https://moodle.cs.ubbcluj.ro/pluginfile.php/50138/mod_resource/content/1/DBMSs_Lecture4.pdf

https://moodle.cs.ubbcluj.ro/pluginfile.php/50141/mod_resource/content/1/DBMSs_Lecture5.pdf

Recovery Manager

• ensures:

  • atomicity - effects of uncommitted transactions are undone
  • durability - effects of committed transactions survive system crashes

  

  • the system comes back up:
    • the effects of T1 & T2 must persist
    • T3 & T4 are undone (their effects are not persisted in the DB)

Transaction Failure

Causes

• system failure (hardware failures, bugs in the operating system, database system, etc.)
  • all running transactions terminate
  • contents of internal memory - affected (i.e. lost)
  • contents of external memory - not affected
• application error (“bug”, e.g. division by 0, infinite loop, etc.) ⇒ transaction fails; it should be executed again only after the error is corrected
• action by the Transaction Manager (TM)
• e.g. deadlock resolution scheme
  • a transaction is chosen as the deadlock victim and terminated
  • the transaction might complete successfully if executed again
• self-abort
  • based on some computations, a transaction can decide to terminate and undo its actions
  • there are special statements for this purpose, e.g. ABORT, ROLLBACK
  • can be seen as a special case of action by the TM

Normal Execution

• during normal execution, transactions read / write database objects
• reading database object O:
  • bring O from disk into a frame in the Buffer Pool (BP)
  • copy O’s value into a program variable
• writing database object O:
  • modify an in-memory copy of O in the Buffer Pool (BP)
  • write the in-memory copy to disk

Buffer Manager

Writing Objects

Transaction T changes object O (in frame F in the BP)