Implementing Stripe Checkout Billing in PatentLLM

#webdev #devops #infrastructure

Introduction

To commercialize PatentLLM, we implemented a billing system using Stripe Checkout. Existing FTS5 search, web search, and analysis prompts were already complete. The remaining challenge was to "build a billing gate."

Design Policy: Do Not Hold Card Information

Reasons for Not Directly Calling the API

We considered directly calling the Stripe API to handle billing, but decided against it.

Managing card information on the server increases security risks.
Risk of the entire application becoming unusable during payment errors.

Adoption of Stripe Checkout

We adopted a method where a "Subscribe" button is placed within the application, transitioning the user to Stripe's payment screen.

Card information is managed solely by Stripe (not held on our server at all).
Subscription status is cached in local SQLite upon successful payment.
Handles network failures with fallback to Stripe API.

Implementation Highlights

Graceful Degradation

import os
import stripe

stripe_key = os.getenv('STRIPE_SECRET_KEY', '')

def is_billing_enabled():
    """課金ゲートの有効/無効を判定"""
    if not stripe_key:
        return False  # キー未設定→課金無効（開発環境）
    if stripe_key.startswith('sk_test_'):
        return False  # テストキー→課金無効
    return True

If STRIPE_SECRET_KEY is not set in .env or if a test key is used, the billing gate is automatically disabled. This simplifies testing in development environments and prevents service outages due to misconfigurations during production deployment.

Reducing API Calls with Local Cache

import sqlite3
from datetime import datetime, timedelta

def check_subscription(user_id):
    """サブスクリプション状態を確認（ローカルキャッシュ優先）"""
    conn = sqlite3.connect('subscriptions.db')

    # ローカルキャッシュを確認
    row = conn.execute(
        'SELECT status, expires_at FROM subscriptions WHERE user_id = ?',
        (user_id,)
    ).fetchone()

    if row and row[0] == 'active':
        expires = datetime.fromisoformat(row[1])
        if expires > datetime.now():
            return True  # キャッシュ有効

    # キャッシュが無いか期限切れ→Stripe APIで確認
    sub = stripe.Subscription.retrieve(user_id)
    # 結果をキャッシュに保存
    conn.execute(
        'INSERT OR REPLACE INTO subscriptions VALUES (?, ?, ?)',
        (user_id, sub.status,
         (datetime.now() + timedelta(hours=1)).isoformat())
    )
    conn.commit()
    return sub.status == 'active'

The local SQLite cache is referenced when valid, and the Stripe API is re-checked only when expired. This minimizes the frequency of API calls.

UI: Subscription Management

We added the following UI for unregistered users:

"Subscription" section in the sidebar (displaying remaining days + Customer Portal link).
"Subscribe" button placed on the main screen.
Users can manage cancellations and plan changes themselves via the Stripe Customer Portal.

Summary

Designed not to hold card information on our server using Stripe Checkout.
Billing gate ON/OFF controlled by environment variables (easy switching between development/production).
Minimized Stripe API calls with local SQLite cache.

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