Your browser sends the quantity to the server and waits — it never changes the numbers itself. Flask looks up the current price, checks you have enough cash, writes one new row to the ledger table, then re-adds the whole list to work out your new cash and shares, and sends those numbers back. The page updates just that one card — no reload, no page jump.

A Python web framework, run by gunicorn (2 worker processes) behind nginx. It decides what happens when you click Buy or Sell, builds the page you see, and is the only thing allowed to change your cash or shares — the page in your browser can never change them on its own.

Every deposit, buy, sell, and reset is written as a new row in one growing list (a "ledger") that's never edited or erased. Your cash and shares aren't stored directly — they're worked out by adding up that list, so there's always an honest record of everything that happened. Six tables in total:

The server asks Finnhub for each company's current price and saves the answer for about a minute before asking again; company logos and performance stats (5-day, 1-month, 1-year returns, 52-week range) are cached too, refreshed daily since they change slowly. Your API key lives only on the server and is never sent to the browser. Finnhub's free plan doesn't include multi-year historical prices — that's a real limit of the free API, not something this app fakes. The "years of growth" numbers you see are genuine trailing returns Finnhub does provide for free; the price chart's long-term view instead grows organically from the moment this app started tracking it.

gunicorn runs the Flask app as two worker processes on 127.0.0.1:8000, not exposed to the internet directly. systemd (part of the server's operating system) manages it as a service named paper-trader — restarts it if it ever crashes, and starts it automatically if the server reboots. PostgreSQL and nginx are separate systemd services managed the same way.

nginx listens on ports 80 and 443, redirects plain HTTP to HTTPS, and forwards everything else to gunicorn. The certificate is a free one from Let's Encrypt, obtained and installed by certbot; a systemd timer checks daily and renews it automatically before it expires — no one has to remember to do it by hand.

The price and portfolio charts are plain JavaScript drawing simple shapes (SVG) directly from the numbers in the database — no chart library, no other outside service, nothing downloaded from anywhere but this server.