Seminar 12

DISTRIBUTED TRANSACTION MANAGEMENT & RECOVERY

S.SANTHI PRIYA

Properties of Transactions •  Atomicity - Either all or none of the transaction’s operations are performed. Reasons why a transaction is not completed : » transaction aborts - Transaction recovery

» system crashes - crash recovery

•

Durability - Once the transaction has committed, the system must guarantee that the results of its operations will never be lost, independent of subsequent failures.

» database recovery

•

Serializability - If several transactions are executed concurrently, the result must be the same as if they were executed serially in same order. » concurrency control

•

Isolation -  An incomplete transaction cannot revel its results to other transactions before its commitment.

» Cascading aborts

Distributed Transactions • There exists a root agent which starts the whole transaction,

so that when the user request the execution of an application, the root agent is started; the site of the root agent is called the site of origin of the transaction. • The root agent has the responsibility of issuing the

begin_transaction, commit and abort primitives. • Only the root agent can request the creation of new agent.

Example – bank transactions

fund transfer at global level Read( terminal, $AMOUNT, $FROM_ACC), $TO_ACC); Begin _transaction; select AMOUNT into $FROM_AMOUNT from ACCOUNT where ACCOUNT_NUMBER=$FROM_ACC; If $FROM_AMOUNT-$AMOUNT