Beyond Mutex: Distributed Locking and Coordination in Microservices
Introduction: The Double-Spending Problem When you develop E-commerce platforms, you may commonly face a coordination problem: refunds, for example, can be triggered simultaneously by customers, sellers, or automated fraud detection systems. Without proper distributed coordination, the same order gets refunded multiple times. Consider a real-world scenario: an order with a total value of $200 receives three concurrent $200 partial refund requests within a 1-second window. The Customer requests a $200 refund for a damaged item via the app. The Seller initiates a $200 courtesy refund due to a shipping delay. The Loyalty System triggers an automated $200 refund as part of a promotional price-match guarantee. Three separate microservices process these requests simultaneously. Each service reads the order state from the database, seeing a “remaining refundable balance” of $1,000. Because the requests overlap before the database can update the balance, all three services validate the transaction as “safe.” ...