cs4432recovery1 cs4432: database systems ii database consistency and violations?
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Transactions, etc.
• Crash recovery Chapter 17• Concurrency control Chapter 18• Transaction processing Chapter 19
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Integrity or correctness of data ?
• We would like data in our database to be “accurate” ( “correct” ) at all times.
EMP
• How DBMS decides if data is consistent?
Name
WhiteGreenGray
Age
523421
1
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Integrity or consistency constraints• Utilize predicates data must
satisfy• Examples:
- x is key of relation R- x y holds in R- Domain(x) = {Red, Blue, Green}- no employee should make more than
twice the average salary
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Definitions:
• Consistent state: satisfies all constraints
• Consistent DB: DB in consistent state
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Such constraints may not capture “full
correctness”
Example 1 : Transaction constraints• When salary is updated,
new salary > old salary• When account record is deleted,
balance = 0
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Example 2 Database should reflect real world
DBReality
Constraints (as we use here) may not capture “full
correctness”
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in any case, continue with constraints ...
Observation: DB cannot be consistent always
Example: Constraint : a1 + a2 +…. an = TOT Action:
Deposit $100 in a2: a2 a2 + 100TOT TOT + 100
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a2
TOT
.
.50
.
.1000
.
.150
.
.1000
.
.150
.
.1100
Example: a1 + a2 +…. an = TOT (constraint)Deposit $100 in a2: a2 a2 + 100
TOT TOT + 100
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Transaction: a collection of actions
that preserve consistency
Consistent DB Consistent DB’T
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Big assumption:
If T starts with consistent state ANDT executes in isolation
T leaves consistent state
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Correctness (informally)
• If we stop running transaction(s),DB left consistent
• Each transaction sees a consistent DB
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How can constraints be violated?
• Transaction bug• DBMS bug• Hardware failure
e.g., disk crash alters balance of account
• Data sharinge.g.: T1: give 10% raise to programmers
T2: change programmers systems analysts
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Will not consider:
• How to write correct transactions• How to write correct DBMS system• Constraint checking & repair
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How can we prevent/fix violations?
• Chapter 17: due to failures only• Chapter 18: due to data sharing only• Chapter 19: due to failures and
sharing
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Desired events: see product manuals….
Undesired expected events:System crash
- memory lost- cpu halts, resets
Undesired Unexpected: Everything else!
that’s it!!
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Examples:• Disk data is lost• Memory lost without CPU halt• CPU implodes wiping out
universe….• You name it …
Undesired Unexpected: Everything else!
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Operations reStorage Hierarchy:• Input (x): block containing x memory• Output (x): block containing x disk
• Read (x,t): do input(x) if necessary t value of x in
block• Write (x,t): do input(x) if necessary
value of x in block t
Memory Disk
x x
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T1: Read (A,t); t t2Write (A,t);Read (B,t); t t2Write (B,t);Output-to-Disk (A);Output-to-Disk (B);
A: 8B: 8
A: 8B: 8
memory disk
1616
16
failure!
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One solution: undo logging (immediate
modification)
A la Hansel and Gretel recording their navigation through forest via bread crumbs …
Must have durable undo logging !!!
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T1: Read (A,t); t t2 A=B
Write (A,t);Read (B,t); t t2Write (B,t);Output-to-disk (A);Output-to-disk (B);
A:8B:8
A:8B:8
memory disk log
Undo logging (Immediate modification)
1616
<T1, start><T1, A, 8>
<T1, commit>16 <T1, B, 8>
16
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One “complication” : First disk, then log• Log is first written in memory• Not written to disk on every action
memory
DB
Log
A: 8 16B: 8 16Log:<T1,start><T1, A, 8><T1, B, 8>
A: 8B: 8
16BAD STATE
# 1
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One “complication” : first log, then disk.• Log is first written in memory• Not written to disk on every action
memory
DB
Log
A: 8 16B: 8 16Log:<T1,start><T1, A, 8><T1, B, 8><T1, commit>
A: 8B: 8
16BAD STATE
# 2
<T1, B, 8><T1, commit>
...
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Undo logging rules
(1)For every action generate undo log record (containing old value)
(2) Before x is modified on disk, log records pertaining to x must be
on disk (write ahead logging)
(3) Before commit is written to log on disk, all writes of transaction must be
reflected on disk
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Recovery rules: Undo logging
(1) Let S = set of transactions with<Ti, start> in log, but no<Ti, commit> (or <Ti, abort>) record
in log(2) For each <Ti, X, v> in log,
in reverse order (latest earliest) do: - write old value from log back to disk:
- if Ti S then - write (X, v) - output (X)
(3) For each Ti S do- write <Ti, abort> to log
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Redo logging (deferred modification)
T1: Read(A,t); t t2; write (A,t); Read(B,t); t t2; write (B,t);
Output(A); Output(B)
A: 8B: 8
A: 8B: 8
memory DB LOG
1616
<T1, start><T1, A, 16><T1, B, 16>
<T1, commit>
output16
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Redo logging rules
(1) For every action, generate redo logrecord (containing new value)
(2) Before X is modified on disk (DB),all log records for transaction that
modified X (including commit) must be on disk
(3) Flush log at commit
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(1) Let S = set of transactions with<Ti, commit> in log
(2) For each <Ti, X, v> in log, in forward order (earliest latest) do:
- if Ti S then Write(X, v) Output(X)
optional
Recovery rules: Redo logging
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Recovery is very, very
SLOW !Redo log:
First T1 wrote A,B LastRecord Committed a year ago
Record(1 year ago) --> STILL, Need to redo after crash!!
... ... ...
Crash
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Solution: Checkpoint (simple version)
Periodically:(1) Do not accept new transactions(2) Wait until all transactions finish(3) Flush all log records to disk (log)(4) Flush all buffers to disk (DB) (do not discard buffers)
(5) Write “checkpoint” record on disk (log)
(6) Resume transaction processing
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Example: what to do at recovery?
Redo log (disk):
<T1
,A,1
6>
<T1
,com
mit
>
Ch
eck
poin
t
<T2
,B,1
7>
<T2
,com
mit
>
<T3
,C,2
1>
Crash... ... ... ...
...
...
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Key drawbacks:
• Undo logging: Too many disk IOs• Redo logging: need to keep all
modified blocks in memory
until commit
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Solution: undo/redo logging!
Combine Undo/Redo Logging, namely:
Update <Ti, Xid, New X val, Old X val>page X
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Rules
• Page X can be flushed before orafter Ti commit
• Log record flushed before corresponding updated page (WAL)
• Flush at commit (log only)
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Non-quiescent checkpoint
LOG
for undo dirty buffer
pool pagesflushed
Start-ckptactive TR:
Ti,T2,...
endckpt
.........
...
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Examples what to do at recovery time?
no T1 commit
LOG
T1,-a
...CkptT1
...Ckptend
...T1-b
...
Undo T1 (undo a,b)
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Example
LOG
...T1
a... ...
T1
b... ...
T1
c...
T1
cmt...
ckpt-end
ckpt-sT1
Redo T1: (redo b,c)
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Recovery process:• Backwards pass (end of log latest checkpoint
start)
– construct set S of committed transactions– undo actions of transactions not in S
• Undo pending transactions– follow undo chains for transactions in
(checkpoint active list) - S
• Forward pass (latest checkpoint start end of log)
– redo actions of S transactions
backward pass
forward passstart
check-point