File Processing : Transaction Management 2015, Spring Pusan National University Ki-Joune Li.

File Processing : Transaction Management

2015, Spring

Pusan National University

Ki-Joune Li

STEMPNU

Basic Concepts of Transaction

Transaction A set of operations Atomic :

All or Nothing : Consistent State of Database Example : Flight Reservation Cf. Partially Done : Inconsistent State

STEMPNU

Transaction States

Active

PartiallyCommitted

Failed Aborted

Committedthe initial state; the transaction stays in this state while it is

executing

after the discovery that normal execution can no longer proceed. after the transaction has been rolled back and

the database restored to its state prior to the start of the transaction.

- restart the transaction or - kill the transaction

ALL

NOTHING

STEMPNU

ACID Properties

Atomicity. All or Nothing Not Partially Done Example : Failure in Flight Reservation

Consistency. Execution of a transaction preserves the consistency of the

database.

State 1 State 2

All

Nothing

State 2’PartiallyDone

Consistent

Consistent

STEMPNU

ACID Properties

Isolation. Although multiple transactions may execute concurrently, each

transaction must be unaware of other concurrently executing transactions.

Intermediate transaction results must be hidden from other concurrently executed transactions.

Durability. After a transaction completes successfully, the changes it has

made to the database persist, even if there are system failures.

DB

Transation 1

Transation 2

No Effect

STEMPNU

Example

Transaction : Transfer $50 from account A to account B:1. read(A)

2. A := A – 50

3. write(A)

4. read(B)

5. B := B + 50

6. write(B)

Consistency requirement the sum of A and B is unchanged after the transaction.

Atomicity requirement Durability Isolation

STEMPNU

Example : Concurrent Execution

Two Transactions T1 : transfer $50 from A to B,

T2 transfer 10% of the balance from A to B

Serial Schedule Concurrent Schedule

STEMPNU

Serializability

What happens after these transactions ? Serial Schedule :

Always Correct T1 T2 and T2 T1

Concurrent Schedule Serializable if

Result (T1 || T2) = Result(T1 T2) or

Result(T2 T1)

STEMPNU

Transaction Management

Transaction Management Guarantee ACID Properties of Transaction by

Concurrency Control : Isolation and Consistency Recovery : Atomicity and Durability

File Processing : Concurrency Control

STEMPNU

Serializability

For given transactions T1, T2,.., Tn, Schedule (History) S is serializable if

Result(S) Result(Sk) where Sk is a serial excution schedule.

Note that Result(Si ) may be different from Result(Sj ) (i j )

How to detect whether S is serializable Conflict Graph

STEMPNU

Conflict Graph

T1

T2

r(a)

w(a)

affects

r(b)

w(b)

S1S1

T1

T2

r(a)

w(a)

affects

r(b)

w(b)

S2S2

Res(S1) Res( (T1, T2) )Res(S1) Res( (T1, T2) )

Res(S1) Res( (T1, T2) )Res(S1) Res( (T1, T2) )

Res(S1) Res( (T2, T1) )Res(S1) Res( (T2, T1) )

STEMPNU

Detect Cycle in Conflict Graph

T1

T2

r(a)

w(a)

affects

r(b)

w(b) T2T2

T1T1

If Cycle in Conflict Graph Then Not Serializable Otherwise Serializable

STEMPNU

How to make it serializable

How to make it serializable Control the order of execution of operations in concurrent transactions.

Two Approaches Two Phase Locking Protocol

Locking on each operation

Timestamping : Ordering by timestamp on each transaction and each operation

STEMPNU

Lock-Based Protocols

A lock mechanism to control concurrent access to a data item

Data items can be locked in two modes : Exclusive (X) mode : Data item can be both read as well as written. X-lock is requested using lock-X instruction. Shared (S) mode : Data item can only be read. S-lock is requested using lock-S instruction.

Lock requests are made to concurrency-control manager.

Transaction can proceed only after request is granted.

STEMPNU

Lock-Based Protocols (Cont.)

Lock-compatibility matrix

A transaction may be granted a lock on an item if the requested lock is compatible with locks already held Any number of transactions can hold shared locks

If a lock cannot be granted, the requesting transaction is made to wait till all incompatible locks held have been released. the lock is then granted.

