How to prevent race conditions when updating user balance in PHP with PDO transactions?
I'm prototyping a solution and I'm deploying to production and This might be a silly question, but I tried several approaches but none seem to work... I tried several approaches but none seem to work. I'm working with a race condition scenario in my PHP application while updating user balances in a MySQL database using PDO. The application allows users to make transactions that modify their balance, and during high traffic times, I sometimes get incorrect balances due to concurrent updates. To manage database interactions, I’m using PDO with transactions. Here's the relevant code snippet where I perform the balance update: ```php try { $this->pdo->beginTransaction(); $stmt = $this->pdo->prepare('SELECT balance FROM users WHERE id = :id FOR UPDATE'); $stmt->execute([':id' => $userId]); $currentBalance = $stmt->fetchColumn(); if ($currentBalance === false) { throw new Exception('User not found.'); } // Assuming $amount is the transaction amount $newBalance = $currentBalance + $amount; $updateStmt = $this->pdo->prepare('UPDATE users SET balance = :balance WHERE id = :id'); $updateStmt->execute([':balance' => $newBalance, ':id' => $userId]); $this->pdo->commit(); } catch (Exception $e) { $this->pdo->rollBack(); echo 'Transaction failed: ' . $e->getMessage(); } ``` I’ve tried using `FOR UPDATE` in my SQL query to lock the row, but the scenario continues. Sometimes, users see incorrect balances reflected on their accounts. I've also checked for other processes that might be updating user balances but couldn't find any. I would like to understand how to effectively manage concurrent updates and ensure that transactions are safe from race conditions. Should I consider using additional locking mechanisms or perhaps redesign my transaction logic? Any recommendations or best practices would be greatly appreciated. For context: I'm using Php on macOS. My development environment is macOS. Any ideas what could be causing this? For context: I'm using Php on Ubuntu. For reference, this is a production microservice.