What is an engineering interview testing when the work is market infrastructure rather than a generic service? This note describes the test, using a short exercise about a Nasdaq message sequence as the public example, and it applies to conversations in New York, London, and Hong Kong.
The puzzle is a sample of the job
An engineering interview is a sample of how a person behaves when a specification and a failure are both in front of them. Fluency matters. Taste matters more. The interviewer wants to see whether the candidate notices an underspecified point, says so, and picks a boring, testable interpretation instead of a clever one. The same habit is the job: feeds have specifications, logs have gaps, and cleverness that cannot be replayed is a cost. Interviews in the New York, London, and Hong Kong offices are looking for that habit. They are not three different professions.
Internships are the same test at a different depth. An intern conversation still asks whether the person can read a rule and notice when the data breaks it. The difference is the amount of surrounding system the candidate is expected to invent. Inventing a whole trading architecture in an intern loop is a miss. Reading one rule carefully is a hit. Live capital decisions stay with the firm either way. The interview is not a ceremony that transfers those decisions.
An engineering interview tests whether a person will name a hole in the record and test the behavior around it, instead of filling the hole so the demo looks smooth.
A sequence of equities messages
A fair exercise uses a public shape. Suppose a short log of Nasdaq equities messages arrives with sequence numbers, and one number is missing. The candidate who fills the gap with a guessed quote has failed the exercise even if the code runs. The candidate who stops, names the hole, and asks what the consumer is supposed to see has started the job. The public example is the sequence check, not a trick about a particular stock. Apple or any other name would do. The point is the hole.
The interview also listens for how the candidate talks about tests. A replay of yesterday's log, a comparison against a reference decoder, and a flag when the clock source is unknown are the kinds of sentences that match the work. A sentence about being fast, with a number attached, does not. The firm’s public engineering notes refuse to define speed as a bare figure. An interview answer that leads with a figure has not read the work. That is true whether the room is in New York, London, or Hong Kong.
Intern candidates are not expected to have operated a production feed. They are expected to be honest about what they have operated. A class project that parsed a public file and kept a test for a missing row is relevant. A project described as if it sent orders, when it did not, is a problem. The internship exists so a person can ship a research tool or a carefully reviewed piece of supporting code. It does not exist so a person can skip the wall between a notebook and an order. An interviewer who blurs that wall is off script, in every office.
What a good answer sounds like
A good answer is specific, reversible, and bored by heroics. It says what the program should do when the sequence jumps, how that choice would be tested, and what the candidate does not know. It does not pretend that three offices are three cultures of engineering. New York, London, and Hong Kong share the specification and the replay. Geography changes the hour of the trading day a person can see out the window. It does not change the meaning of a missing sequence number.
An engineering interview is testing whether the candidate protects the record. The Nasdaq sequence is the example used here because a reader can understand the failure without a proprietary system. Internships use the same standard more gently. None of this is an offer of employment, and none of it describes a live order.
This piece does not promise an interview format, a hiring volume, or an outcome. It describes the habit being sampled, in New York, London, and Hong Kong, for both experienced conversations and internships. A missing message in a public equities feed is the example, not a riddle to prepare by rote.
