Recovery

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Recovery
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Integrity or correctness of data

• Would like data to be accurate or correct at
  all times
• EMP

Name
Age
White Green Gray
52 3421 1
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Integrity or consistency constraints

• Predicates data must satisfy
• Examples
• - x is key of relation R
• - x ? y holds in R
• - Domain(x) Red, Blue, Green
• - a is valid index for attribute x of R
• - no employee should make more than twice the
  average salary

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Definition

• Consistent state satisfies all constraints
• Consistent DB DB in consistent state

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• Observation DB cannot be consistent always!
• Example a1 a2 . an TOT (constraint)
• Deposit 100 in a2 a2 ? a2 100
• TOT ? TOT 100

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Example a1 a2 . an TOT (constraint) Deposi
t 100 in a2 a2 ? a2 100 TOT ?
TOT 100

• a2
• TOT

. .
. .
. .
50
150
150
. .
. .
. .
1000
1000
1100
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Transaction collection of actions that
preserve consistency

Consistent DB
Consistent DB
T
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How can we prevent/fix violations?

• cc due to data sharing only
• recovery due to failures only

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Will not consider

• How to write correct transactions
• How to write correct DBMS
• Constraint checking repair
• That is, solutions studied here do not need to
  know constraints

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• Storage hierarchy

x
x
Memory Disk
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Operations

• Input (x) block with x ? memory
• Output (x) block with 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
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Key problem Unfinished transaction

• Example Constraint AB
• T1 A ? A ? 2
• B ? B ? 2

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• T1 Read (A,t) t ? t?2
• Write (A,t)
• Read (B,t) t ? t?2
• Write (B,t)
• Output (A)
• Output (B)

failure!
A 8 B 8
A 8 B 8
16
memory
disk
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Undo logging

• T1 Read (A,t) t ? t?2 AB
• Write (A,t)
• Read (B,t) t ? t?2
• Write (B,t)
• Output (A)
• Output (B)

A8 B8
A8 B8
ltT1, B, 8gt
ltT1, commitgt
disk
memory
log
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One complication

• Log is first written in memory
• Not written to disk on every action
• memory
• DB
• Log

A 8 B 8
A 8 16 B 8 16 Log ltT1,startgt ltT1, A, 8gt ltT1,
B, 8gt
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One complication

• Log is first written in memory
• Not written to disk on every action
• memory
• DB
• Log

A 8 B 8
A 8 16 B 8 16 Log ltT1,startgt ltT1, A, 8gt ltT1,
B, 8gt ltT1, commitgt
...
ltT1, B, 8gt ltT1, commitgt
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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 WAL)
• (3) Before commit is flushed to log, all writes
  of transaction must be reflected on disk

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Recovery rules Undo logging

• (1) Let S set of transactions with ltTi, startgt
  in log, but no
• ltTi, commitgt (or ltTi, abortgt) record in log
• (2) For each ltTi, X, vgt in log,
• in reverse order (latest ? earliest) do
• - if Ti ? S then - write (X, v)
• - output (X)
• (3) For each Ti ? S do
• - write ltTi, abortgt to log

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• What if failure during recovery?
• No problem! ? Undo idempotent

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To discuss

• Redo logging
• Undo/redo logging, why both?
• Checkpoints

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Redo logging (deferred modification)

• T1 Read(A,t) t t?2 write (A,t)
• Read(B,t) t t?2 write (B,t)
• Output(A) Output(B)

A 8 B 8
A 8 B 8
DB
memory
LOG
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Redo logging rules

• (1) For every action, generate redo log record
  (containing new value)
• (2) Before anything is modified on disk (DB), all
  log records for transaction that modify things
  (including commit) must be on disk

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Recovery rules Redo logging

• (1) Let S set of transactions with ltTi,
  commitgt in log
• (2) For each ltTi, X, vgt in log, in forward
• order (earliest ? latest) do
• - if Ti ? S then Write(X, v)
• Output(X)

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Recovery is very, very SLOW !

• Redo log
• First T1 wrote A,B Last
• Record Committed a year ago Record
• (1 year ago) --gt STILL, Need to redo after crash!!

