15.1. 15.2 General Overview Where we’ve been... DBA skills for relational DB’s: Logical Schema Design E/R diagrams Decomposition and Normalization.

15.1

15.2

General OverviewGeneral Overview

Where we’ve been...

DBA skills for relational DB’s:

Logical Schema Design

E/R diagrams

Decomposition and Normalization

Query languages

RA, RC, SQL

Integrity Constraints

Transactions

Database Implementation Issues

Files, buffering, indexing

Today: Data Integrity

15.3

What Does a DBMS Manage?What Does a DBMS Manage?1. Data organization

E/R Model

Relational Model

2. Data Retrieval Relational Algebra

Relational Calculus

SQL

3. Data Integrity Integrity Constraints

Transactions

15.4

15.5

Updates in SQLUpdates in SQLAn example:

UPDATE accountSET balance = balance -50WHERE acct_no = A102

account

Dntn: A102: 300

Dntn: A15: 500

Mian: A142: 300

…

…

(1) Read

(2) update

(3) writeTransaction:

1. Read(A)2. A <- A -503. Write(A)

What takes place:

memory

Disk

15.6

The Threat to Data IntegrityThe Threat to Data IntegrityConsistent DB

Name Acct bal-------- ------ ------Joe A-33 300Joe A-509 100

Joe’s total: 400

Consistent DB

Name Acct bal-------- ------ ------Joe A-33 250Joe A-509 150

Joe’s total: 400

transaction

Inconsistent DB

Name Acct bal-------- ------ ------Joe A-33 250Joe A-509 100

Joe’s total: 350

What a Xaction shouldlook like to Joe

What actually happensduring execution

15.7

TransactionsTransactions

What?: Updates to db that can be executed concurrently

Why?:

(1) Updates can require multiple reads, writes on a db

e.g., transfer $50 from A-33 to A509 = read(A) A A -50 write(A) read(B) BB+50 write(B)

(2) For performance reasons, db’s permit updates to be executed concurrently

Concern: concurrent access/updates of data can compromise data integrity

15.8

ACID PropertiesACID Properties

Atomicity: either all operations in a Xaction take effect, or none

Consistency: operations, taken together preserve db consistency

Isolation: intermediate, inconsistent states must be concealed from other Xactions

Durability. If a Xaction successfully completes (“commits”), changes made to db must persist, even if system crashes

Properties that a Xaction needs to have:

15.9

Demonstrating ACIDDemonstrating ACIDTransaction to transfer $50 from account A to account B:

1. read(A)2. A := A – 503. write(A)4. read(B)5. B := B + 506. write(B)

Consistency: total value A+B, unchanged by Xaction

Atomicity: if Xaction fails after 3 and before 6, 3 should not affect db

Durability: once user notified of Xaction commit, updates to A,B shouldnot be undone by system failure

Isolation: other Xactions should not be able to see A, B between steps 3-6

15.10

Threats to ACIDThreats to ACID1. Programmer Errore.g.: $50 substracted from A, $30 added to B threatens consistency

2. System Failurese.g.: crash after write(A) and before write(B) threatens atomicitye.g.: crash after write(B) threatens durability

3. ConcurrencyE.g.: concurrent Xaction reads A, B between steps 3-6

threatens isolation

15.11

IsolationIsolationSimplest way to guarantee: forbid concurrent Xactions!

But, concurrency is desirable:

(1) Achieves better throughput (TPS: transactions per second)

one Xaction can use CPU while another is waiting for disk to service request

(2) Achieves better average response time

short Xactions don’t need to get stuck behind long ones

Prohibiting concurrency is not an option

15.12

IsolationIsolation Approach to ensuring Isolation:

Distinguish between “good” and “bad” concurrency

Prevent all “bad” (and sometime some “good”) concurrency from happening OR

Recognize “bad” concurrency when it happens and undo its effects (abort some transactions)

Pessimistic vs Optimistic CC

Both pessimistic and optimistic approaches require distinguishing between good and bad concurrency

