Audit trails and reconciliation for trading systems

In a system that can place thousands of orders a second, memory is not a record. When a regulator asks what happened, when a client disputes a fill, or when you are reconstructing a bad afternoon, the only thing that matters is the trail the system left behind. A complete, honest, tamper-evident audit trail is the difference between explaining an incident and merely apologising for one.

§01Record the decision, not just the order

A useful audit trail captures more than the orders that left the box. It records the inputs — the market data, the signal, the risk-check result — that led to each action, timestamped to the resolution the system actually operates at. Reconstructing why a decision was made, not just what was sent, is what turns a log into evidence. The goal is that any order can be traced back to the exact state of the world that produced it.

§02Tamper-evidence

A record that can be quietly edited after the fact is not an audit trail; it is a story. Serious systems make their logs append-only and tamper-evident — hash-chained, write-once, or independently mirrored — so that any alteration is detectable. The point is not paranoia but credibility: a trail you can prove was not changed is worth infinitely more than one you merely claim was not.

§03Reconciliation catches the drift

Independent reconciliation is where audit trails earn their keep operationally. Continuously comparing the system’s own view of positions, orders, and fills against the venue’s and the broker’s catches the silent divergences — a missed fill, a dropped cancel, a double-count — before they compound. Reconciliation that runs outside the trading loop, on an independent view of the truth, is what stops a small discrepancy from becoming a large surprise.

§04Boring until it is everything

Audit and reconciliation are the least exciting parts of a trading system, right up until the moment they are the only parts that matter. Record the reasoning, make the record tamper-evident, and reconcile against an independent source of truth. This advisory is educational and illustrative and is not compliance, legal, or investment advice for any specific situation.