G4G0-1/Semester 2/Database Systems/Week 8/Week 8 Database Systems.md

Undoing & Redoing Transactions

Deferred Update Protocol

Updates not written until after transaction has reached commit point. If transaction fails before committing, no modification is made to database, hence no undoing is required. May be necessary to redo updates of committed transactions as effect may not have reached database.

Procedure

Redo transaction is both <T1 start> and <T1 commit> are present in the log

Example

Failure in different places: In case(a), no commit is present, so no redo or undo is required. case(b), T0 commits, but T1 does not, hence T0 is redone. case(b), T0 and T1 commits, so both are redone.

Immediate Update Protocol

Updates applied as they occur. Following a failure, redo updates of committed transactions. Undo effects of transactions uncommitted. If no commit record, transacation was active and must be undone. Undo operations performed in reverse order in log. In recovery, use log to undo or redo.

Protocol

Undo

• If <Ti,start> is in the log but <Ti,commit> is not then restore the value of all data items updated by Ti to their old values, going backwards from the last log record for Ti. Redo
• If <Ti,start> and <Ti,commit> are both in the log then set the value of all data items updated by Ti to the new values, going forward from the first log record for Ti

Example

Checkpoints

After failure, may not know how far back to search log. Checkpoints limit log searching, made automatically by DBMS Creation of checkpoints schedules at predetermined intervals. Definition: Checkpoints are points of sync between database and log file. All buffers are written to secondary storage.

Procedure

1. Write to secondary storage
   • All log records
   • All modified buffer blocks to database.
   • Checkpoint record to log file which contains identities of transactions active at time of checkpoint
2. Resume processing.

• 

Example 1 ( Immediate )

• Starts at t0, fails at tf.
• T1 and T6 have to be undone
• In absence of other info, recovery manager has to redo T2, 3, 4, and 5.
• 

Example 2 ( serial with checkpoint, Immediate )

• Dotted lines = checkpoints
• T1 is safe since updates are written to disk
• T4 needs to be undone
• T2 and 3 need to be redone
• 

Example 3 ( concurrent with checkpoint, Immediate )

• T2 and 3 safe since written to disk
• T1 and 6 need to be undone.
• Since checkpoint, redo T4 and 5.
• 

Purpose of Recovery Log File

• Scan log backwards
• Create undo / redo lists
• Undo List: Transactions active at time of crash
  • Perform undo(T) for every transaction in list
  • Stop when reaching <T, start> for every transaction in list.
• Redo List: Transactions committed after last checkpoint
  • Redo for each transaction in list.

Example of Log Use ( Immediate )

Shadow Paging

Alternative technique for providing atomicity and durability. DBMS maintains two page tables during the life of a transaction.

1. Current Page
2. Shadow Page Table

Transaction Start: Two pages are the same

• Shadow page table is never changed, only used to restore database. Transaction Execution: current page table records all updates to database Transaction End: Current page table becomes shadow page table and all modified pages from database buffers written to secondary storage. Transaction Failure: new pages ignored, shadow page table becomes current page table.

Tutorial

1. State whether each of the following is true or false. If it is false then briefly explain why.
   1. True
   2. True
   3. True
   4. False: REDO / NO UNDO
   5. True
2. The DBMS maintains a log file containing transaction records that identify the start and end of transactions and the before and after images of the write operations. Consider the log files shown in Tables 32.1 and 32.2. For each one, explain what the DBMS recovery system would do, and why, if there was a failure at the time shown below each table and the DBMS was using:
   1. Deferred update protocol
      1. 32.1 Fail @10:24
         • Redo T1 as commit before failure. Ignore T2 and 3, active at point of failure. T4 not active at point of failure, Ignore.
      2. 32.2 Fail @9.21
         • Redo T2 and 4, since no commit occurs. T3 commits after the checkpoint, and anything before has no effect.
   2. Immediate update protocol
      1. 32.1 Fail @10.24
         • Undo T2 and 3, since they are active, and do not commit. T1 must be redone, and T4 is ignored, as it never started.
      2. 32.2 Fail @9:21
         • Undo T2 and 4, since they are active and do not commit. Redo T3, as it is active after the checkpoint.

Workshop

1. 
2. 
3. 

SELECT SUM([UnitPrice]) AS TotalPrice
FROM [Products] 
WHERE [SupplierID] = (
	SELECT [SupplierID] 
	FROM [Suppliers]
	WHERE [Country] = 'UK'
	);
	

4. 1.