a lock and zero or more condition variables for managing concurrent access to shared data –Use of Monitors is a programming paradigm –Some languages like Java provide monitors in the language The lock provides mutual exclusion to shared data: –Always acquire before accessing shared data structure –Always release after finishing with shared data –Lock initially free

6. Lec 7.6 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Simple Monitor Example (version 1) Here is an (infinite) synchronized queue Lock lock; Queue queue; AddToQueue(item) { lock.Acquire();// Lock shared data queue.enqueue(item);// Add item lock.Release();// Release Lock } RemoveFromQueue() { lock.Acquire();// Lock shared data item = queue.dequeue();// Get next item or null lock.Release();// Release Lock return(item);// Might return null }

7. Lec 7.7 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Condition Variables How do we change the RemoveFromQueue() routine to wait until something is on the queue? –Could do this by keeping a count of the number of things on the queue (with semaphores), but error prone Condition Variable: a queue of threads waiting for something inside a critical section –Key idea: allow sleeping inside critical section by atomically releasing lock at time we go to sleep –Contrast to semaphores: Can’t wait inside critical section Operations: –Wait(&lock) : Atomically release lock and go to sleep. Re-acquire lock later, before returning. –Signal() : Wake up one waiter, if any –Broadcast() : Wake up all waiters Rule: Must hold lock when doing condition variable ops! –In Birrell paper, he says can perform signal() outside of lock – IGNORE HIM (this is only an optimization)

8. Lec 7.8 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Complete Monitor Example (with condition variable) Here is an (infinite) synchronized queue Lock lock; Condition dataready; Queue queue; AddToQueue(item) { lock.Acquire();// Get Lock queue.enqueue(item);// Add item dataready.signal();// Signal any waiters lock.Release();// Release Lock } RemoveFromQueue() { lock.Acquire();// Get Lock while (queue.isEmpty()) { dataready.wait(&lock); // If nothing, sleep } item = queue.dequeue();// Get next item lock.Release();// Release Lock return(item); }

9. Lec 7.9 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Mesa vs. Hoare monitors Need to be careful about precise definition of signal and wait. Consider a piece of our dequeue code: while (queue.isEmpty()) { dataready.wait(&lock); // If nothing, sleep } item = queue.dequeue();// Get next item –Why didn’t we do this? if (queue.isEmpty()) { dataready.wait(&lock); // If nothing, sleep } item = queue.dequeue();// Get next item Answer: depends on the type of scheduling –Hoare-style (most textbooks): »Signaler gives lock, CPU to waiter; waiter runs immediately »Waiter gives up lock, processor back to signaler when it exits critical section or if it waits again –Mesa-style (Nachos, most real operating systems): »Signaler keeps lock and processor »Waiter placed on ready queue with no special priority »Practically, need to check condition again after wait

10. Lec 7.10 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Readers/Writers Problem Motivation: Consider a shared database –Two classes of users: »Readers – never modify database »Writers – read and modify database –Is using a single lock on the whole database sufficient? »Like to have many readers at the same time »Only one writer at a time R R R W

11. Lec 7.11 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Basic Readers/Writers Solution Correctness Constraints: –Readers can access database when no writers –Writers can access database when no readers or writers –Only one thread manipulates state variables at a time Basic structure of a solution: –Reader() Wait until no writers Access data base Check out – wake up a waiting writer –Writer() Wait until no active readers or writers Access database Check out – wake up waiting readers or writer –State variables (Protected by a lock called “lock”): »int AR: Number of active readers; initially = 0 »int WR: Number of waiting readers; initially = 0 »int AW: Number of active writers; initially = 0 »int WW: Number of waiting writers; initially = 0 »Condition okToRead = NIL »Conditioin okToWrite = NIL

12. Lec 7.12 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Code for a Reader Reader() { // First check self into system lock.Acquire(); while ((AW + WW) > 0) {// Is it safe to read? WR++;// No. Writers exist okToRead.wait(&lock);// Sleep on cond var WR--;// No longer waiting } AR++;// Now we are active! lock.release(); // Perform actual read-only access AccessDatabase(ReadOnly); // Now, check out of system lock.Acquire(); AR--;// No longer active if (AR == 0 && WW > 0)// No other active readers okToWrite.signal();// Wake up one writer lock.Release(); } Why Release the Lock here?

