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CSE 444: Database Internals
Lectures 17-19Transactions: Recovery
CSE 444 - Winter 2019
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The Usual Reminders
• HW3 is due tonight– Only a single problem
• Lab3 is due on Monday
• Quiz grades should be returned on Gradescope tomorrow
CSE 444 - Winter 2019 2
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Readings for Lectures 17-19
Main textbook (Garcia-Molina)• Ch. 17.2-4, 18.1-3, 18.8-9Second textbook (Ramakrishnan)• Ch. 16-18Also: M. J. Franklin. Concurrency Control and Recovery. The Handbook of Computer Science and Engineering, A. Tucker, ed., CRC Press, Boca Raton, 1997.
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Transaction Management
Two parts:
• Concurrency control: ACID
• Recovery from crashes: ACID
We already discussed concurrency control
You are implementing locking in lab3
Today, we start recovery
CSE 444 - Winter 2019 4
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System Crash
Client 1:BEGIN TRANSACTIONUPDATE Account1SET balance= balance – 500
UPDATE Account2SET balance = balance + 500COMMIT
Crash !
CSE 444 - Winter 2019
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RecoveryType of Crash Prevention
Wrong data entry Constraints andData cleaning
Disk crashes Redundancy: e.g. RAID, archive
Data center failures Remote backups or replicas
System failures:e.g. power
DATABASERECOVERY
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System Failures
• Each transaction has internal state
• When system crashes, internal state is lost– Don’t know which parts executed and which didn’t– Need ability to undo and redo
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Buffer Manager Review
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Disk
Main
memory
Page requests from higher-level code
Buffer pool
Disk page
Free frame
1 page corresponds
to 1 disk block
Disk = collection
of blocks
Buffer pool manager
Files and access methods
READ
WRITE
INPUT
OUTPUT
choice of frame dictated
by replacement policy
Data must be in RAM for DBMS to operate on it!
Buffer pool = table of <frame#, pageid> pairs
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Buffer Manager Review
• Enables higher layers of the DBMS to assume that needed data is in main memory
• Caches data in memory. Problems when crash occurs:– If committed data was not yet written to disk– If uncommitted data was flushed to disk
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Transactions
• Assumption: the database is composed of elements.
• 1 element can be either:– 1 page = physical logging– 1 record = logical logging
• Aries uses physiological logging– (will discuss later)
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Primitive Operations of Transactions
• READ(X,t)– copy element X to transaction local variable t
• WRITE(X,t)– copy transaction local variable t to element X
• INPUT(X)– read element X to memory buffer
• OUTPUT(X)– write element X to disk
CSE 444 - Winter 2019
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Running Example
Initially, A=B=8.
Atomicity requires that either (1) T commits and A=B=16, or(2) T does not commit and A=B=8.
CSE 444 - Winter 2019
BEGIN TRANSACTIONREAD(A,t); t := t*2;WRITE(A,t); READ(B,t); t := t*2;WRITE(B,t)COMMIT;
Will look at various crash scenarios
What behavior do we want in each case?
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Buffer pool DiskTransaction
READ(A,t); t := t*2; WRITE(A,t); READ(B,t); t := t*2; WRITE(B,t)
CSE 444 - Winter 2019
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t)
t:=t*2
WRITE(A,t)
INPUT(B)
READ(B,t)
t:=t*2
WRITE(B,t)
OUTPUT(A)
OUTPUT(B)
COMMIT
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Buffer pool DiskTransaction
READ(A,t); t := t*2; WRITE(A,t); READ(B,t); t := t*2; WRITE(B,t)
CSE 444 - Winter 2019
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2
WRITE(A,t)
INPUT(B)
READ(B,t)
t:=t*2
WRITE(B,t)
OUTPUT(A)
OUTPUT(B)
COMMIT
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Buffer pool DiskTransaction
READ(A,t); t := t*2; WRITE(A,t); READ(B,t); t := t*2; WRITE(B,t)
CSE 444 - Winter 2019
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t)
INPUT(B)
READ(B,t)
t:=t*2
WRITE(B,t)
OUTPUT(A)
OUTPUT(B)
COMMIT
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Buffer pool DiskTransaction
READ(A,t); t := t*2; WRITE(A,t); READ(B,t); t := t*2; WRITE(B,t)
CSE 444 - Winter 2019
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B)
READ(B,t)
t:=t*2
WRITE(B,t)
OUTPUT(A)
OUTPUT(B)
COMMIT
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Buffer pool DiskTransaction
READ(A,t); t := t*2; WRITE(A,t); READ(B,t); t := t*2; WRITE(B,t)
CSE 444 - Winter 2019
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B) 16 16 8 8 8
READ(B,t)
t:=t*2
WRITE(B,t)
OUTPUT(A)
OUTPUT(B)
COMMIT
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Buffer pool DiskTransaction
READ(A,t); t := t*2; WRITE(A,t); READ(B,t); t := t*2; WRITE(B,t)
CSE 444 - Winter 2019
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2
