Transaction processing

Book, chapter 6.6

Problem: With a single user….

you run a query, you get the results, you run the next, etc.

But database life is rarely that simple.

Multi user systems where users can be accessing data at the same time.

Protecting the database

• Recovery – restoring the database to a correct state after a failure

• Concurrency control – allow simultaneous use without unwanted interference

Passenger 1

Opens seat assignment

querySeat 13 C shows as available

Thinks

Takes a phone call

Select 13 Cupdate query

Passenger 2

Opens seat assignment

querySees Seat 13 C as available

Select 13Cupdate query

Who gets the seat?

Transaction

• “a group of operations that need to be performed together”

• Serializability – The property that a transaction with respect to other transactions must appear to be run in serial (one after another)…or in other words, one transaction completes before the next can begin.

In the prior case

• If Passenger 1 and 2 are both shown the seat, passenger 2 chooses it first, passenger 1 should not be allowed to complete his/her transaction.

• If passenger 1 is told, “I’m sorry, but that seat is now taken” and the choice is blocked, passenger 1 will need to make another choice and more importantly, the database is left in a consistent state.

Options

• We could block passenger 2 from seeing anything until passenger 1 is done (phone call)

• We could display both, but not let P2 make a choice until P1 is done

• We could let P2 make a choice and then alert P1 that their choice failed

• We could let P1’s choice go through and negate P2’s

Atomicity

• A transaction is considered an atomic statement – it is an all or nothing proposition.

• Another example

Banking example

• Customer transfers money

• Check account 1• Deduct from account 1• Credit account 2

• What if the last bit doesn’t go through?

Transaction

• One or more statements that must execute as a group.

• START TRANSACTION begins a group of statements

• COMMIT tells the system to make the transaction permanent (everything is ok)

• ROLLBACK tells the system to go back to the original state (problem)

Properties

• A – Atomic• C – Consistent• I – Isolated• D - Durable

Problems

• Lost update problem – See airline example

• Uncommitted update problem – One transaction “looks at” a change, but the change never goes through because of a problem; it’s result is based on wrong information.

• Inconsistent analysis – Query once , query a second time after data has changed

Problems

• Non repeatable read• 1 reads• 2 changes• 1 reads again and its different

• Phantom transaction• 1 reads• 2 inserts• 1 reads again and now new row

No problem

• 2 Transactions may read data at the same time

• 2 Transactions may operate on completely different data elements

• 2 Transactions operate on the same data set in serial time

Serial process

Transaction 1

Transaction 2

Transaction 3

Transaction 4

Interleaved process

Transaction 1A

Transaction 2A

Transaction 1B

Transaction 2B

Preventing concurrency issues

• Locking – halt operation of one transaction while the resource required is in use

• Timestamping – transactions are stamped with a start time and checked against the resource change time.