Architecture Overview

Tina Diet has two user-facing surfaces (LINE bot chat + LIFF web app) sharing one backend, with SQLite for state and three external services for AI, payment, and event delivery.

System diagram

                                                ┌──────────────────┐
                                                │  OpenAI API      │
                                                │  gpt-4o,         │
                                                │  gpt-4o-mini     │
                                                └─────────▲────────┘
                                                          │
                  ┌─────────────────┐    ┌────────────────┴────────────────┐
                  │ LINE Messaging  │    │                                 │
                  │ API (Cloud)     │◀──▶│                                 │
                  └────────▲────────┘    │     Railway backend             │
                           │             │     api.tinadiet.com            │
                           │webhook      │                                 │
                  ┌────────┴────────┐    │  ┌──────────────────────────┐   │
                  │   User on LINE  │    │  │  Express                 │   │
                  │   (Thai phone)  │    │  │  /webhook/line           │   │
                  └────────┬────────┘    │  │  /webhooks/omise         │   │
                           │             │  │  /api/v1/*               │   │
                  ┌────────▼────────┐    │  │  /internal/jobs/*        │   │
                  │   LIFF app      │    │  └────────┬─────────────────┘   │
                  │ app.tinadiet.com│───▶│           │                     │
                  │ (Cloudflare     │    │  ┌────────▼────────┐            │
                  │   Workers)      │    │  │ better-sqlite3  │            │
                  └─────────────────┘    │  │ /data/app.db    │            │
                                         │  │ (volume)        │            │
                                         │  └─────────────────┘            │
                                         └────────────────▲────────────────┘
                                                          │webhook
                                                ┌─────────┴────────┐
                                                │  Omise API       │
                                                │  PromptPay +     │
                                                │  TrueMoney       │
                                                └──────────────────┘

Components

LINE bot chat surface

• User sends a message in LINE → LINE webhook → POST /webhook/line on backend
• Backend signature-verifies, classifies intent (greeting / show-logs / log-weight / consult / parse), runs handler, replies via LINE Reply API
• Push messages: backend → LINE Push API for daily/weekly summaries, renewal reminders, welcome message

LIFF (LINE Front-end Framework) app

• A React SPA hosted at app.tinadiet.com (Cloudflare Workers Static Assets)
• Loaded by LINE app’s embedded webview, gets the user’s lineUserId via LIFF SDK
• Exchanges lineUserId for a backend session JWT at /api/v1/auth/exchange
• All subsequent API calls authenticate with Authorization: Bearer <jwt>

Backend (Railway)

• Single Express process, Node 22 ESM, tsx watch for dev
• Mounted routers in order (order matters for express.json body parser):
  1. /webhook/line — express.json (after sig verify)
  2. /webhooks/stripe — express.raw (dormant)
  3. /webhooks/omise — express.raw (for HMAC sig verify)
  4. CORS for /api/v1
  5. express.json global
  6. /api/v1/* — all LIFF-facing API
  7. /internal/jobs/* — cron triggers (x-jobs-secret guarded)
  8. /healthz — public health check
• Cron jobs (node-cron, Asia/Bangkok timezone):
  • 0 21 * * * daily summary
  • 0 8 * * 1 weekly summary (Monday morning)
  • 0 2 * * * expire premium

SQLite database

• File at /data/app.db on Railway volume (backend-volume, 1 GB)
• Accessed synchronously via better-sqlite3 — no async/await for queries
• Migrations applied at boot via runMigrations() (idempotent)
• See Data model for schema

External services

• OpenAI — text parsing (food logs from chat), vision parsing (food photos), coach suggestions, consultation Q&A
• Omise — PromptPay QR + TrueMoney Wallet charges + webhooks
• LINE Cloud — Messaging API (bot) + Login API (LIFF auth)

Data flow: AI food log via chat

1. User sends “ผัดกะเพราไก่ + ไข่ดาว” in LINE
2. LINE → POST /webhook/line (signed request)
3. Backend verifies signature, parses event, classifies intent → attempt_parse
4. Backend calls OpenAI gpt-4o-mini with system prompt + user text + strict JSON schema
5. AI returns { items: [{food_name_th: 'ผัดกะเพราไก่', kcal: 450, ...}, {food_name_th: 'ไข่ดาว', kcal: 90, ...}] } (or needs_clarification / not_food)
6. Backend inserts each item into food_logs table
7. Backend asks for proactive meal suggestion via coach.ts (also OpenAI)
8. Backend replies to LINE with confirmation + suggestion in single message

Data flow: payment (Omise PromptPay)

1. User taps “Premium” in LIFF Rich Menu → PremiumSection
2. User picks PromptPay, taps “ชำระ 150 ฿”
3. LIFF POST /api/v1/billing/omise/charge body {method:"promptpay"}
4. Backend calls Omise API POST /charges with source.type=promptpay
5. Omise creates charge, returns qr_image_uri
6. Backend inserts payments row (status=pending), returns charge to LIFF
7. LIFF shows QR modal with countdown + polls GET /omise/charge/:id every 2 seconds
8. Meanwhile Omise fires charge.create webhook → backend ACKs (no grant)
9. User scans QR in bank app, pays
10. Omise fires charge.complete webhook with HMAC signature
11. Backend verifies signature, marks payment successful, extends users.premium_expires_at (stacks if user was already premium)
12. LIFF polling sees new status → modal transitions to success → reload billing status

Where things live

Concept	Path
Database schema	backend/src/db/migrations.ts
Repositories	backend/src/repositories/
HTTP routes	backend/src/routes/{api,webhook,internal}/
Domain logic	backend/src/domain/
AI services	backend/src/services/{food_parser,coach,consultation}.ts
Payment service	backend/src/services/{omise,stripe}.ts
Background jobs	backend/src/jobs/
LIFF pages	liff/src/pages/
LIFF components	liff/src/components/
LIFF API client	liff/src/api/
Brand assets	liff/public/

Read next

• Data model — every table
• Key invariants — patterns that must not be broken
• Backend stack — Express conventions
• LIFF stack — React conventions