Concurrency Issues ka Funda - Fullstackgada Interactive Demo

⚡ Concurrency Problem Simulator

Problem: दो users एक ही product को एक साथ खरीदने की कोशिश कर रहे हैं!

📱 iPhone 15 Pro
Stock Available: 1
Price: ₹1,29,999

Purchase Logs:

Ready to simulate purchases...

🛡️ Concurrency Solutions

जेठालाल के Solutions: Different techniques to handle concurrent access properly.

-- Pessimistic Locking Approach
BEGIN TRANSACTION
SELECT stock FROM products WHERE id = 1 FOR UPDATE;
-- Row is now locked, other users must wait
IF stock > 0 THEN
  UPDATE products SET stock = stock - 1 WHERE id = 1;
  INSERT INTO orders (user_id, product_id) VALUES (?, ?);
COMMIT;

Advantage: Guaranteed consistency
Disadvantage: Performance impact, potential deadlocks

-- Optimistic Concurrency Control
SELECT stock, version FROM products WHERE id = 1;
-- Client performs business logic
UPDATE products SET
  stock = stock - 1,
  version = version + 1
WHERE id = 1 AND version = @original_version;
-- If affected rows = 0, then conflict occurred

Advantage: Better performance, no blocking
Disadvantage: Retry logic needed

-- Atomic Transaction
CREATE PROCEDURE BuyProduct(@userId INT, @productId INT)
AS BEGIN
  BEGIN TRANSACTION
  DECLARE @stock INT
  SELECT @stock = stock FROM products WHERE id = @productId
  IF @stock > 0
    UPDATE products SET stock = stock - 1 WHERE id = @productId
    INSERT INTO orders VALUES(@userId, @productId, GETDATE())
    COMMIT
  ELSE
    ROLLBACK
END

// Frontend Concurrency Handling
let isProcessing = false;
function buyNow() {
  if (isProcessing) return;
  isProcessing = true;
  document.getElementById('buyBtn').disabled = true;
  
  fetch('/api/purchase', { method: 'POST' })
    .then(response => {
      isProcessing = false;
      document.getElementById('buyBtn').disabled = false;
    });
}

🔍 Concurrency Debugging & Monitoring

🕒 Timestamp Analysis

/* Simulate purchases to see debug logs */

📈 Concurrency Metrics

Metric	Without Protection	With Protection
Success Rate	60%	100%
Data Integrity	❌ Compromised	✅ Maintained
Response Time	150ms	280ms
Error Rate	40%	0%

Solution	✅ Pros	❌ Cons	Use Case
🔒 Pessimistic Locking	100% consistency guaranteed	Performance impact, deadlock risk	Critical operations, low concurrency
🎯 Optimistic Locking	Better performance, no blocking	Retry logic needed, conflict resolution	High read, low conflict scenarios
⚛️ Atomic Transactions	ACID compliance, data integrity	Resource intensive	Financial transactions, inventory
🖥️ UI Controls	User-friendly, immediate feedback	Client-side only, not foolproof	User experience enhancement