13. Lec 7.13 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Writer() { // First check self into system lock.Acquire(); while ((AW + AR) > 0) {// Is it safe to write? WW++;// No. Active users exist okToWrite.wait(&lock);// Sleep on cond var WW--;// No longer waiting } AW++;// Now we are active! lock.release(); // Perform actual read/write access AccessDatabase(ReadWrite); // Now, check out of system lock.Acquire(); AW--;// No longer active if (WW > 0){// Give priority to writers okToWrite.signal();// Wake up one writer } else if (WR > 0) {// Otherwise, wake reader okToRead.broadcast();// Wake all readers } lock.Release(); } Why Give priority to writers? Code for a Writer Why broadcast() here instead of signal()?

14. Lec 7.14 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Simulation of Readers/Writers solution Consider the following sequence of operators: –R1, R2, W1, R3 On entry, each reader checks the following: while ((AW + WW) > 0) {// Is it safe to read? WR++;// No. Writers exist okToRead.wait(&lock);// Sleep on cond var WR--;// No longer waiting } AR++;// Now we are active! First, R1 comes along: AR = 1, WR = 0, AW = 0, WW = 0 Next, R2 comes along: AR = 2, WR = 0, AW = 0, WW = 0 Now, readers may take a while to access database –Situation: Locks released –Only AR is non-zero

15. Lec 7.15 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Simulation(2) Next, W1 comes along: while ((AW + AR) > 0) {// Is it safe to write? WW++;// No. Active users exist okToWrite.wait(&lock);// Sleep on cond var WW--;// No longer waiting } AW++; Can’t start because of readers, so go to sleep: AR = 2, WR = 0, AW = 0, WW = 1 Finally, R3 comes along: AR = 2, WR = 1, AW = 0, WW = 1 Now, say that R2 finishes before R1: AR = 1, WR = 1, AW = 0, WW = 1 Finally, last of first two readers (R1) finishes and wakes up writer: if (AR == 0 && WW > 0)// No other active readers okToWrite.signal();// Wake up one writer

16. Lec 7.16 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Simulation(3) When writer wakes up, get: AR = 0, WR = 1, AW = 1, WW = 0 Then, when writer finishes: if (WW > 0){ // Give priority to writers okToWrite.signal();// Wake up one writer } else if (WR > 0) {// Otherwise, wake reader okToRead.broadcast();// Wake all readers } –Writer wakes up reader, so get: AR = 1, WR = 0, AW = 0, WW = 0 When reader completes, we are finished

17. Lec 7.17 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Questions Can readers starve? Consider Reader() entry code: while ((AW + WW) > 0) {// Is it safe to read? WR++;// No. Writers exist okToRead.wait(&lock);// Sleep on cond var WR--;// No longer waiting } AR++;// Now we are active! What if we erase the condition check in Reader exit? AR--;// No longer active if (AR == 0 && WW > 0)// No other active readers okToWrite.signal(); // Wake up one writer Further, what if we turn the signal() into broadcast() AR--;// No longer active okToWrite.broadcast(); // Wake up one writer Finally, what if we use only one condition variable (call it “ okToContinue ”) instead of two separate ones? –Both readers and writers sleep on this variable –Must use broadcast() instead of signal()

18. Lec 7.18 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Can we construct Monitors from Semaphores? Locking aspect is easy: Just use a mutex Can we implement condition variables this way? Wait() { semaphore.P(); } Signal() { semaphore.V(); } –Doesn’t work: Wait() may sleep with lock held Does this work better? Wait(Lock lock) { lock.Release(); semaphore.P(); lock.Acquire(); } Signal() { semaphore.V(); } –No: Condition vars have no history, semaphores have history: »What if thread signals and no one is waiting? NO-OP »What if thread later waits? Thread Waits »What if thread V’s and noone is waiting? Increment »What if thread later does P? Decrement and continue

19. Lec 7.19 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Construction of Monitors from Semaphores (con’t) Problem with previous try: –P and V are commutative – result is the same no matter what order they occur –Condition variables are NOT commutative Does this fix the problem? Wait(Lock lock) { lock.Release(); semaphore.P(); lock.Acquire(); } Signal() { if semaphore queue is not empty semaphore.V(); } –Not legal to look at contents of semaphore queue –There is a race condition – signaler can slip in after lock release and before waiter executes semaphore.P() It is actually possible to do this correctly –Complex solution for Hoare scheduling in book –Can you come up with simpler Mesa-scheduled solution?

