I co-founded a startup and spent 5 years as its founding engineer — taking a B2B multi-tenant SaaS from zero to 10+ business customers, absorbing event-day peaks of ~25K concurrent users with zero downtime, and operating everything I shipped, from the UI down to the infrastructure and the on-call that follows.

• 🌱 Going deeper on system design & architecture that stays simple and reliable as it scales
• 🚀 Looking to help build ambitious global products where the engineering bar is high — developer tools, AI, or deep tech — and own problems from zero to one
• 🛠 Polyglot by choice: shipped production systems in TypeScript, Rust, Java, and C# — the problem picks the tool
• 📫 Reach me at rayleighko2@gmail.com · linkedin.com/in/rayleighko
• 📄 Résumé — English (PDF) · 국문 (PDF)
• ⚡ Fun fact — I built and ran a company for 7 years before returning to full-time engineering, and I'm a devoted cat person 🐈

The Bearings — which economic regime is your portfolio betting on? A regime read for what you already hold. Solo-built and operated: Next.js 16 · React 19 · Supabase (Postgres RLS) · Claude-powered briefings behind a 3-layer safety filter · PWA on Vercel. Docs-first engineering with architecture decision records.

Breaking Point — an interactive software engineering lab for breaking browser-based simulations, reasoning about system limits, and defending architecture decisions with evidence in the AI era.

• Self-Service Domains: Letting Customers Provision DNS Without Talking to Us — slug uniqueness in Postgres, a four-step Vercel domain pipeline, and the day-2 gaps that stayed manual
• The 403 That Redesigned Our Auth: Three Login Systems, One Rust Middleware — how three identity classes accumulated, why a valid token still failed, and unifying enforcement without merging identity
• 1,000 on a Normal Day, 25,000 on Event Day: Capacity Planning When Your Spikes Are Scheduled — absorbing a 25× event-day spike with no autoscaler: caches, serverless bursts, and a capped worker, layer by layer
• When Serverless Stops Making Sense: Moving Bulk PDF Generation to a Rust Worker — why per-invocation Chromium broke down, the axum worker that replaced it, and a reproduction benchmark (3.2× on bulk exports)
• The Event Is the Tenant: Multi-Tenancy for a Business Where Customers Are Projects, Not People — identity, isolation, and billing scoped to events; ~$100/mo infra serving ₩5–10M-per-event contracts, and what I'd redesign

More at ray-k.medium.com

const ray = {
  name: "MyungJin Ko (Ray)",
  role: "Software Engineer",
  edge: "builds and operates what I ship — adapts to any stack",
  exFounder: true,            // 7 years, took products 0 → 1
  nowBuilding: "cohort.co.kr",
  nowChasing: "system design × scale",
  catPerson: true,            // 🐈
  motto: "If things are not failing, you are not innovating enough.",
};