Database Management Systems (2130703)

Log based recovery

• The most widely used structure for recording database modification is the log.
• The log is a sequence of log records, recording all the update activities in the database.
• In short Transaction log is a journal or simply a data file, which contains history of all transaction performed and maintained on stable storage.
• Since the log contains a complete record of all database activity, the volume of data stored in the log may become unreasonable large.
• For log records to be useful for recovery from system and disk failures, the log must reside on stable storage.
• Log contains
  1. Start of transaction
  2. Transaction-id
  3. Record-id
  4. Type of operation (insert, update, delete)
  5. Old value, new value
  6. End of transaction that is committed or aborted.
• All such files are maintained by DBMS itself. Normally these are sequential files.
• Recovery has two factors Rollback (Undo) and Roll forward (Redo).
• When transaction T_i starts, it registers itself by writing a < T_i start >log record
• • Before T_i executes write(X), a log record < T_i, X, V₁, V₂> is written, where V₁ is the value of X before the write, and V₂ is the value to be written to X.
  • Log record notes that T_i has performed a write on data item X_j
  • X_j had value V₁ before the write, and will have value V₂ after the write.
• When T_i finishes it last statement, the log record < T_i commit> is written.

Immediate Database Modification

Concept

• Updates (changes) to the database are applied immediately as they occur without waiting to reach to the commit point.
• Also, these updates are recorded in the transaction log.
• It is possible here that updates of the uncommitted transaction are also written to the database. And, other transactions can access these updated values.

When failure occurs

• If transaction has not committed, then it may have modified the database. And so, undo the updates of the transaction.
• If transaction has committed, then still it may not have modified the database. And so, redo the updates of the transaction.