STEMPNU

Lock-Based Protocols

Example of a transaction performing locking:

T2: lock-S(A);

read (A);

unlock(A);

lock-S(B);

read (B);

unlock(B);

STEMPNU

The Two-Phase Locking Protocol

This is a protocol which ensures conflict-serializable schedules. Phase 1: Growing Phase

transaction may obtain locks transaction may not release locks

Phase 2: Shrinking Phase transaction may release locks transaction may not obtain locks

The protocol assures serializability

STEMPNU

Lock Conversions

Two-phase locking with lock conversions: First Phase:

can acquire a lock-S on item can acquire a lock-X on item can convert a lock-S to a lock-X (upgrade)

Second Phase: can release a lock-S can release a lock-X can convert a lock-X to a lock-S (downgrade)

This protocol assures serializability

STEMPNU

Where to insert Lock related operations ?

A transaction Ti issues the standard read/write instruction, without explicit locking calls.

Automatic Acquisition of Locks The operation read(D) is processed as:

if Ti has a lock on D

then read(D) else

begin

wait until no other transaction has a lock-X on D

grant Ti a lock-S on D;

read(D) end

STEMPNU

Automatic Acquisition of Locks

write(D) is processed as: if Ti has a lock-X on D then write(D) else begin

if necessary wait until no other trans. has any lock on D,

if Ti has a lock-S on D then upgrade lock on D to lock-X else grant Ti a lock-X on D

write(D) end;

All locks are released after commit or abort

STEMPNU

Lock Manager

Transaction Transaction sends lock request to Lock

Manager Lock Manager determines whether to

allow or not

Lock Table Keeps granted locks and pending locks

for each item In-memory Hash Table

STEMPNU

Problem of Two Phase Locking Protocol

Deadlock Growing Phase and Shrinking Phase

Prevention and Avoidance : Impossible Only Detection may be possible

When a deadlock occurs Detection of Deadlock : Wait-For-Graph Abort a transaction

How to choose a transaction to kill ?

STEMPNU

Tree Lock : A Special Locking Mechanism

Only exclusive locks are allowed. The first lock by T may be on any data

item. Subsequently, a data Q can be locked

by T only if the parent of Q is currently locked by T.

Data items may be unlocked at any time.

Pairwise lock

Not Allowed

STEMPNU

Timestamp-Based Protocols

Each transaction is issued a timestamp when TS(Ti) <TS(Tj) :

old transaction Ti and new transaction Tj

Each data Q, two timestamp : W-timestamp(Q) : largest time-stamp for successful write(Q) R-timestamp(Q) : largest time-stamp for successful read(Q)

STEMPNU

Timestamp-Based Protocols : Read

Transaction Ti issues a read(Q) If TS(Ti) W-timestamp(Q),

then Ti needs to read a value of Q that was already overwritten.

Hence, the read operation is rejected, and Ti is rolled back.

If TS(Ti) W-timestamp(Q), then the read operation is executed, and R-timestamp(Q) is set set

STEMPNU

Timestamp-Based Protocols : Write

Transaction Ti issues write(Q). If TS(Ti) < R-timestamp(Q),

then the value of Q that Ti is producing was needed previously, and the system assumed that that value would never be produced. Hence, the write operation is rejected, and Ti is rolled back.

If TS(Ti) < W-timestamp(Q), then Ti is attempting to write an obsolete value of Q.

Hence, this write operation is rejected, and Ti is rolled back.

Otherwise, the write operation is executed, and W-timestamp(Q) is reset

STEMPNU

The timestamp-ordering protocol guarantees serializability since all the arcs in the precedence graph are of the form:

Thus, there will be no cycles in the conflict graph Timestamp protocol : free from deadlock

transactionwith smallertimestamp

transactionwith largertimestamp

Correctness of Timestamp-Ordering Protocol

STEMPNU

Problem of Time-Stamping Protocol

Rollback and Restarting Overhead Rollback and Restarting

relatively more frequent than deadlock Instead of Deadlock Detection

Cost ? pRB(CRB+CRStart) > pDL CDL + CDLDetect

STEMPNU

Long Duration Transaction

Transaction of Long Duration with large number of operations

Problem Expensive rollback and restart Degradation of concurrency

Approach Nested Transaction Semantic Consistency rather than Serializability

File Processing : Recovery

STEMPNU

Failure Classification

Transaction failure : Logical errors: internal error condition System errors: system error condition (e.g., deadlock)

System crash: a power failure or other hardware or software failure Fail-stop assumption: non-volatile storage contents are assumed to

not be corrupted by system crash Database systems have numerous integrity checks to prevent corruption

of disk data

Disk failure

STEMPNU

Recovery Algorithms

Recovery algorithms : should ensure database consistency transaction atomicity and durability despite failures

Recovery algorithms have two parts1. Preparing Information for Recovery : During normal transaction

2. Actions taken after a failure to recover the database

STEMPNU

Storage Structure

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

STEMPNU

Recovery and Atomicity

Modifying the database must be committed Otherwise it may leave the database in an inconsistent state.

