We will explore Transactions, Deadlocks, and Log-Based Recovery using SQL.
We use the Accounts table as our base schema and demonstrate ACID concepts with rollback, simulate a deadlock, and discuss log-based recovery.
Step 1: Create Table & Insert Data
CREATE TABLE CustomerAccounts (
acc_no INT PRIMARY KEY,
name VARCHAR2(50),
balance INT
);
INSERT INTO CustomerAccounts VALUES (1, 'Alice', 1000);
INSERT INTO CustomerAccounts VALUES (2, 'Bob', 1500);
INSERT INTO CustomerAccounts VALUES (3, 'Charlie', 2000);
SELECT * FROM CustomerAccounts;
Transaction – Atomicity & Rollback
Task: Transfer 500 from Alice to Bob, but rollback before committing.
Deduct 500 from Alice
UPDATE CustomerAccounts
SET balance = balance - 500
WHERE name = 'Alice';
-- Add 500 to Bob
UPDATE CustomerAccounts
SET balance = balance + 500
WHERE name = 'Bob';
Rollback transaction
ROLLBACK;
Check balances
SELECT * FROM CustomerAccounts;
Deadlock Simulation (Conceptual)
Session 1 (conceptual)
-- Lock Alice
UPDATE CustomerAccounts SET balance = balance - 100 WHERE name = 'Alice';
-- Do NOT commit
Session 2 (conceptual)
-- Lock Bob
UPDATE CustomerAccounts SET balance = balance - 200 WHERE name = 'Bob';
-- Do NOT commit
Continuing Session 1
-- Try updating Bob (held by Session 2)
UPDATE CustomerAccounts SET balance = balance + 100 WHERE name = 'Bob';
Continuing Session 2
-- Try updating Alice (held by Session 1)
UPDATE CustomerAccounts SET balance = balance + 200 WHERE name = 'Alice';
Expected in a real multi-session DB:
ORA-00060: deadlock detected while waiting for resource
Log-Based Recovery
Task: Show rollback and explain internal logs (undo/redo).
-- Update Charlie
UPDATE CustomerAccounts SET balance = balance + 300 WHERE name = 'Charlie';
-- Rollback
ROLLBACK;
-- Verify balances
SELECT * FROM CustomerAccounts;
Conclusion:-
Atomicity: Rollback prevents partial updates.
Deadlock: Occurs when two sessions wait for each other (conceptual in LiveSQL).
Log-Based Recovery: Oracle maintains undo/redo logs for safe recovery.
Special thanks to @santhoshnc Sir for guidance throughout this assignment.
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