Overview

The terms “Continuous Monthly Subscription”, “Periodic Deduction”, and “Auto-Renewal” refer to the same core functionality:

• Periodic Deduction - Official Alipay terminology
• Continuous Monthly Subscription - Common implementation pattern (also supports daily/quarterly)
• Auto-Renewal - User-facing behavior of automatic payment

Deduction Scenarios

Two implementation models:

1. Sign-then-Deduct

   • Redirect to agreement signing page
   • User signs agreement
   • Alipay async callback + immediate deduction

2. Pay-then-Sign

   • Redirect to payment page (user can disable auto-renewal)
   • User completes payment
   • Receive two async notifications: payment & agreement

Recommendation: Prefer Sign-then-Deduct to prevent users from disabling renewal during payment.

Platform Requirements

• Alipay: Requires enabling “Periodic Deduction” via Alipay Open Platform
• WeChat Pay: More complex application process requiring additional Template ID

Agreement Signing Flow

Signature Generation (Go Implementation)

We use modified gopay library (PR #295 merged 2023-01-01):

Key implementation notes:

1. Generate contract_code as merchant agreement ID
2. Create signature parameters for mobile app deep linking:

// Example signature parameters
bm := gopay.BodyMap{}
bm.Set("personal_product_code", "GENERAL_WITHHOLDING")
bm.Set("sign_scene", "INDUSTRY|OTHER")
bm.Set("external_agreement_no", contractCode)
// ... other required fields

3. Construct deep link URL:

alipays://platformapi/startapp?appId=60000157&sign_params={URL-Encoded_Params}

Database Design

Core subscription table structure (simplified):

CREATE TABLE subscriptions (
    id INT PRIMARY KEY,
    order_no VARCHAR(64) UNIQUE,      -- Merchant order ID
    trade_no VARCHAR(64),             -- Payment platform ID
    user_id INT,                      -- User reference
    contract_code VARCHAR(64) UNIQUE, -- Custom agreement ID
    contract_id VARCHAR(64),          -- Platform agreement ID
    expire_time DATETIME              -- Next deduction date
);

Order Processing Flow

1. Create order with generated contract_code
2. Generate signing parameters and return deep link to client
3. Handle async notification to store contract_id
4. Initial payment via alipay.trade.pay (synchronous API)
5. Client polling for payment status
6. Scheduled task for recurring deductions:

// Cron job example (executed daily)
func CheckSubscriptions() {
    // Query subscriptions expiring in 5 days
    subscriptions := GetExpiringSubscriptions()
    
    for _, sub := range subscriptions {
        // Initiate payment via Alipay API
        resp, err := alipay.TradePay(sub)
        // Handle payment result
    }
}

Platform-Specific Notes

Key Implementation Points

1. Use separate async handlers for payment & agreement notifications
2. Implement idempotency checks for all payment operations
3. Maintain detailed audit logs for deduction attempts
4. Provide clear cancellation UI/API endpoints

Complete implementation example available at: github.com/yourrepo/subscription-service