Design a Stock Exchange — Visual Design Journey

What You Should Understand Now

• Price-time priority is deterministic: when two resting orders share a price (S1 and S2 both at $101.00), the one with the earlier sequence number is always filled first — fairness comes from sequencing, not from which request "arrives" fastest over the network.
• An accepted order is never the same thing as an executed trade — a limit buy below the best ask simply rests on the book, and a limit price is a ceiling on what you will pay, never a promise of what you will pay, as the multi-level sweep in Test B demonstrates.
• IOC and FOK orders answer the same question — "what if the book can't fill me completely?" — in opposite ways: IOC takes the partial fill and cancels the rest, while FOK requires the entire order to be fillable right now or it executes nothing at all.
• A cancel request racing a matching order is resolved by whichever the sequencer serialized first — a completed fill can never be retroactively undone by a cancel that is sequenced after it.
• A single-writer matching engine per instrument is what makes price-time priority provably correct without locks, but durability requires a journal committed before acknowledgment, and failover requires epoch fencing so two engines never believe they own the same book at once.
• Market data is a broadcast of the truth, not the truth itself: it has its own sequence numbers, a subscriber detects gaps and resynchronizes from a snapshot, and none of this is allowed to sit on the matching engine's hot path.

Concepts in This Visual

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