20. Lec 7.20 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Monitor Conclusion Monitors represent the logic of the program –Wait if necessary –Signal when change something so any waiting threads can proceed Basic structure of monitor-based program: lock while (need to wait) { condvar.wait(); } unlock do something so no need to wait lock condvar.signal(); unlock Check and/or update state variables Wait if necessary Check and/or update state variables

21. Lec 7.21 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Administrivia First design document due 9/26 –Has to be in by 11:59pm –Good luck! Design reviews: [polls] –Everyone must attend! (no exceptions) –2 points off for one missing person –1 additional point off for each additional missing person –Penalty for arriving late (plan on arriving 5—10 mins early)

22. Lec 7.22 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Java Language Support for Synchronization Java has explicit support for threads and thread synchronization Bank Account example: class Account { private int balance; // object constructor public Account (int initialBalance) { balance = initialBalance; } public synchronized int getBalance() { return balance; } public synchronized void deposit(int amount) { balance += amount; } } –Every object has an associated lock which gets automatically acquired and released on entry and exit from a synchronized method.

23. Lec 7.23 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Java Language Support for Synchronization (con’t) Java also has synchronized statements: synchronized (object) { … } –Since every Java object has an associated lock, this type of statement acquires and releases the object’s lock on entry and exit of the body –Works properly even with exceptions: synchronized (object) { … DoFoo(); … } void DoFoo() { throw errException; }

24. Lec 7.24 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Java Language Support for Synchronization (con’t 2) In addition to a lock, every object has a single condition variable associated with it –How to wait inside a synchronization method of block: »void wait(long timeout); // Wait for timeout »void wait(long timeout, int nanoseconds); //variant »void wait(); –How to signal in a synchronized method or block: »void notify();// wakes up oldest waiter »void notifyAll(); // like broadcast, wakes everyone –Condition variables can wait for a bounded length of time. This is useful for handling exception cases: t1 = time.now(); while (!ATMRequest()) { wait (CHECKPERIOD); t2 = time.new(); if (t2 – t1 > LONG_TIME) checkMachine(); } –Not all Java VMs equivalent! »Different scheduling policies, not necessarily preemptive!

25. Lec 7.25 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Concurrency Summary Semaphores: Like integers with restricted interface –Two operations: »P(): Wait if zero; decrement when becomes non-zero »V(): Increment and wake a sleeping task (if exists) »Can initialize value to any non-negative value –Use separate semaphore for each constraint Monitors: A lock plus zero or more condition variables –Always acquire lock before accessing shared data –Use condition variables to wait inside critical section »Three Operations: Wait(), Signal(), and Broadcast() Readers/Writers –Readers can access database when no writers –Writers can access database when no readers –Only one thread manipulates state variables at a time Language support for synchronization: –Java provides synchronized keyword and one condition- variable per object (with wait() and notify() )

26. Lec 7.26 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Query Compiler query Execution Engine Logging/Recovery LOCK TABLE Concurrency Control Storage Manager BUFFER POOL BUFFERS Buffer Manager Schema Manager Data Definition DBMS: a set of cooperating software modules Transaction Manager transaction Components of a DBMS

27. Lec 7.27 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Data Models DBMS models real world Data Model is link between user’s view of the world and bits stored in computer Many models exist We will concentrate on the Relational Model 10101 11101 Student (sid: string, name: string, login: string, age: integer, gpa:real)

28. Lec 7.28 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Why Study the Relational Model? Most widely used model “Legacy systems” in older models –e.g., IBM’s IMS Object-oriented concepts merged in –“Object-Relational” model »Early work done in POSTGRES research project at Berkeley XML features in most relational systems –Can export XML interfaces –Can embed XML inside relational fields