How: concurrency characterized in terms of possible Xaction “schedules”

15.13

SchedulesSchedules

Schedules – sequences that indicate the chronological order in which instructions of concurrent transactions are executed a schedule for a set of transactions must consist of all instructions of

those transactions

must preserve the order in which the instructions appear in each individual transaction

T1123

T2ABCD

T11

23

T2

AB

CD

one possible schedule:

15.14

Example SchedulesExample Schedules

Constraint: The sum of A+B must be the same

Before: 100+50

After: 45+105

T1read(A)A <- A -50write(A)read(B)B<-B+50write(B)

T2

read(A)tmp <- A*0.1A <- A – tmpwrite(A)read(B)B <- B+ tmpwrite(B)

Transactions: T1: transfers $50 from A to B T2: transfers 10% of A to B

=150, consistent

Example 1: a “serial” schedule

15.15

Example ScheduleExample Schedule

Another “serial” schedule:

T1

read(A)A <- A -50write(A)read(B)B<-B+50write(B)

T2read(A)tmp <- A*0.1A <- A – tmpwrite(A)read(B)B <- B+ tmpwrite(B)

Before: 100+50

After: 40+110

Consistent but not the same as previous schedule..

Either is OK!

=150, consistent

15.16

Example Schedule (Cont.)Example Schedule (Cont.)Another “good” schedule:

T1read(A)A <- A -50write(A)

read(B)B<-B+50write(B)

T2

read(A)tmp <- A*0.1A <- A – tmpwrite(A)

read(B)B <- B+ tmpwrite(B)

Effect: Before After A 100 45 B 50 105

Same as one of the serial schedulesSerializable

15.17

Example Schedules (Cont.)Example Schedules (Cont.) A “bad” schedule

Before: 100+50 = 150

After: 50+60 = 110 !!

Not consistent

T1read(A)A <- A -50

write(A)read(B)B<-B+50write(B)

T2

read(A)tmp <- A*0.1A <- A – tmpwrite(A)read(B)

B <- B+ tmpwrite(B)

15.18

Example Schedule (Schedule B)Example Schedule (Schedule B)T1 T2 T3 T4 T5

read(X)read(Y)read(Z)

read(V)read(W)

read(Y)write(Y)

write(Z)read(U)

read(Y)write(Y)read(Z)write(Z)

read(U)write(U)

15.19

Transaction Definition in SQLTransaction Definition in SQL Data manipulation language must include a construct for

specifying the set of actions that comprise a transaction. In SQL, a transaction begins implicitly. A transaction in SQL ends by:

Commit work commits current transaction and begins a new one.

Rollback work causes current transaction to abort.

Levels of consistency specified by SQL-92: Serializable — default (more conservative than conflict

serializable) Repeatable read Read committed Read uncommitted

15.20

Transactions in SQLTransactions in SQL

Serializable — default

- can read only committed records

- if T is reading or writing X, no other Xaction can change X until T commits

- if T is updating a set of records (identified by WHERE clause), no other Xaction can change this set until T commits

Weaker versions (non-serializable) levels can also be declared

Idea: tradeoff: More concurrency => more overhead to ensure

valid schedule.

Lower degrees of consistency useful for gathering approximateinformation about the database, e.g., statistics for query optimizer.

15.21

15.22

General OverviewGeneral Overview

Relational model - SQL Formal & commercial query languages

Functional Dependencies

Normalization

Physical Design

Indexing

Query Processing and Optimization

Transaction Processing and CC

15.23

Transaction ConceptTransaction Concept

A transaction is a unit of program execution that accesses and possibly updates various data items.

A transaction must see a consistent database. During transaction execution the database may be

inconsistent. A transaction ends with a Commit or an Abort When the transaction is committed, the database must

be consistent.

State 1 State 2

15.24

Prevent P(S) cycles from occurring using a concurrency control manager: ensures interleaving of operations amongst concurrent xactions only result in serializable schedules.