...
...
...
Crash
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Undo Recovery Better?

• The first record without commit or abort should
  not be too far away from crash
• But immediate update itself is expensive

...
...
...
first record without commit or abort
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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)

Crash
...
...
...
...
...
...
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Example what to do at recovery?

• Undo log (disk)

Crash
...
...
...
...
...
...
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Nonquiescent Checkingpointing

• Quiescent checkpointing stalls DB
• Nonquiescent checkpointing admits new
  transactions while checkpointing

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Nonquiescent Checkpoint Rules - undo

• Write and flush a log record ltSTART CKPT(T1, ,
  Tk)gt where T1,,Tk are active transactions
• When all T1, , Tk have completed, write and
  flush a log record ltEND CKPTgt

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Example what to do at recovery?

• Undo log (disk)

Crash
ltEND CKPTgt
ltT2,B,12gt
ltT2,commitgt
...
...
...
...
Crash after end of checkpoint Only need to undo
all the incomplete transactions started after
ltSTART CKPTgt
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Example what to do at recovery?

• Undo log (disk)

Crash
ltT2,B,12gt
ltT2,commitgt
...
...
...
...

Crash before end of checkpoint Only need to undo
all incomplete transactions started after ltSTART
CKPTgt and those in ltCKPT ()gt
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Nonquiescent Checkpoint Rules - redo

• Write and flush a log record ltSTART CKPT(T1, ,
  Tk) where T1,,Tk are active transactions
• Flush all the updates of the transactions
  committed before ltSTART CKPT(T1,,Tk)gt
• Write and flush a log record ltEND CKPTgt

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Example what to do at recovery?

• Redo log (disk)

Crash
ltEND CKPTgt
ltT2,B,12gt
ltT2,commitgt
...
...
...
...
Crash after end of checkpoint Only need to redo
all the transactions committed after the latest
ltSTART CKPTgt
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Example what to do at recovery?

• Redo log (disk)

Crash
ltT2,B,12gt
ltT2,commitgt
...
...
...
...

Crash before end of checkpoint Only need to redo
all the transactions committed after the latest
successful ltSTART CKPTgt
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Key drawbacks

• Undo logging need to update immediately,
  increasing I/O
• Redo logging need to keep all modified blocks in
  memory until commit

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Solution undo/redo logging!

• Update ? ltTi, Xid, New X val, Old X valgt

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Rules

• Page X can be flushed before or after Ti commit
• Log record flushed before corresponding updated
  page (WAL)
• Flush log at commit

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Non-quiesce checkpoint

• L
• O
• G
• for
• undo

Start-ckpt active TR Ti,T2,...
end ckpt
...
...
...
Flush dirty buffers
...
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Examples what to do at recovery time?

• no T1 commit
• L
• O
• G

T1,- a
...
Ckpt T1
...
Ckpt end
...
T1- b
...
? Undo T1 (undo a,b)
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Example

• L
• O
• G

...
T1 a
...
...
T1 b
...
...
T1 c
...
T1 cmt
...
Ckpt end
ckpt-s T1
? Redo T1 (redo b,c)
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Examples what to do at recovery time?

• no T1 commit
• L
• O
• G

T1,- a
...
Ckpt T1
...
...
T1- b
...
? Undo T1 (all the actions)
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Example

• L
• O
• G

...
T1 a
...
...
T1 b
...
...
T1 c
...
T1 cmt
...
ckpt-s T1
? Redo T1 (redo b,c, a and the actions after
last successful ckp-s)
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Recovery process

• Backwards pass (end of log -gt 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 successful checkpoint start
  -gt end of log)
• redo actions of S transactions

backward pass
start check- point
forward pass
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Recovery with Checkpoint Summary

• Logging still obey the logging rules
• undo, redo, undo-redo
• Recovery still obey rules
• Undo undo all incomplete transactions
• Redo redo all completed transactions
• Undo-redo combined
• Checkpoint provides a way to limit the
  transactions needing undo or redo

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Summary
WAL
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Summary

• Consistency of data
• One source of problems failures
• - WAL