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Properties of an undo/redo-log

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Slideshow:

Important property imposed by the UNDO/REDO log write rule
 
Important property of the UNDO/REDO log for incomplete (= uncommited) transactions
   
I.e.:   the UNDO/REDO log has the property of an UNDO (only) log (pretty obvious)
Important property of the UNDO/REDO log for committed transactions
Prelude - some facts about the (UNDO/REDO) log file:
 
Consequence:

Important property of the UNDO/REDO log for committed transactions
Consequence:
 
Example:  

Important property of the UNDO/REDO log for committed transactions
Consequence:
 
Summary of finding of properties of the UNDO/REDO log
 
I.e.:   the UNDO/REDO log can provide atomic execution of incomplete transactions by UNDO-ing all its updates
 
 

I.e.:   the UNDO/REDO log can provide atomic execution of committed transactions by REDO-ing all its updates
The recovery procedure of the UNDO/REDO log
 
The general algorithm to recover from a system failure using logging is:
 
 

Comparing the recovery procedure of UNDO/REDO logging with UNDO (only) logging
 
I.e.:   we sacrificed some flexibility and gain some efficiency (= omit the REDO step !) by incorporating updated made by committed transactions !!!
Comparing the recovery procedure of UNDO/REDO logging with REDO (only) logging
 
I.e.:   we sacrificed some flexibility and gain some efficiency (= omit the UNDO step !) by disallowing disk write operations made by uncommitted transactions !!!
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• Property of the uncommitted transactions in undo/redo logging
  • Due the undo/redo write rule:

    Write the log record: < T, X, v, w > to disk before writing the DB element X to disk (= OUTPUT(X))

    The undo/redo-log file will contain undo information to undo any updates made by uncommitted transactions:

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  • Property of the uncommitted transactions using undo/redo logging:

    Updates to disk made by uncommitted transactions can always be undone in undo/redo logging

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• Property of the committed transactions in undo/redo logging

  • Ordering of records in a log file:

    Because: Order in which the log records for transaction T is created is: <START T> <T, A, vA, wA> <T, B, vB, wB> ... <COMMIT T> A log file is written sequential a log file has the following important property: If the <COMMIT T> log record has been written to disk            then we must also have written the all undo/redo log records of the transaction T to disk: < T, A, 1000, 1100 > <---- (2) We must have written all the < T, B, 2000, 1900 > <---- undo/redo log records too !!! .... < COMMIT T > <---- (1) If < COMMIT T > is written

    Example:

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    Therefore:

    When we see a <COMMIT T> log record on disk (= transaction T is committed): The log file will certainly (= for sure !!!) contain all the redo log records to redo the updates made by a committed transaction T !!!

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  • Property of the committed transactions using undo/redo logging:

    Updates to disk made by committed transactions can always be redone in undo/redo logging

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• Algorithm for recovery in undo/redo logging
  • The general algorithm to recover from a system failure:

    Find all the uncommitted transactions Find all the committed transactions Undo all updates made by the uncommitted transactions Redo all updates made by the committed transactions

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  • In undo logging:

    We do not need to redo the updates made by the committed transactions (Because the updates made by committed transactions in undo-logging are written to disk before the <COMMIT> record can be written !!!)

    And the recovery procedure for undo logging was simplified to:

    Find all the uncommitted transactions Undo all updates made by the uncommitted transactions

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  • In redo logging:

    We do not need to undo the updates (on disk) made by the uncommitted transactions (Because the updates made by uncommitted transactions in redo-logging are never written to disk --- redo-log write rule !!!)

    And the recovery procedure for redo logging was simplified to:

    Find all the committed transactions Redo all updates made by the committed transactions

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  • In undo/redo logging:

    We must use the general recovery algorithm