ACID Properties, Properties of Database Transactions, Atomicity Explained, Example for Atomic transactions
Isolation 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;
|
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.
Isolation – This ensures that the instructions of a
transaction should not be visible to any other ongoing transactions. A transaction
should be isolated from other transactions. In other words, the partial effects
of incomplete transactions should not be visible to other transactions. It is
the responsibility of the concurrency control subsystem to ensure isolation.
Isolation property requires serial execution of transactions to ensure consistency.
For example, in T1, first we change
A from 10000 to 8000. Then we change B from 14000 to 16000. During T1, we
should not allow other transactions to see or to use the old values of A or B
and the new values of A or B before executing commit (step 7). T1 should not be interfered by other
transactions during its execution.
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