Transactions 1

Concurrency controlusing transactions

Transactions 2

Transaction, the concept

• Logical unit of work on the database– Examples• Transfer money from one bank account to another • Reserve a seat on a flight

• Transaction boundaries are defined by the database user / application programmer– That’s us!

Transactions 3

Interleaved processingversus parallel processing

Transactions 4

Transaction operations• Reading data

– Read_item(X)• Reads data X from the database

1. Find the address of the disk block containing X2. Copy the disk block from hard disk to buffer in main memory3. Copy data from the buffer to program variable X

• Writing data– Write_item(X)

• Writes data X to the database1. Find the address of the disk block containing X2. Copy the disk block from hard disk to buffer in main memory3. Copy item X from program variable to buffer4. Store the updated buffer on hard disk

• SELECT = Read_item(X)• INSERT = Write_item(X)• UPDATE = {Read_item(X)} + Write_item(X)

Transactions 5

Example transactions

a) Transferring N kroner from account X to account Y

b) Updating account X, adding M kroner

Transactions 6

Concurrency problems: Lost update + dirty read

Transactions 7

Concurrency problems:Incorrect analysis

Transactions 8

Concurrency problems:Unrepeatable reads

• A transaction reads the same data item twice, with different results!

• Example: Ticket reservation1. The seat is free2. After at little while, your decide to buy the ticket3. But now the seat is not longer free• Someone else bought it while you are ”thinking”.

Transactions 9

Transaction states• Transactions can end in two ways– COMMIT_TRANSACTION• Successful end of transaction

– ROLLBACK / ABORT• Unsuccessful end of transaction

Transactions 10

ACID properties of a transaction

• Atomicity– A transaction is an atomic unit of processing: It should either be

performed in its entirety OR not performed at all.– Rollback means rollback the transaction as if it never happened.

• Consistency– A transaction takes the database from one consistent state to

another• Isolation

– Transactions should be executed in isolation from other transactions

• Durability– The results of a committed transaction can never be undone

Transactions 11

Schedules of transactions• Schedule, a definition

– A schedule of a set of transactions is an ordering of the operations of the transactions

• Examples, fig. 20.3– Sa: r1(X); r2(X); w1(X);r1(Y); w2(X); w1(Y)

• T1 reads X, T2 reads X, T1 writes X, etc.

– Sb: r1(X); w1(X); r2(X); w2(X); r1(Y); abort T1

• Serial schedule– No concurrency permitted

• T1 finishes before T2 starts, or vice versa

– Not good: Too much waiting time• (Conflict) serializable schedule

– Schedule where the end result is (conflict) equal to the result of some serial schedule

– This is OK. But how do we achieve it?

Transactions 12

Serial and non-serial schedules

Transactions 13

Testing for (conflict) serializability of a schedule

• A simple(?) way to test if a schedule is serializable– Make a precedence graph

• Nodes: Transactions• Edges

– Tj does Read_item(X) AFTER Ti does Write_item(X)

» Ti → Tj

– Tj does Write_item(X) AFTER Ti does Read_item(X)

» Ti → Tj

– Tj does Write_item(X) AFTER Ti does Write_item(X)

» Ti → Tj

– Schedule is serializable if the precedence graph has no cycles

Transactions 14

Example: Precedence graphs

Transactions 15

Transaction support in Microsoft SQL Server

• BEGIN TRANSACTION• COMMIT TRANSACTION• ROLLBACK TRANSACTION• Documentation– http://msdn.microsoft.com/en-us/library/

ms174377.aspx