inter-ans/INTERVIEWER_NOTES.md

Interviewer Notes

The candidate's README is vague on purpose. Share the details below only when the candidate asks for them.

Answers to give if asked

1. A Trade has trade_id, symbol, side ("BUY" or "SELL"), qty, and price.
2. qty is always entered as a positive whole number. The side gives the direction.
3. A trade with a qty or price of zero or less, or an unknown side, is invalid. It must be rejected and logged at ERROR.
4. Positions use weighted-average cost.
5. Realized P&L is booked only when a position is reduced.
6. Ledger.history is the record of truth for every accepted trade. The EOD job rebuilds a fresh ledger from it.
7. The two ledgers reconcile when every symbol has the same qty and avg_price.
8. Every log line must name the trade it is about.
9. A logger only writes messages at or above its configured level.
10. Single-threaded. You don't need to handle persistence, partial fills, or fees.
11. Short selling is allowed, and one trade can move a position from long, through zero, to short.
12. After a flip, avg_price is the price of the trade that caused the flip.
13. Treat a Trade as immutable once it has been submitted.
14. Rejected trades do not go into history.
15. The EOD rebuild may skip bad trades, but each one it skips must be logged at ERROR.
16. Losing records that were never flushed when the process exits is out of scope.
17. The 1e-9 price tolerance is fine.

Details to share only if the candidate is stuck

• Ops reports that the EOD reconcile shows breaks between the live ledger and the rebuild.
• Some desks say their average prices look wrong after large sells.
• The logs show no errors, and the log lines don't match the trades they're about.
• CI is green.

Signals to watch for

• Does the candidate ask about the assumptions before changing code?
• Does the candidate reproduce the problem before trying a fix?
• Does the candidate question why the tests pass, or trust them?
• Does the candidate check the AI's suggestions, or accept them as they are?