DBMS

Transaction Concepts 

A transaction is a unit of program execution that accesses and possibly updates various data items.

E.g. transaction to transfer $50 from account A to account B:
 

1. read(A)

2. A := A – 50

3. write(A)

4. read(B)

5. B := B + 50

6. write(B)

 

Two main issues to deal with:

• Failures of various kinds, such as hardware failures and system crashes
• Concurrent execution of multiple transactions

 

Example of Fund Transfer
 

Transaction to transfer $50 from account A to account B:
 

1. read(A)

2. A := A – 50

3. write(A)

4. read(B)

5. B := B + 50

6. write(B)

 

Atomicity requirement

if the transaction fails after step 3 and before step 6, money will be "lost" leading to an inconsistent database state

•  Failure could be due to software or hardware
   
•  The system should ensure that updates of a partially executed transaction are not reflected in the database

 

Durability requirement

once the user has been notified that the transaction has completed (i.e., the transfer of the $50 has taken place), the updates to the database by the transaction must persist even if there are software or hardware failure

 

Consistency requirement

• The sum of A and B is unchanged by the execution of the transaction
• In general, consistency requirements include
  • Explicitly specified integrity constraints such as primary keys and foreign keys
  • Implicit integrity constraint  e.g. sum of balances of all accounts, minus sum of loan amounts must equal value of cash-in-hand
• A transaction must see a consistent database.
• During transaction execution the database may be temporarily inconsistent.
• When the transaction completes successfully the database must be consistent erroneous transaction logic can lead to inconsistency

 

Isolation requirement 

• if between steps 3 and 6, another transaction T2 is allowed to access the partially updated database, it will see an inconsistent database (the sum A + B will be less than it should be).
  

 

T1	T2
1. read(A)
2. A := A – 50
3. write(A)
	read(A), read(B), print(A+B)
4. read(B)
5.  B := B + 50
6.  write(B)

• Isolation can be ensured trivially by running transactions serially that is, one after the other

 

ACID Properties

 

A transaction is a unit of program execution that accesses and possibly updates various data items.To preserve the integrity of data the database system must ensure

1. Atomicity- Either all operations of the transaction are properly reflected in the database or none are.
2. Consistency -Execution of a transaction in isolation preserves the consistency of the database.
3. Isolation- Although multiple transactions may execute concurrently, each transaction must be unaware of other concurrently executing transactions.Intermediate transaction results must be hidden from other concurrently executed transactions.That is, for every pair of transactions Ti and Tj, it appears to Ti that either Tj, finished execution before Ti started, or Tj started execution after Ti finished.
4. Durability. After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures

 

Transaction State

• Active – the initial state; the transaction stays in this state while it is executing
• Partially committed – after the final statement has been executed.
• Failed -- after the discovery that normal execution can no longer proceed.
• Aborted – after the transaction has been rolled back and the database restored to its state prior to the start of the transaction. Two options after it has been aborted:
  • restart the transaction
  • can be done only if no internal logical error
  • kill the transaction
• Committed – after successful completion