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Example:; transfers transfer_amount from x's account to y's account
; Supposes x&y both accounts exist
TRANSACTION (x, y, transfer_amount)
Start transaction
IF X AND Y exist then
READ x.balance
IF x.balance > transfer_amount THEN
x.balance = x.balance - transfer_amount
READ y.balance
y.balance = y.balance + transfer_amount
COMMIT
ELSE DISPLAY ("BALANCE IN X NOT OK") ROLLBACK
ELSE DISPLAY ("ACCOUNT X OR Y DOES NOT EXIST") End_transaction
Please note the use of 2 keywords here COMMIT and ROLLBACK. Commit makes sure that all the modifications made by transactions are made permanent. ROLLBACK ends the transactions and discards any change made by the transaction. Transactions have definite desirable properties. Let us see into those properties of a transaction.
Lost Updates : Suppose the two transactions T3 and T4 run simultaneously and they happen to be interleaved in the following way (suppose the initial value of X as 10000):
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