A financial trading terminal is two systems sharing one screen. The market-data side consumes a fast, ordered, entitlement-scoped stream and turns it into a bounded visual projection. The order side handles private, safety-critical commands whose states come only from an authoritative service. Keeping those lifecycles separate is the foundation of a trustworthy design.
A demo terminal can subscribe to a WebSocket and set React state for every quote. It looks convincing with ten symbols and a calm feed. Production pressure appears when thousands of messages arrive each second, one packet is missed, a reconnect starts a new session, a hidden tab stops painting, or an order acknowledgement crosses a stale market snapshot.
The interview is not a contest to name chart libraries. It is a test of whether the design preserves sequence, precision, account ownership, and user control while still feeling immediate.
Clarify the trading product and its authority
Assume a browser terminal for active but human-paced trading. Users maintain watchlists, inspect quotes and charts, create price and volume alerts, enter and amend orders, and monitor positions, balances, fills, and connection health. Market data may update far faster than the screen needs to paint. Orders are submitted to a trusted service that applies authentication, permissions, trading-session rules, tick and lot validation, risk checks, and venue routing.
Ask which instruments and markets are supported, whether data is delayed or real time, how entitlements vary by user, which order types exist, whether pre-trade risk checks are synchronous, and what actions are allowed when prices are stale. Also ask about desktop density, mobile scope, multi-window use, regulatory audit needs, and recovery objectives.
The design below gives desktop users a dense workspace and mobile users a focused monitor with guarded order actions. It does not put exchange credentials, risk rules, or authoritative balances in the browser.
The public interview prompt
Design a browser-based financial trading terminal. Explain:
- snapshots, incremental market-data events, sequence numbers, gaps, reconnects, and stale-state policy;
- workers, binary decoding, normalization, coalescing, rendering cadence, charts, watchlists, and memory limits;
- decimal precision, currency, tick size, lot size, price formatting, and derived values;
- order tickets, validation, idempotent submission, acknowledgements, fills, cancellation, and reconciliation;
- private account streams, entitlements, risk controls, alerts, audit receipts, and multi-tab behavior;
- keyboard navigation, color-independent status, live-region policy, responsive layouts, degraded states, telemetry, testing, and rollout.
What the premium solution covers
The complete solution begins with a feed-session receipt and follows ordered deltas through gap detection, snapshot repair, worker reduction, frame-bounded publication, and an accessible quote surface. It then separates that public market projection from private order and account state, including exact-value contracts, idempotent commands, risk gates, reconciliation, alerts, hidden tabs, and failure policy.
Original media includes a frame-verified sequence-recovery animation, a market-versus-account trust map, a rate-conversion visual, and a degraded-mode action matrix. A deterministic render-budget lab lets the reader vary feed rate, subscriptions, visible rows, paint cadence, decode cost, paint cost, and alert pressure.