Crypto Exchange HLD: The Double-Entry Ledger That Never Loses a Coin

You send a friend ₹500 through your bank app. The instant your balance ticks down to ₹4,500, theirs ticks up by exactly ₹500 — there is never a flicker where the money sits in both accounts at once, and never a gap where it has left yours but not yet landed in theirs. One movement, two sides, and the total across the two of you is unchanged to the last paisa. That single, boring guarantee is the hardest thing a crypto exchange has to keep — because on an exchange billions of dollars move like that every day, and a single arithmetic slip that creates or destroys value is catastrophic and, on a …