ACID Properties, Properties of Database Transactions, Atomicity Explained, Example for Atomic transactions
Consistency 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;
|
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Step 6: Write (B);
|
Write the new
value of B into database
|
B’s new balance
is 16000.
|
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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.
Consistency – This property
ensures that whether the old values of all the data items of a transaction is
successfully transformed into new values in the exact manner as it is specified
in the instructions of that transaction.
In other words, a transaction should
transform a database from one consistent state to a new one. This would happen
only if all the instructions are executed without any interference from other
transactions.
For example, the old values of A and
B are 10000 and 14000 respectively. The instructions or calculations that
change A and B values are A := A-2000 and B := B+2000. If these statements are
executed successfully and there was no other transactions that change A or B
during the execution of T1, then we would see the new values of A and B as 8000
and 16000.
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