Walk me through the lifecycle of an equity order from the click to settlement.

Order entry and validation

The click produces an order instruction, usually a FIX NewOrderSingle (35=D) carrying a client order ID, symbol, side, quantity, order type and time in force. The broker's order management system stamps it, assigns its own order ID, and runs pre-trade validation before anything reaches a venue. That validation is not decoration. It checks the instrument is tradable and not suspended, that the client is permissioned for it, that quantity and price are within fat-finger tolerances, that the order does not breach the client's buying power or short-sale constraints, and that it does not push the desk past a credit or position limit. Brokers with direct market access are required by their regulator to apply these controls on the broker's own systems rather than trusting the client's, because the broker carries the risk of a runaway algorithm.

What fails here: a reject. A rejected order never reaches the market, so the client sees an OrdStatus of rejected with a reason, and the interesting engineering question is whether your rejection is idempotent and whether the client can distinguish "rejected by us" from "we never got it".

Routing

An accepted order is rarely sent to one place. A smart order router decides where to send it, and in how many pieces, based on displayed liquidity, expected fill probability, fee schedules and the broker's best-execution obligation. A large order may be sliced by an algorithm over minutes or hours, sent partly to a lit exchange and partly to a dark venue or an internal crossing engine where it might match against another client of the same broker.

This is where a candidate first has to hold two ideas at once. The client has one order. The market sees many child orders. The parent-child relationship, and the aggregation of child fills back into the parent, is real state your system owns and must be able to reconstruct.

What fails here: the venue rejects the child order, the session drops mid-flight leaving an order whose state you do not know, or a child order times out. A dropped session is the nastiest case, because an order you cannot see is still an order that can trade. Recovery relies on the venue's order-status request and drop-copy feed rather than on your own optimism.

Matching and price-time priority

At the venue, the order joins the limit order book. A buy order that crosses the best offer trades immediately; one that does not rests. The matching rule on most equity venues is price-time priority: better prices trade first, and within a price level the order that arrived first trades first. The consequence candidates often miss is that time priority is lost the moment you amend a resting order's price or increase its size, so an algorithm that continually repegs is repeatedly sending itself to the back of the queue.

Some derivatives venues use pro-rata or a size-weighted allocation instead, which changes the optimal order-placement behaviour entirely. Knowing that price-time is a choice rather than a law is a good signal.

Fills and the execution report

Each match produces an ExecutionReport (35=8) back to the broker and on to the client. A 100,000-share order may generate hundreds of them. Each carries the incremental fill quantity and price plus the running state: CumQty filled so far, LeavesQty still working, AvgPx across fills. The order is partially filled until LeavesQty reaches zero, at which point it is filled; if the day ends first, the remainder expires or is cancelled depending on time in force.

Two engineering realities follow. Execution reports are a sequenced stream, so out-of-order or duplicated delivery must be handled by sequence number rather than by trusting arrival order. And the client's economics are the average price, not any single fill, which is why the average has to be computed to the venue's price precision using exact arithmetic rather than accumulated floating-point.

Allocation

An institutional order is usually placed as a block by a fund manager acting for many underlying accounts. After execution the manager sends an allocation instruction (35=J) splitting the block across those accounts, typically at the block average price so no account is advantaged. The broker enriches the trade with settlement instructions, fees, commission and tax, and confirms the allocated trades to the manager. This is also where the trade is affirmed: both sides agree the economics before it goes to the clearing house.

What fails here: a break. The allocation quantities do not sum to the executed quantity, an account has no standing settlement instruction, or the manager's price differs from yours by a rounding convention. Breaks found here are cheap; the same break found on settlement date is expensive.

Clearing and novation

The affirmed trade goes to the clearing house, and this is the step whose significance candidates most often understate. The clearing house novates: it replaces the original bilateral contract with two new contracts, becoming buyer to the seller and seller to the buyer. Your counterparty is no longer the broker on the other side of the trade, and their failure is no longer directly your problem. In exchange, you post initial margin, meet variation margin as prices move, and contribute to a default fund.

The clearing house also nets. All of a member's buys and sells in a security across the day collapse into a single net obligation to receive or deliver, which is why a member trading millions of shares may settle only a small fraction of that gross volume.

What fails here: a margin call the member cannot meet, which escalates from a funding problem to a default management problem, or a trade the clearing house will not accept because it fails validation and drops back to the members to fix bilaterally.

Settlement on T+1

On settlement date the securities move on the books of the central securities depository and the cash moves in the payment system, ideally simultaneously as delivery versus payment so neither side is exposed to the other's failure mid-transfer. The custodian is the client's agent here: it holds the securities, receives or delivers against the net obligation, updates the client's position, and handles subsequent income and corporate-action entitlements. The custodian is not the clearing house, and conflating the two is a common and visible error.

The United States, Canada and Mexico moved to T+1 in May 2024, and India completed its move earlier. The EU and UK have committed to following. Compressing the cycle removes a full day of slack from everything upstream: allocation and affirmation that could once drift into the next morning now have to complete on trade date, funding and securities lending recalls have to be arranged the same day, and any manual exception process becomes a same-day process. That is the real engineering consequence of T+1, not the arithmetic.

What fails here: a settlement fail. The seller does not deliver, usually because the securities are on loan, unrecalled, or expected from an unsettled purchase. The trade does not vanish; it stays open, is marked to market daily, and the buyer may eventually be made whole through a buy-in or a cash compensation regime, with penalties in some markets.

Where candidates lose the thread

The single thing separating a strong answer from an adequate one is treating execution as a milestone rather than the finish line. A candidate who says "the order fills and the trade is done" cannot then answer who owes what to whom on Tuesday morning, what the clearing house changed about that answer, or which failure the buyer is exposed to when the seller cannot deliver. Ownership does not transfer when the matching engine matches. It transfers when the depository moves the security against the cash, and everything between those two moments exists to make that transfer certain.

Watch where the counterparty changes hands: the broker faces the venue while executing, but from novation onwards the obligation is owed to the clearing house.

sequenceDiagram
    participant Cl as Client
    participant B as Broker OMS
    participant V as Venue
    participant CH as Clearing house
    participant Cu as Custodian
    Cl->>B: NewOrderSingle
    B->>B: Pre-trade risk checks
    B->>V: Child orders from the router
    V-->>B: ExecutionReport per fill
    B-->>Cl: Fills and average price
    Cl->>B: Allocation instruction
    B->>CH: Affirmed trade
    Note over CH: Novation and netting
    CH->>Cu: Net delivery versus payment on T plus 1

Execution establishes an obligation; settlement discharges it, and the clearing house exists so that the party you are obliged to is not the one who might fail. If you can hold those three roles apart, the rest of the lifecycle is detail.