WATCH for optimistic locking in Redis - Mini Project: Build & Apply

Using WATCH for Optimistic Locking in Redis

📖 Scenario: You are managing a simple inventory system where multiple users can update the stock count of a product. To avoid conflicts when two users try to update the stock at the same time, you will use Redis's WATCH command for optimistic locking.

🎯 Goal: Build a Redis transaction that safely updates the stock count of a product using WATCH to detect changes and retry if the stock was modified by another user.

📋 What You'll Learn

Create a Redis key product_stock with initial stock value 10

Use WATCH on the product_stock key before starting the transaction

Get the current stock value and check if it is enough to reduce by 3

Use MULTI and EXEC to decrement the stock by 3 atomically

If the transaction fails due to concurrent modification, retry the operation

💡 Why This Matters

🌍 Real World

Optimistic locking with WATCH is useful in inventory systems, booking systems, or any application where multiple users update shared data concurrently.

💼 Career

Understanding Redis transactions and optimistic locking is important for backend developers working with caching, real-time data, and distributed systems.

1

Set initial stock value

Create a Redis key called product_stock and set its value to 10 using the SET command.

Redis

# Your code here

Need a hint?

Use the SET command to create the key with the initial stock.

2

Watch the stock key

Use the WATCH command on the product_stock key to monitor it for changes before starting the transaction.

Redis

SET product_stock 10
# Your code here

Need a hint?

Use WATCH product_stock to watch the key.

3

Get the current stock value

Use GET product_stock to retrieve the current stock value (executed immediately). In a real application or client script, check here if the value is at least 3 before proceeding to the transaction.

Redis

SET product_stock 10
WATCH product_stock
# Your code here

Need a hint?

Use GET product_stock before MULTI to read and check the value client-side.

4

Start transaction, decrement, and execute

Start the transaction with MULTI, decrement the stock atomically with DECRBY product_stock 3, and execute with EXEC. If EXEC returns (nil), the WATCH was triggered by a concurrent change—retry the entire operation from WATCH.

Redis

SET product_stock 10
WATCH product_stock
GET product_stock
# Your code here

Need a hint?

Add MULTI, then DECRBY product_stock 3, followed by EXEC.