WRITE(B,t)
OUTPUT(A)
OUTPUT(B)
COMMIT
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Buffer pool DiskTransaction
READ(A,t); t := t*2; WRITE(A,t); READ(B,t); t := t*2; WRITE(B,t)
CSE 444 - Winter 2019
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t)
OUTPUT(A)
OUTPUT(B)
COMMIT
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Buffer pool DiskTransaction
READ(A,t); t := t*2; WRITE(A,t); READ(B,t); t := t*2; WRITE(B,t)
CSE 444 - Winter 2019
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
OUTPUT(A)
OUTPUT(B)
COMMIT
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Buffer pool DiskTransaction
READ(A,t); t := t*2; WRITE(A,t); READ(B,t); t := t*2; WRITE(B,t)
CSE 444 - Winter 2019
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
OUTPUT(A) 16 16 16 16 8
OUTPUT(B)
COMMIT
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Buffer pool DiskTransaction
READ(A,t); t := t*2; WRITE(A,t); READ(B,t); t := t*2; WRITE(B,t)
CSE 444 - Winter 2019
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16COMMIT
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Is this bad ?
CSE 444 - Winter 2019 23
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
COMMIT
Crash !
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Is this bad ? Yes it’s bad: A=16, B=8….
CSE 444 - Winter 2019 24
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
COMMIT
Crash !
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Is this bad ?
CSE 444 - Winter 2019 25
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
COMMIT
Crash !
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Is this bad ? Yes it’s bad: A=B=16, but not committed
CSE 444 - Winter 2019 26
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
COMMIT
Crash !
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Is this bad ?
CSE 444 - Winter 2019 27
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
COMMIT
Crash !
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Is this bad ? No: that’s OK
CSE 444 - Winter 2019 28
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
COMMIT
Crash !
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OUTPUT can also happen after COMMIT (details coming)
CSE 444 - Winter 2019 29
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
COMMIT
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
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OUTPUT can also happen after COMMIT (details coming)
CSE 444 - Winter 2019 30
Action t Mem A Mem B Disk A Disk BINPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
COMMIT
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
Crash !
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Atomic Transactions
• FORCE or NO-FORCE– Should all updates of a transaction be forced to
disk before the transaction commits?• STEAL or NO-STEAL
– Can an update made by an uncommitted transaction overwrite the most recent committed value of a data item on disk?
CSE 444 - Winter 2019 31
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Force/No-steal
• FORCE: Pages of committed transactions must be forced to disk before commit
• NO-STEAL: Pages of uncommitted transactions cannot be written to disk
CSE 444 - Winter 2019 32
Easy to implement (how?) and ensures atomicity
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No-Force/Steal
• NO-FORCE: Pages of committed
transactions need not be written to disk
• STEAL: Pages of uncommitted
transactions may be written to disk
CSE 444 - Winter 2019 33
In either case, need a Write Ahead Log (WAL)
to provide atomicity in face of failures
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Write-Ahead Log (WAL)The Log: append-only file containing log records• Records every single action of every TXN• Forces log entries to disk as needed• After a system crash, use log to recoverThree types: UNDO, REDO, UNDO-REDOAries: is an UNDO-REDO log
CSE 444 - Winter 2019
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Policies and Logs
CSE 444 - Winter 2019 35
NO-STEAL STEALFORCE Lab 3 Undo LogNO-FORCE Redo Log Undo-Redo Log
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UNDO Log
CSE 444 - Winter 2019 36
FORCE and STEAL
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Undo LoggingLog records• <START T>
– transaction T has begun• <COMMIT T>
– T has committed• <ABORT T>
– T has aborted• <T,X,v>
– T has updated element X, and its old value was v– Idempotent, physical log records
CSE 444 - Winter 2019
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Action t Mem A Mem B Disk A Disk B UNDO Log
<START T>
INPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8 <T,A,8>
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8 <T,B,8>
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
COMMIT <COMMIT T>CSE 444 - Winter 2019 38
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Action t Mem A Mem B Disk A Disk B UNDO Log
<START T>
INPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8 <T,A,8>
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8 <T,B,8>
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
COMMIT <COMMIT T>
Crash !