T1 T2 ….. Tn

CC Scheduler

How to enforce serializable schedules?How to enforce serializable schedules?

Serializableschedule

15.25

Concurrency Via LocksConcurrency Via Locks

Idea:

Data items modified by one xaction at a time

Locks Control access to a resource

Can block a xaction until lock granted

Two modes:

Shared (read only)

eXclusive (read & write)

15.26

Granting LocksGranting Locks

Requesting locks Must request before accessing a data item

Granting Locks No lock on data item? Grant

Existing lock on data item?

Check compatibility:

Compatible? Grant

Not? Block xaction shared exclusive

shared Yes No

exclusive No No

15.27

Lock instructionsLock instructions

New instructions

- lock-S: shared lock request

- lock-X: exclusive lock request

- unlock: release previously held lock

Example: lock-X(B)read(B)B B-50write(B)unlock(B)lock-X(A)read(A)A A + 50write(A)unlock(A)

lock-S(A)read(A)unlock(A)lock-S(B)read(B)unlock(B)display(A+B)

T1 T2

15.28

Locking IssuesLocking Issues

Starvation T1 holds shared lock on Q

T2 requests exclusive lock on Q: blocks

T3, T4, ..., Tn request shared locks: granted

T2 is starved!

Solution?

Do not grant locks if other xaction is waiting

15.29

Locking IssuesLocking Issues

No xaction proceeds:

Deadlock

- T1 waits for T2 to unlock A

- T2 waits for T1 to unlock B

T1 T2

lock-X(B)

read(B)

B B-50

write(B)

lock-X(A)

lock-S(A)

read(A)

lock-S(B)

More later…

15.30

Locking IssuesLocking Issues Does not ensure serializability by itself:

lock-X(B)read(B)B B-50write(B)unlock(B)

lock-X(A)read(A)A A + 50write(A)unlock(A)

lock-S(A)read(A)unlock(A)lock-S(B)read(B)unlock(B)display(A+B)

T1

T2

T2 displays 50 less!!

15.31

Two-Phase Locking: 2PLTwo-Phase Locking: 2PL

Each xaction has two phases Growing : only lock request

Shrinking: only unlocks

xactions start in growing phase

First unlock begins the shrinking phase

Ensures serializabile schedules ! Order concurrent xactions on the moment of final lock is granted

15.32

2PL2PL Example: T1 in 2PL

T1

lock-X(B)

read(B)

B B - 50

write(B)

lock-X(A)

read(A)

A A - 50

write(A)

unlock(B)

unlock(A)

Growing phase

Shrinking phase

15.33

2PL Issues2PL Issues

2PL does not prevent deadlock

> 2 xactions involved?

- Rollbacks expensive

T1 T2

lock-X(B)

read(B)

B B-50

write(B)

lock-X(A)

lock-S(A)

read(A)

lock-S(B)

15.34

Dealing with DeadlocksDealing with Deadlocks

How do you detect a deadlock? Wait-for graph

Directed edge from Ti to Tj

Ti waiting for Tj

T1 T2 T3 T4

S(V)

X(V)

S(W)

X(Z)

S(V)

X(W)

T1

T2

T4

T3

Suppose T4 requests lock-S(Z)....

15.35

Detecting DeadlocksDetecting Deadlocks

Wait-for graph has a cycle deadlock

T2, T3, T4 are deadlocked

T1

T2

T4

T3•Build wait-for graph, check for cycle

•How often?- Tunable

Expect many deadlocks or many xactions involvedRun often to avoid aborts

Else run less often to reduce overhead

15.36

Recovering from DeadlocksRecovering from Deadlocks

Rollback one or more xaction Which one?

Rollback the cheapest ones

Cheapest ill-defined

Was it almost done?

How much will it have to redo?

Will it cause other rollbacks?

How far?

May only need a partial rollback

Avoid starvation

Ensure same xaction not always chosen to break deadlock

15.37

15.38

Review: The ACID propertiesReview: The ACID properties

AA tomicity: All actions in the Xaction happen, or none happen.

