ACID Properties, Properties of Database Transactions, Atomicity Explained, Example for Atomic transactions
Durability property - ACID Properties
Let us consider a transaction T1 through
which you want to transfer 2000 from your account ‘A’ (source account) to your
friend’s account ‘B’ (destination account). Assume that the account balance of
A is 10000 and B is 14000. This transaction can be represented as follows;
|
Instructions in Transaction T1
|
Meaning
|
Example
|
|
Step 0:
Begin_Transaction
|
The starting
point of the current transaction.
|
T1 started. At this
stage A = 10000 and B = 14000.
Old consistent state
|
|
Step 1: Read (A);
|
Read the current balance
of A from database.
|
A’s current balance
is 10000.
|
|
Step 2: A := A – 2000;
|
Deduce the amount
to be transferred from the source account.
|
A := 10000 –
2000;
|
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Step 3: Write (A);
|
Write the new
value of A into database.
|
A’s new balance
is 8000.
|
|
Step 4: Read (B);
|
Read the current
balance of B from database.
|
B’s current
balance is 14000.
|
|
Step 5: B := B + 2000;
|
Add the
transferred amount (amount deduced from A) to destination account B.
|
B := 14000 +
2000;
|
|
Step 6: Write (B);
|
Write the new
value of B into database
|
B’s new balance
is 16000.
|
|
Step 7: Commit;
|
Permanently write
values of A and B into database.
|
At the end of the
transaction,
A = 8000 and B =
16000.
New consistent state
|
Table: Transaction T1
From the above example, we would
know that if the transaction executed the instructions in Step 0 to Step 7,
then we would say that this transaction T1 is successfully completed. All these
steps have to be executed successfully, safely and without any other external
intervention. To ensure this we use to check the transaction properties.
Durability – The changes made to the database during
a successful transaction must persist in the database. In other words, the updates
carried out by a successful transaction must be permanently reflected in the database
even if any failure occurs after commit. It is the responsibility of the
recovery subsystem to ensure durability.
For example, in T1, we must see 8000
and 16000 as the current balances of account A and B respectively after the
commit has reached. After commit has reached, if any failure occurs, we must
not lose the updates.
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