CSE 444 - Winter 2019 39WHAT DO WE DO ?
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40
Action t Mem A Mem B Disk A Disk B UNDO Log
<START T>
INPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8 <T,A,8>
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8 <T,B,8>
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
COMMIT <COMMIT T>
Crash !
CSE 444 - Winter 2019WHAT DO WE DO ? We UNDO by setting B=8 and A=8
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41
Action t Mem A Mem B Disk A Disk B UNDO Log
<START T>
INPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8 <T,A,8>
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8 <T,B,8>
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
COMMIT <COMMIT T>Crash !CSE 444 - Winter 2019What do we do now ?
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Action t Mem A Mem B Disk A Disk B UNDO Log
<START T>
INPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8 <T,A,8>
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8 <T,B,8>
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
COMMIT <COMMIT T>Crash !CSE 444 - Winter 2019 42What do we do now ? Nothing: log contains COMMIT
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Action t Mem A Mem B Disk A Disk B UNDO Log
<START T>
INPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8 <T,A,8>
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8 <T,B,8>
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
COMMITCSE 444 - Winter 2019 43
Crash !
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CSE 444 - Winter 2019 44
<START T><T,A,8><T,B,8>
Disk A Disk B8 16
After Crash
• This is all we see (for example):
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CSE 444 - Winter 2019 45
<START T>
<T,A,8><T,B,8>
Disk A Disk B8 16
After Crash
• This is all we see (for example):
• Need to step through the log
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CSE 444 - Winter 2019 46
<START T><T,A,8><T,B,8>
Disk A Disk B8 16
After Crash
• This is all we see (for example):• Need to step through the log
• What direction?
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47
<START T><T,A,8><T,B,8>
Disk A Disk B8 16
After Crash
• This is all we see (for example):• Need to step through the log
• What direction?• In UNDO log, we start at the most
recent and go backwards in time
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• This is all we see (for example):• Need to step through the log
• What direction?• In UNDO log, we start at the most
recent and go backwards in time 48
<START T><T,A,8><T,B,8>
Disk A Disk B8 16
After Crash
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• This is all we see (for example):• Need to step through the log
• What direction?• In UNDO log, we start at the most
recent and go backwards in time 49
<START T><T,A,8><T,B,8>
Disk A Disk B8 16
After Crash
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• This is all we see (for example):• Need to step through the log
• What direction?• In UNDO log, we start at the most
recent and go backwards in time 50
<START T><T,A,8><T,B,8>
Disk A Disk B8 8
After Crash
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• This is all we see (for example):• Need to step through the log
• What direction?• In UNDO log, we start at the most
recent and go backwards in time 51
<START T><T,A,8><T,B,8>
Disk A Disk B8 8
After Crash
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After Crash• If we see NO Commit statement:
– We UNDO both changes: A=8, B=8– The transaction is atomic, since none of its actions have
been executed
• In we see that T has a Commit statement– We don’t undo anything– The transaction is atomic, since both it’s actions have been
executed
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Recovery with Undo LogAfter system’s crash, run recovery manager
• Decide for each transaction T whether it is completed or not– <START T>….<COMMIT T>…. = yes– <START T>….<ABORT T>……. = yes– <START T>……………………… = no
• Undo all modifications by incompletetransactions
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Recovery with Undo Log
Recovery manager:• Read log from the end; cases:
<COMMIT T>: mark T as completed<ABORT T>: mark T as completed<T,X,v>: if T is not completed
then write X=v to diskelse ignore
<START T>: ignore
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Recovery with Undo Log……<T6,X6,v6>……<START T5><START T4><T1,X1,v1><T5,X5,v5><T4,X4,v4><COMMIT T5><T3,X3,v3><T2,X2,v2>
Question1: Which updatesare undone ?
Question 2:How far back do we need toread in the log ?
Question 3:What happens if second crash during recovery?
Crash !
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Recovery with Undo Log……<T6,X6,v6>……<START T5><START T4><T1,X1,v1><T5,X5,v5><T4,X4,v4><COMMIT T5><T3,X3,v3><T2,X2,v2>
Question1: Which updatesare undone ?