29. Lec 7.29 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Relational Database: Definitions Relational database: a set of relations Relation: made up of 2 parts: –Schema : specifies name of relation, plus name and type of each column. »E.g. Students(sid: string, name: string, login: string, age: integer, gpa: real) –Instance : a table, with rows and columns. »#rows = cardinality »#fields = degree / arity Can think of a relation as a set of rows or tuples –i.e., all rows are distinct

30. Lec 7.30 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Ex: Instance of Students Relation sid name login age gpa 53666 Jones jones@cs 18 3.4 53688 Smith smith@eecs 18 3.2 53650 Smith smith@math 19 3.8 Cardinality = 3, arity = 5, all rows distinct Do all values in each column of a relation instance have to be distinct?

31. Lec 7.31 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 SQL - A language for Relational DBs SQL (a.k.a. “Sequel”), standard language Data Definition Language (DDL) –create, modify, delete relations –specify constraints –administer users, security, etc. Data Manipulation Language (DML) –Specify queries to find tuples that satisfy criteria –add, modify, remove tuples

32. Lec 7.32 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 SQL Overview CREATE TABLE (, … ) INSERT INTO ( ) VALUES ( ) DELETE FROM WHERE UPDATE SET = WHERE SELECT FROM WHERE

33. Lec 7.33 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Creating Relations in SQL Creates the Students relation. –Note: the type (domain) of each field is specified, and enforced by the DBMS whenever tuples are added or modified. CREATE TABLE Students (sid CHAR(20), name CHAR(20), login CHAR(10), age INTEGER, gpa FLOAT)

34. Lec 7.34 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Table Creation (continued) Another example: the Enrolled table holds information about courses students take CREATE TABLE Enrolled (sid CHAR(20), cid CHAR(20), grade CHAR(2))

35. Lec 7.35 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Adding and Deleting Tuples Can insert a single tuple using: INSERT INTO Students (sid, name, login, age, gpa) VALUES (‘53688’, ‘Smith’, ‘smith@ee’, 18, 3.2) Can delete all tuples satisfying some condition (e.g., name = Smith): DELETE FROM Students S WHERE S.name = ‘Smith’ Powerful variants of these commands are available; more later!

36. Lec 7.36 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Keys Keys are a way to associate tuples in different relations Keys are one form of integrity constraint (IC) sidnameloginagegpa 53666Jonesjones@cs183.4 53688Smithsmith@eecs183.2 53650Smithsmith@math193.8 sidcidgrade 53666Carnatic101C 53666Reggae203B 53650Topology112A 53666History105B Enrolled Students PRIMARY Key FOREIGN Key

37. Lec 7.37 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Primary Keys A set of fields is a superkey if: –No two distinct tuples can have same values in all key fields A set of fields is a key for a relation if : –It is a superkey –No subset of the fields is a superkey what if >1 key for a relation? –One of the keys is chosen (by DBA) to be the primary key. Other keys are called candidate keys. E.g. –sid is a key for Students. –What about name? –The set {sid, gpa} is a superkey.

38. Lec 7.38 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Primary and Candidate Keys in SQL Possibly many candidate keys (specified using UNIQUE ), one of which is chosen as the primary key. Keys must be used carefully! “For a given student and course, there is a single grade.” “Students can take only one course, and no two students in a course receive the same grade.” CREATE TABLE Enrolled (sid CHAR(20) cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid)) CREATE TABLE Enrolled (sid CHAR(20) cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid), UNIQUE (cid, grade)) vs.

39. Lec 7.39 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Foreign Keys, Referential Integrity Foreign key: Set of fields in one relation that is used to `refer’ to a tuple in another relation. –Must correspond to the primary key of the other relation. –Like a `logical pointer’. If all foreign key constraints are enforced, referential integrity is achieved (i.e., no dangling references.)

