DBMS-DATABASE TRANSACTION MANAGEMENT AS PER SPPU

 Transaction Concept:

 A transaction is a unit of program execution that accesses and possibly updates various detail terms.

•  A transaction must see a consistent database.
•  During transaction execution, the database may be inconsistent.
•  When the transaction is committed, the database must be consistent.

 Two main issues to deal with:

-Failures of various kinds, such as hardware failures and system crashes

- Concurrent execution of multiple transactions.

ACID Properties

 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 between 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.

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)

 Consistency requirement – the sum of A and B is unchanged by the  execution of the transaction.

 Atomicity requirement — if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency will result.

 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 despite failures.

 Isolation requirement — if between steps 3 and 6, another transaction 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). Can be ensured trivially by running transactions serially, that is one after the other. However, executing multiple transactions concurrently has significant benefits, as we will see.

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 – only if no internal logical error

- kill the transaction

 Committed: after successful completion.

Concurrent Executions:

 Multiple transactions are allowed to run concurrently in the system. Advantages are:  increased processor and disk utilization, leading to better transaction throughput: one transaction can be using the CPU while another is reading from or writing to the disk  reduced average response time for transactions: short transactions need not wait behind long ones. 

 Concurrency control schemes – mechanisms to achieve isolation, i.e., to control the interaction among the concurrent transactions in order to prevent them from destroying the consistency of the database

Schedules:

 Schedules – sequences that indicate the chronological order in which instructions of concurrent transactions are executed 

-a schedule for a set of transactions must consist of all instructions of those transactions.

-must preserve the order in which the instructions appear in each individual transaction.