Question 2:How far back do we need toread in the log ?To the beginning.
Question 3:What happens if second crash during recovery?
Crash !
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Recovery with Undo Log……<T6,X6,v6>……<START T5><START T4><T1,X1,v1><T5,X5,v5><T4,X4,v4><COMMIT T5><T3,X3,v3><T2,X2,v2>
Question1: Which updatesare undone ?
Question 2:How far back do we need toread in the log ?To the beginning.
Question 3:What happens if second crash during recovery?No problem! Log records areidempotent. Can reapply.
Crash !
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Action t Mem A Mem B Disk A Disk B UNDO Log
<START T>
INPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8 <T,A,8>
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8 <T,B,8>
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
COMMIT <COMMIT T>
When mustwe force pagesto disk ?
58CSE 444 - Winter 2019
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59
Action t Mem A Mem B Disk A Disk B UNDO Log
<START T>
INPUT(A) 8 8 8
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8 <T,A,8>
INPUT(B) 16 16 8 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8 <T,B,8>
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
COMMIT <COMMIT T>FORCECSE 444 - Winter 2019
RULES: log entry before OUTPUT before COMMIT
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60
Undo-Logging Rules
U1: If T modifies X, then <T,X,v> must be written to disk before OUTPUT(X)
U2: If T commits, then OUTPUT(X) must be written to disk before <COMMIT T>
• Hence: OUTPUTs are done early, before the transaction commits
CSE 444 - Winter 2019
FORCE
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Checkpointing
Checkpoint the database periodically• Stop accepting new transactions• Wait until all current transactions complete• Flush log to disk• Write a <CKPT> log record, flush• Resume transactions
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Undo Recovery with Checkpointing
……<T9,X9,v9>……(all completed)<CKPT><START T2><START T3<START T5><START T4><T1,X1,v1><T5,X5,v5><T4,X4,v4><COMMIT T5><T3,X3,v3><T2,X2,v2>
During recovery,Can stop at first<CKPT>
transactions T2,T3,T4,T5
other transactions
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Nonquiescent Checkpointing
• Problem with checkpointing: database freezes during checkpoint
• Would like to checkpoint while database is operational
• Idea: nonquiescent checkpointing
Quiescent = being quiet, still, or at rest; inactiveNon-quiescent = allowing transactions to be active
CSE 444 - Winter 2019 63
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Nonquiescent Checkpointing
• Write a <START CKPT(T1,…,Tk)>where T1,…,Tk are all active transactions. Flush log to disk
• Continue normal operation
• When all of T1,…,Tk have completed, write <END CKPT>, flush log to disk
64
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Undo Recovery with Nonquiescent Checkpointing
………………<START CKPT T4, T5, T6>………<END CKPT>………
T4, T5, T6, pluslater transactions
earlier transactions plusT4, T5, T6
later transactions
65
Need to readBack to start ofT4, T5, T6
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Undo Recovery with Nonquiescent Checkpointing
………………<START CKPT T4, T5, T6>………<END CKPT>………
T4, T5, T6, pluslater transactions
earlier transactions plusT4, T5, T6
later transactionsQ: do we need <END CKPT> ? 66
Need to readBack to start ofT4, T5, T6
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Undo Recovery with Nonquiescent Checkpointing
………………<START CKPT T4, T5, T6>………<END CKPT>………
T4, T5, T6, pluslater transactions
earlier transactions plusT4, T5, T6
later transactionsQ: do we need <END CKPT> Not really, it’s implicit in seeing T4,T5,T6 commits
Need to readBack to start ofT4, T5, T6
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Implementing ROLLBACK• Recall: a transaction can end in COMMIT
or ROLLBACK• Idea: use the undo-log to implement
ROLLBACK• How ?
– LSN = Log Sequence Number– Log entries for the same transaction are
linked, using the LSN’s– Read log in reverse, using LSN pointers
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Implementing ROLLBACK• Recall: a transaction can end in COMMIT
or ROLLBACK• Idea: use the undo-log to implement
ROLLBACK• How ?
– LSN = Log Sequence Number– Log entries for the same transaction are
linked, using the LSN’s– Read log in reverse, using LSN pointers
CSE 444 - Winter 2019 69
……<T9,X9,v9>……(all completed)<CKPT><START T2><START T3<START T5><START T4><T1,X1,v1><T5,X5,v5><T2,X1,v2><T4,X4,v4><COMMIT T5><T3,X3,v3><T2,X2,v2>
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REDO Log
CSE 444 - Winter 2019 70
NO-FORCE and NO-STEAL
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71
Action t Mem A Mem B Disk A Disk B
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
COMMIT
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
CSE 444 - Winter 2019
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72
Action t Mem A Mem B Disk A Disk B
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
COMMIT
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
Is this bad ?