40. Lec 7.40 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Foreign Keys in SQL E.g. Only students listed in the Students relation should be allowed to enroll for courses. – sid is a foreign key referring to Students: CREATE TABLE Enrolled (sid CHAR(20),cid CHAR(20),grade CHAR(2), PRIMARY KEY (sid,cid), FOREIGN KEY (sid) REFERENCES Students ) sidcidgrade 53666Carnatic101C 53666Reggae203B 53650Topology112A 53666History105B Enrolled sidnameloginagegpa 53666Jonesjones@cs183.4 53688Smithsmith@eecs183.2 53650Smithsmith@math193.8 Students 11111 English102 A

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42. Lec 7.42 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Enforcing Referential Integrity Consider Students and Enrolled; sid in Enrolled is a foreign key that references Students. What should be done if an Enrolled tuple with a non- existent student id is inserted? (Reject it!) What should be done if a Students tuple is deleted? –Also delete all Enrolled tuples that refer to it? –Disallow deletion of a Students tuple that is referred to? –Set sid in Enrolled tuples that refer to it to a default sid? –(In SQL, also: Set sid in Enrolled tuples that refer to it to a special value null, denoting `unknown’ or `inapplicable’.) Similar issues arise if primary key of Students tuple is updated.

43. Lec 7.43 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Integrity Constraints (ICs) IC: condition that must be true for any instance of the database; e.g., domain constraints. –ICs are specified when schema is defined. –ICs are checked when relations are modified. A legal instance of a relation is one that satisfies all specified ICs. –DBMS should not allow illegal instances. If the DBMS checks ICs, stored data is more faithful to real-world meaning. –Avoids data entry errors, too!

44. Lec 7.44 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 ICs are based upon the semantics of the real- world that is being described in the database relations We can check a database instance to see if an IC is violated, but we can NEVER infer that an IC is true by looking at an instance –An IC is a statement about all possible instances! –From example, we know name is not a key, but the assertion that sid is a key is given to us Key and foreign key ICs are the most common; more general ICs supported too Where do ICs Come From?

45. Lec 7.45 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 A major strength of the relational model: supports simple, powerful querying of data Queries can be written intuitively, and the DBMS is responsible for efficient evaluation –The key: precise semantics for relational queries. –Allows the optimizer to extensively re-order operations, and still ensure that the answer does not change Relational Query Languages

46. Lec 7.46 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 The SQL Query Language The most widely used relational query language. –Current std is SQL:2003; SQL92 is a basic subset To find all 18 year old students, we can write: SELECT * FROM Students S WHERE S.age=18 To find just names and logins, replace the first line: SELECT S.name, S.login

47. Lec 7.47 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Querying Multiple Relations What does the following query compute? SELECT S.name, E.cid FROM Students S, Enrolled E WHERE S.sid=E.sid AND E.grade='A' sidcidgrade 53831Carnatic101C 53831Reggae203B 53650Topology112A 53666History105B Given the following instance of Enrolled S.nameE.cid SmithTopology112 we get:

48. Lec 7.48 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Semantics of a Query A conceptual evaluation method for the previous query: 1. do FROM clause: compute cross-product of Students and Enrolled 2. do WHERE clause: Check conditions, discard tuples that fail 3. do SELECT clause: Delete unwanted fields Remember, this is conceptual. Actual evaluation will be much more efficient, but must produce the same answers.

49. Lec 7.49 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Cross-product of Students and Enrolled Instances Smith smith@ee 18 3.2 53650 Topology112 A 53688 Smith smith@ee 18 3.2 53666 History105 B 53650 Smith smith@math 19 3.8 53831 Carnatic101 C 53650 Smith smith@math 19 3.8 53831 Reggae203 B 53650 Smith smith@math 19 3.8 53650 Topology112 A 53650 Smith smith@math 19 3.8 53666 History105 B

50. Lec 7.50 9/19/07Joseph CS194-3/16x ©UCB Fall 2007 Relational Model: Summary A tabular representation of data Simple and intuitive, currently the most widely used –Object-relational support in most products –XML support added in SQL:2003, most systems Integrity constraints can be specified by the DBA, based on application semantics. DBMS checks for violations. –Two important ICs: primary and foreign keys –In addition, we always have domain constraints. Powerful query languages exist –SQL is the standard commercial one »DDL - Data Definition Language »DML - Data Manipulation Language