CC onsistency: If each Xaction is consistent, and the DB starts consistent, it ends up consistent.

II solation: Execution of one Xaction is isolated from that of other Xacts.

D D urability: If a Xaction commits, its effects persist.

CC guarantees Isolation and Atomicity.

The Recovery Manager guarantees Atomicity & Durability.

15.39

Why is recovery system necessary?Why is recovery system necessary?

Transaction failure : Logical errors: application errors (e.g. div by 0, segmentation fault) System errors: deadlocks Aborts

System crash: hardware/software failure causes the system to crash.

Disk failure: head crash or similar disk failure destroys all or part of disk storage

The lost data can be in main memory or in disk

15.40

Storage MediaStorage Media

Volatile storage: does not survive system crashes

examples: main memory, cache memory

Nonvolatile storage: survives system crashes

examples: disk, tape, flash memory, non-volatile (battery backed up) RAM

Stable storage: a “mythical” form of storage that survives all failures

approximated by maintaining multiple copies on distinct nonvolatile media

15.41

Recovery and DurabilityRecovery and Durability

To achieve Durability: Put data on stable storage

To approximate stable storage make two copies of data

15.42

Stable-Storage ImplementationStable-Storage Implementation

Solution: Write to the first disk

Write to the second disk when the first disk completes

The process is complete only after the second write completes successfully

Recovery (from disk failures, etc): Detect bad blocks with the checksum (e.g. parity)

Two good copies, equal blocks: done

One good, one bad : copy good to bad

Two bad copies: ignore write

Two good, unequal blocks?

Ans: Copy the second to the first

15.43

Recovery and AtomicityRecovery and Atomicity

Example: transfer $50 from account A to account B

goal is either to perform all database modifications made by Ti or none at all.

Requires several inputs (reads) and outputs (writes)

Failure after output to account A and before output to B…. DB is corrupted!

15.44

Recovery AlgorithmsRecovery Algorithms

Recovery algorithms are techniques to ensure database consistency and transaction atomicity and durability despite failures

Recovery algorithms have two parts

1. Actions taken during normal transaction processing to ensure enough information exists to recover from failures

2. Actions taken after a failure to recover the database contents to a state that ensures atomicity, consistency and durability

ARIES: Algorithms for Recovery and Isolation Exploiting Semantics

http://en.wikipedia.org/wiki/Algorithms_for_Recovery_and_Isolation_Exploiting_Semantics

15.45

Log-Based RecoveryLog-Based Recovery

Simplifying assumptions: Transactions run serially

logs are written directly on the stable storage

Log: a sequence of log records; maintains a record of update activities on the database. (Write Ahead Log, W.A.L.)

W.A.L. Write-Ahead Log: record the operation on the log, before you write it on the db.

Record everything else on the log before commit

15.46

Log based approachLog based approach

Log records for transaction Ti:<Ti start ><Ti, X, V1, V2>

<Ti commit >

Two approaches using logsDeferred database modificationImmediate database modification

15.47

Log exampleLog example

Transaction T1

Read(A) A =A-50 Write(A) Read(B) B = B+50 Write(B)

Log

<T1, start><T1, A, 1000, 950><T1, B, 2000, 2050><T1, commit>

15.48

Deferred Database ModificationDeferred Database Modification

Ti starts: write a <Ti start> record to log.

Ti write(X)

write <Ti, X, V> to log: V is the new value for X

The write is deferred

Note: old value is not needed for this scheme

Ti partially commits:

Write <Ti commit> to the log

DB updates by reading and executing the log:

<Ti start> …… <Ti commit>

15.49

Deferred Database ModificationDeferred Database Modification How to use the log for recovery after a crash?

Redo: if both <Ti start> and <Ti commit> are there in the log. Crashes can occur while

the transaction is executing the original updates, or while recovery action is being taken

example transactions T0 and T1 (T0 executes before T1):

T0: read (A) T1 : read (C)

A: - A - 50 C:- C- 100

Write (A) write (C)

read (B)

B:- B + 50

write (B)

15.50

Deferred Database Modification (Cont.)Deferred Database Modification (Cont.) Below we show the log as it appears at three instances of time.