Crash !
CSE 444 - Winter 2019
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73
Action t Mem A Mem B Disk A Disk B
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
COMMIT
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
Is this bad ? Yes, it’s bad: A=16, B=8
Crash !
CSE 444 - Winter 2019
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74
Action t Mem A Mem B Disk A Disk B
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
COMMIT
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
Is this bad ?
Crash !
CSE 444 - Winter 2019
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Action t Mem A Mem B Disk A Disk B
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
COMMIT
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
Is this bad ?
Crash !
Yes, it’s bad: lost update
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Action t Mem A Mem B Disk A Disk B
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
COMMIT
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
Is this bad ?
Crash !
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Action t Mem A Mem B Disk A Disk B
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8
COMMIT
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
Is this bad ? No: that’s OK.
Crash !
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Redo Logging
One minor change to the undo log:
• <T,X,v>= T has updated element X, and its new value is v
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Action t Mem A Mem B Disk A Disk B REDO Log
<START T>
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8 <T,A,16>
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8 <T,B,16>
COMMIT <COMMIT T>
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
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Action t Mem A Mem B Disk A Disk B REDO Log
<START T>
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8 <T,A,16>
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8 <T,B,16>
COMMIT <COMMIT T>
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
How do we recover ?
Crash !
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Action t Mem A Mem B Disk A Disk B REDO Log
<START T>
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8 <T,A,16>
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8 <T,B,16>
COMMIT <COMMIT T>
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
How do we recover ?
Crash !
CSE 444 - Winter 2019We REDO by setting A=16 and B=16
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Recovery with Redo Log
After system’s crash, run recovery manager
• Step 1. Decide for each transaction T whether
it is committed or not
– <START T>….<COMMIT T>…. = yes
– <START T>….<ABORT T>……. = no
– <START T>……………………… = no
• Step 2. Read log from the beginning, redo all
updates of committed transactions
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Recovery with Redo Log
<START T1><T1,X1,v1><START T2><T2, X2, v2><START T3><T1,X3,v3><COMMIT T2><T3,X4,v4><T1,X5,v5>
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Show actionsduring recovery
Crash !
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Nonquiescent Checkpointing
• Write a <START CKPT(T1,…,Tk)>where T1,…,Tk are all active txn’s
• Flush to disk all blocks of committed transactions (dirty blocks)
• Meantime, continue normal operation• When all blocks have been written, write
<END CKPT>
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END CKPT has different meaning here than in Undo log
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Nonquiescent Checkpointing…<START T1>…<COMMIT T1>…<START T4>…<START CKPT T4, T5, T6>…………<END CKPT>………<START CKPT T9, T10>…
Step 1: look forThe last<END CKPT>
Step 2: redofrom theearlieststart ofT4, T5, T6ignoringtransactionscommittedearlier
All OUTPUTsof T1 areknown to be on disk
Cannotuse 85CSE 444 - Winter 2019
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Action t Mem A Mem B Disk A Disk B REDO Log
<START T>
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8 <T,A,16>
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8 <T,B,16>
COMMIT <COMMIT T>
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
When mustwe force pagesto disk ?