Example Consider transaction Ti that transfers $50 from account A to

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

Several output operations may be required for Ti For example : output(A) and output(B). A failure may occur after one of these modifications have been

made but before all of them are made.

STEMPNU

Recovery and Atomicity (Cont.)

To ensure atomicity despite failures, we first output information describing the modifications to stable

storage without modifying the database itself.

We study two approaches: log-based recovery, and shadow-paging

STEMPNU

Log-Based Recovery

A log : must be kept on stable storage. <Ti, Start>, and <Ti, Start> < Ti, X, V1, V2 >

Logging Method When transaction Ti starts, <Ti start> log record

When Ti finishes, <Ti commit> log record Before Ti executes write(X), <Ti, X, Vold , Vnew > log record We assume for now that log records are written directly to stable storage

Two approaches using logs Deferred database modification Immediate database modification

STEMPNU

Deferred Database Modification

The deferred database modification scheme records all modifications to the log, writes them after commit.

Log Scheme Transaction starts by writing <Ti start> record to log. write(X) :

<Ti, X, V> Note: old value is not needed for this scheme The write is not performed on X at this time, but is deferred.

When Ti commits, <Ti commit> is written to the log Finally, executes the previously deferred writes.

STEMPNU

Deferred Database Modification (Cont.)

Recovering Method During recovery after a crash,

a transaction needs to be redone if and only if both <Ti start> and<Ti commit> are there in the log.

Redoing a transaction Ti ( redoTi) sets the value of all data items updated by the transaction to the new values.

Deletes Ti such that <Ti ,start> exists but <Ti commit> does not.

STEMPNU

Deferred Database Modification : Example

If log on stable storage at time of crash is as in case:(a) No redo actions need to be taken

(b) redo(T0) must be performed since <T0 commit> is present

(c) redo(T0) must be performed followed by redo(T1) since <T0 commit>

and <Ti commit> are present

T0: read (A) T1 : read (C) A: = A - 50 C:= C- 100 write (A) write (C) read (B) B:= B + 50 write (B)

STEMPNU

Immediate Database Modification

Immediate database modification scheme Database updates of an uncommitted transaction For undoing : both old value and new value

Recovery procedure has two operations undo(Ti) : restores the value of all data items updated by Ti

redo(Ti) : sets the value of all data items updated by Ti

When recovering after failure: Undo if the log <Ti start>, but not <Ti commit>.

Redo if the log both the record <Ti start> and <Ti commit>.

STEMPNU

Immediate Database Modification : Example

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

STEMPNU

Idempotent Operation

Result (Op(x)) = Result (Op(Op(x)) Example

Increment(x); : Not Idempotent x=a; write(x); : Idempotent

Operations in Log Record Must be Idempotent, otherwise

Multple Executions (for redo) may cause incorrect results

STEMPNU

Checkpoints

Problems in recovery procedure by Log record scheme :1. searching the entire log records : time-consuming

2. Discard unnecessary redo transactions already executed.

Checkpoint Method Marking Checkpoint Recovery from Checkpoint

Marking Checkpoint1. Output all log records currently residing in main memory onto stable

storage.

2. Output all modified buffer blocks to the disk.

3. Write a log record < checkpoint> onto stable storage.

STEMPNU

Checkpoints (Cont.)

In case of failure

T1 can be ignored

T2 and T3 redone.

T4 undone

TcTf

T1

T2

T3

T4

checkpoint system failure

STEMPNU

Shadow Paging

Mechanism maintain two page tables

the current page table, and the shadow page table

shadow page table state of the database before transaction execution Shadow page table is never modified during execution To start with, both the page tables are identical.

Whenever any page is about to be written for the first time A copy of this page is made onto an unused page. The current page table is then made to point to the copy The update is performed on the copy

STEMPNU

Shadow Page : Example

Old State

New State