Order types and validation, routing, matching by price-time priority, fills and execution reports.
3 questions in Capital Markets.
An equity order is validated and risk-checked at the broker, routed to a venue, matched by price-time priority into one or more fills reported back as execution reports, allocated to end accounts, novated to a clearing house that becomes the counterparty, and finally settled against cash at the custodian on T+1.
Book the fill. The venue decides what happened, so a fill against a cancelled order means your state was wrong, and the position, risk headroom and client record all have to be corrected from the venue's version. The design fix is to make cancelled a state only the venue can grant.
The gateway owns state you cannot drain, namely resting orders at the venue and sequenced sessions, so a rolling deployment leaves a mixed fleet holding live obligations. Verification has to be a position reconciliation rather than a health check, rollback does not undo executions, and dead code must be deleted rather than disabled.