<T0, start><T0, A, 950><T0, B, 2050>

(a)

<T0, start><T0, A, 950><T0, B, 2050><T0, commit><T1, start><T1, C, 600>

(b)

<T0, start><T0, A, 950><T0, B, 2050><T0, commit><T1, start><T1, C, 600><T1, commit>

(c)

15.51

Immediate Database ModificationImmediate Database Modification

Database updates of an uncommitted transaction is allowed

Tighter logging rules are needed to ensure transactions are undoable

Write records must be of the form: <Ti, X, Vold, Vnew >

log record must be written before database item is written

Output of DB blocks can occur:

Before or after commit

In any order

15.52

Immediate Database Modification ExampleImmediate Database Modification Example

Log Write Output

<T0 start>

<T0, A, 1000, 950>

<To, B, 2000, 2050>

A = 950 B = 2050

<T0 commit>

<T1 start><T1, C, 700, 600> C = 600

BB, BC

<T1 commit> BA

Note: BX denotes block containing X.

15.53

Immediate Database Modification (Cont.)Immediate Database Modification (Cont.) Recovery procedure :

Undo : <Ti, start > is in the log but <Ti commit> is not. Undo:

restore the value of all data items updated by Ti to their old values, going backwards from the last log record for Ti

Redo: <Ti start> and <Ti commit> are both in the log.

Redo: sets the value of all data items updated by Ti to the new values, going forward from the first log record for Ti

Both operations must be idempotent: even if the operation is executed multiple times the effect is the same as if it is executed once

15.54

I M Recovery ExampleI M Recovery Example

<T0, start><T0, A, 1000, 950><T0, B, 2000, 2050>

(a)

<T0, start><T0, A, 1000, 950><T0, B, 2000, 2050><T0, commit><T1, start><T1, C, 700, 600>

(b)

<T0, start><T0, A, 1000, 950><T0, B, 2000, 2050><T0, commit><T1, start><T1, C, 700, 600><T1, commit>

(c)

Recovery actions in each case above are:

(a) undo (T0): B is restored to 2000 and A to 1000.

(b) undo (T1) and redo (T0): C is restored to 700, and then A and B are

set to 950 and 2050 respectively.

(c) redo (T0) and redo (T1): A and B are set to 950 and 2050

respectively. Then C is set to 600

15.55

CheckpointsCheckpoints

Problems in recovery procedure as discussed earlier :

1. searching the entire log is time-consuming

2. we might unnecessarily redo transactions which have already output their updates to the database.

How to avoid redundant redoes? Put marks in the log indicating that at that point DB and log are

consistent. Checkpoint!

15.56

CheckpointsCheckpoints

At a checkpoint:

Output all log records currently residing in main memory onto stable storage.

Output all modified buffer blocks to the disk.

Write a log record < checkpoint> onto stable storage.

15.57

Checkpoints (Cont.)Checkpoints (Cont.)

Recovering from log with checkpoints:

1. Scan backwards from end of log to find the most recent <checkpoint> record

2. Continue scanning backwards till a record <Ti start> is found.

3. Need only consider the part of log following above start record. Why?

4. After that, recover from log with the rules that we had before.

15.58

Example of CheckpointsExample of Checkpoints

Tc Tf

T1

T2

T3

T4

checkpoint system failurecheckpoint

T1 can be ignored (updates already output to disk due to checkpoint)

T2 and T3 redone.

T4 undone

15.59

Recovery With Concurrent TransactionsRecovery With Concurrent Transactions

To permit concurrency: All transactions share a single disk buffer and a single log

Concurrency control: Strict 2PL :i.e. Release eXclusive locks only after commit.

Logging is done as described earlier.

The checkpointing technique and actions taken on recovery have to be changed since several transactions may be active when a checkpoint is

performed.