About the author

I build software that has to explain what it did.

I'm johnnyji, a software engineer based in Seoul, South Korea. My work sits around web applications, crypto infrastructure, trading automation, and the operational tooling needed to keep automated systems understandable.

I started johnnyji.dev as a place for practical calculators. While building them, I realized the useful part is rarely the formula alone. Developers and operators also need to know which assumptions can fail, what data should be recorded, and how a result changes when fees, retries, stale data, or provider limits appear.

The articles on this site document those details. They are written from implementation experience and focus on decisions that can be tested: how to prevent duplicate orders, reconcile remote state, estimate API costs from product behavior, and place limits around systems that spend money automatically.

What I write about

Reliable automation

Idempotent jobs, reconciliation, explicit state machines, observability, and recovery after partial failures.

Trading systems

Execution costs, position risk, exchange integrations, inventory tracking, and the operational controls around strategy code.

Small product economics

API usage, infrastructure budgets, free-tier cliffs, and keeping experiments affordable until demand is proven.

Editorial approach

  • Show formulas and assumptions instead of presenting magic numbers.
  • Include failure modes and trade-offs, not only the happy path.
  • Use examples that readers can reproduce or adapt.
  • Update material when the implementation or underlying facts change.

The trading material is educational and does not provide financial or investment advice. Questions, corrections, and technical discussion are welcome through the contact page.