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Action t Mem A Mem B Disk A Disk B REDO Log
<START T>
READ(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8 <T,A,16>
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8 <T,B,16>
COMMIT <COMMIT T>
OUTPUT(A) 16 16 16 16 8
OUTPUT(B) 16 16 16 16 16
NO-STEAL
CSE 444 - Winter 2019RULE: OUTPUT after COMMIT
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Redo-Logging Rules
R1: If T modifies X, then both <T,X,v> and
<COMMIT T> must be written to disk before OUTPUT(X)
• Hence: OUTPUTs are done late
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NO-STEAL
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Comparison Undo/Redo• Undo logging:
– OUTPUT must be done early– If <COMMIT T> is seen, T definitely has written all its data to
disk (hence, don’t need to redo) – inefficient• Redo logging
– OUTPUT must be done late– If <COMMIT T> is not seen, T definitely has not written any
of its data to disk (hence there is not dirty data on disk, no need to undo) – inflexible
• Would like more flexibility on when to OUTPUT: undo/redo logging (next)
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Steal/Force
No-Steal/No-Force
Steal/No-Force
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Undo/Redo Logging
Log records, only one change• <T,X,u,v>= T has updated element X, its old value was u, and its new value is v
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Undo/Redo-Logging Rule
UR1: If T modifies X, then <T,X,u,v> must be written to disk before OUTPUT(X)
Note: we are free to OUTPUT early or late relative to <COMMIT T>
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Action T Mem A Mem B Disk A Disk B Log
<START T>
REAT(A,t) 8 8 8 8
t:=t*2 16 8 8 8
WRITE(A,t) 16 16 8 8 <T,A,8,16>
READ(B,t) 8 16 8 8 8
t:=t*2 16 16 8 8 8
WRITE(B,t) 16 16 16 8 8 <T,B,8,16>
OUTPUT(A) 16 16 16 16 8
<COMMIT T>
OUTPUT(B) 16 16 16 16 16
Can OUTPUT whenever we want: before/after COMMIT 93
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Recovery with Undo/Redo Log
After system’s crash, run recovery manager • Redo all committed transaction, top-down• Undo all uncommitted transactions, bottom-up
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Recovery with Undo/Redo Log
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<START T1>
<T1,X1,v1>
<START T2>
<T2, X2, v2>
<START T3>
<T1,X3,v3>
<COMMIT T2>
<T3,X4,v4>
<T1,X5,v5>
…
…
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ARIES
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Aries
• ARIES pieces together several techniques into a comprehensive algorithm
• Developed at IBM Almaden, by Mohan• IBM botched the patent, so everyone uses it now• Several variations, e.g. for distributed
transactions
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Log GranularityTwo basic types of log records for update operations• Physical log records
– Position on a particular page where update occurred– Both before and after image for undo/redo logs– Benefits: Idempotent & updates are fast to redo/undo
• Logical log records– Record only high-level information about the operation– Benefit: Smaller log– BUT difficult to implement because crashes can occur in
the middle of an operation
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ARIES Recovery Manager
Log entries:
• <START T> -- when T begins
• Update: <T,X,u,v>
– T updates X, old value=u, new value=v
– Logical description of the change
• <COMMIT T> or <ABORT T> then <END>
• <CLR> – we’ll talk about them later.
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ARIES Recovery Manager
Rule:
• If T modifies X, then <T,X,u,v> must be
written to disk before OUTPUT(X)
We are free to OUTPUT early or late w.r.t
commits
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LSN = Log Sequence Number• LSN = identifier of a log entry
– Log entries belonging to the same TXN are linked with extra entry for previous LSN
• Each page contains a pageLSN:– LSN of log record for latest update to that page
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ARIES Data Structures• Active Transactions Table
– Lists all active TXN’s
– For each TXN: lastLSN = its most recent update LSN
• Dirty Page Table– Lists all dirty pages
– For each dirty page: recoveryLSN (recLSN)= first LSN that caused page to become dirty
• Write Ahead Log– LSN, prevLSN = previous LSN for same txn
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ARIES Data Structures
pageID recLSNP5 102P6 103P7 101
LSN prevLSN transID pageID Log entry101 - T100 P7102 - T200 P5103 102 T200 P6104 101 T100 P5
Dirty pages Log (WAL)
transID lastLSNT100 104T200 103
Active transactionsP8 P2 . . .
. . .
P5PageLSN=104
P6PageLSN=103
P7PageLSN=101
Buffer Pool
WT100(P7)WT200(P5)WT200(P6)WT100(P5)
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ARIES Normal Operation
T writes page P• What do we do ?
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ARIES Normal Operation
T writes page P• What do we do ?
• Write <T,P,u,v> in the Log• pageLSN=LSN• prevLSN=lastLSN• lastLSN=LSN• recLSN=if isNull then LSN
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ARIES Normal Operation
Buffer manager wants to OUTPUT(P)
• What do we do ?
Buffer manager wants INPUT(P)
• What do we do ?
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ARIES Normal Operation
Buffer manager wants to OUTPUT(P)• Flush log up to pageLSN• Remove P from Dirty Pages tableBuffer manager wants INPUT(P)• What do we do ?
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ARIES Normal Operation
Buffer manager wants to OUTPUT(P)
• Flush log up to pageLSN• Remove P from Dirty Pages table
Buffer manager wants INPUT(P)
• Create entry in Dirty Pages table
recLSN = NULL
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ARIES Normal Operation
Transaction T starts• What do we do ?
