Date: March 19, 2026 Progress: Phase 3 (System Architecture) Complete · Interactive Push Notification Design Complete Previous Log: See Day 1 AI model: Sonnet 4.6

✅ Today's Achievements

Item Status
Phase 3 — System Architecture Draft (AWS) ✅ Completed then discarded
Phase 3 — Cost Analysis & Stack Review ✅ Done
Phase 3 — Rewritten for Free-tier Stack ✅ Done
Interactive Push Notification Design ✅ Done
Feature Spec F-02 Updated ✅ Done

1. Cost Analysis Results & Stack Migration Decision

Issues with the Original AWS Stack

The initial full AWS stack incurred $196–250/month. Deemed excessive for the MVP stage with fewer than 10 DAU — full review initiated.

Service Monthly Cost
ECS Fargate (API ×2) $55
RDS PostgreSQL $50
ElastiCache Redis $35
ALB $25
CloudFront + WAF $30
SQS + SNS ~$1
Total $196–250

Stack Migration Strategy by Scale

Stage DAU Stack Monthly Cost
Now (Dev / Beta) < 10 Render + Supabase + Upstash $0
Growth 100–3,000 Fly.io + Supabase Pro + Upstash $15–30
Scale-up 3,000+ AWS ECS + RDS + ElastiCache $100–200

Core principle: Keep the codebase identical across stages — swap only the infrastructure as DAU grows. No code changes required during migration.

2. Phase 3 — System Architecture Document (Baseline: DAU 10)

System Architecture

[Flutter App (iOS / Android)]
         │ HTTPS
         ▼
[Cloudflare Free] — CDN · HTTPS · DDoS · WAF
         │
         ▼
[Render Free] — FastAPI Server + APScheduler
    │         │              │
    ▼         ▼              ▼
[Supabase] [Upstash Redis] [Upstash QStash]
PostgreSQL  Session/Token   Notification
            Cache           Queue/Scheduler
    │                        │
    │                        ▼
    │              [FastAPI /notify Webhook]
    │                        │
    │              ┌──────────┴──────────┐
    │           [FCM]               [APNs]
    │         Android Push          iOS Push
    │
[Sentry] [UptimeRobot] [GitHub Actions]
 Error       Uptime          CI/CD
 Tracking    Monitoring

Server Composition Plan

Category Service Spec Count Cost
API + Scheduler Render Free 512MB RAM, 0.5 CPU 1 $0
Database Supabase Free PostgreSQL 500MB 1 $0
Auth Supabase Auth Social login included Included $0
Cache Upstash Redis 256MB, 10K req/day 1 $0
Notification Queue Upstash QStash 500 msg/day 1 $0
CDN + WAF Cloudflare Free Unlimited traffic — $0
Error Monitoring Sentry Free 5,000 events/month — $0
Uptime Monitoring UptimeRobot 1-min check interval — $0
Monthly Total $0

⚠️ Render Free spins down after 15 min of inactivity. Use UptimeRobot to ping every 5 min and prevent sleep.

Final Confirmed Tech Stack