Why NSE closes pre-open order entry at a random moment

transcript

Every morning, NSE stops taking opening orders at a moment nobody knows. On purpose.

9:00. Traders send orders for the opening auction. Somewhere between the 7th and 8th minute, entry closes. Randomly. Then one price: where the most volume can trade. A tie? The least left unmatched. 9:15. Normal trading starts. And after an outage, the market reopens this way.

So why not close at 9:08 sharp? Then the last microsecond is the most valuable. And the fastest traders just wait for it. Close at an unknown moment, and there's no prize for being last.

But here's the catch. A replay after a crash must find the same price. So the close is a message in the sequenced log. Replay it, same price. Random for traders. Exactly repeatable for the exchange.

Same trick for position checks: snapshots at random times, so nobody dodges them. Randomise only where a known instant has value.

Full breakdown in the lecture.