Transaction T commits/aborts• What do we do ?
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ARIES Normal Operation
Transaction T starts• Write <START T> in the log• New entry T in Active TXN;
lastLSN = nullTransaction T commits• What do we do ?
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ARIES Normal Operation
Transaction T starts• Write <START T> in the log• New entry T in Active TXN;
lastLSN = nullTransaction T commits• Write <COMMIT T> in the log• Flush log up to this entry• Write <END>
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Checkpoints
Write into the log
• Entire active transactions table• Entire dirty pages table
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Recovery always starts by analyzing latest checkpoint
Background process periodically flushes dirty pages to disk
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ARIES Recovery1. Analysis pass
– Figure out what was going on at time of crash– List of dirty pages and active transactions
2. Redo pass (repeating history principle)– Redo all operations, even for transactions that will not commit– Get back to state at the moment of the crash
3. Undo pass– Remove effects of all uncommitted transactions– Log changes during undo in case of another crash during undo
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Announcements
• Lab 4 out tomorrow
• Lab 5 due dates extended– No late days allowed (will take that into
consideration when setting deadline)
• HW 6 released tomorrow– On parallel database concepts
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1. Analysis Phase• Goal
– Determine point in log where to start REDO– Determine set of dirty pages when crashed
• Conservative estimate of dirty pages– Identify active transactions when crashed
• Approach– Rebuild active transactions table and dirty pages table– Reprocess the log from the checkpoint
• Only update the two data structures– Compute: firstLSN = smallest of all recoveryLSN
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1. Analysis Phase(crash)Checkpoint
Dirtypages
Activetxn
Log
pageID recLSN
transID lastLSN
firstLSN= ??? Where do we startthe REDO phase ?
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1. Analysis Phase(crash)Checkpoint
Dirtypages
Activetxn
Log
pageID recLSN
transID lastLSN
firstLSN=min(recLSN)
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1. Analysis Phase(crash)Checkpoint
Dirtypages
Activetxn
Log
pageID recLSN
transID lastLSN
pageID recLSN
transID lastLSN
Replayhistory
firstLSN
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2. Redo Phase
Main principle: replay history
• Process Log forward, starting from
firstLSN• Read every log record, sequentially
• Redo actions are not recorded in the log
• Needs the Dirty Page Table
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2. Redo Phase: Details
For each Log entry record LSN: <T,P,u,v>• Redo the action P=u and WRITE(P)
• Only redo actions that need to be redone
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2. Redo Phase: DetailsFor each Log entry record LSN: <T,P,u,v>• If P is not in Dirty Page then no update• If recLSN > LSN, then no update• Read page from disk:
If pageLSN >= LSN, then no update• Otherwise perform update
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2. Redo Phase: DetailsWhat happens if system crashes during
REDO ?
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2. Redo Phase: Details
What happens if system crashes during
REDO ?
We REDO again ! The pageLSN will ensure
that we do not reapply a change twice
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3. Undo Phase
• Cannot “unplay” history, in the same way as we “replay” history
• WHY NOT ?
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3. Undo Phase
• Cannot “unplay” history, in the same
way as we “replay” history
• WHY NOT ?
– Undo only the loser transactions
– Need to support ROLLBACK: selective
undo, for one transaction
• Hence, logical undo v.s. physical redo
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3. Undo Phase
Main principle: “logical” undo• Start from end of Log, move backwards• Read only affected log entries• Undo actions are written in the Log as special
entries: CLR (Compensating Log Records)• CLRs are redone, but never undone
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3. Undo Phase: Details• “Loser transactions” = uncommitted
transactions in Active Transactions Table
• ToUndo = set of lastLSN of loser transactions
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3. Undo Phase: Details
While ToUndo not empty:• Choose most recent (largest) LSN in ToUndo• If LSN = regular record <T,P,u,v>:
– Undo v– Write a CLR where CLR.undoNextLSN = LSN.prevLSN
• If LSN = CLR record:– Don’t undo !
• if CLR.undoNextLSN not null, insert in ToUndootherwise, write <END> in log
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3. Undo Phase: DetailsWhat happens if system crashes during
UNDO ?
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3. Undo Phase: Details
What happens if system crashes during UNDO ?
We do not UNDO again ! Instead, each CLR is a REDO record: we simply redo the undo
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