System Design Methodology

System Design Methodology

Priority: P0 (CRITICAL)

Requirements before solutions. Never draw a full architecture before numbers justify it.

Phase 0 - Classify Depth (always first)

• Quick sketch: exploratory ask, no scale numbers available, answer needed now. Assume defaults, label each one ASSUMED, skip gates.
• Full session: real build, migration, or budget commitment. Run every phase gate.
• State depth and mode (new design | review existing | interview practice) in one line, then continue.
• Escalate quick to full when a hard constraint or irreversible choice appears.

Phase 1 - Intake (gate)

• Parse request: verbs to use cases, nouns to entities, adjectives to constraints.
• Ask max 3 blocking questions per turn, each with a recommended default. See intake checklist.
• Required before design: DAU/actors, top 3 use cases, read:write ratio, latency SLO, consistency need, retention, peak shape, budget, team size.
• Freeze scope: list what is explicitly out of scope.

Phase 2 - Estimation (gate)

• Compute QPS, storage, bandwidth, and working-set memory via system-design-estimation.
• Present the numbers, name the one quantity that shapes the design, confirm before drawing anything.

Phase 3 - High-Level Design (incremental)

• Price the null option first: do nothing, buy it, or let an existing service absorb it. Rejecting it needs a stated reason, not silence.
• Start with the smallest system satisfying functional requirements: client, API, service, store.
• Add one component at a time. For each, state constraint -> component -> cost in one line. No component without a named constraint.
• Define API surface (one endpoint per functional requirement) and data ownership before optimizing.
• Diagram only after the component set is agreed, per system-design-diagramming.

Brownfield Path (review-existing mode)

• Map current state before proposing anything: components, owners, traffic, incidents.
• Measure, do not assume: pull real QPS, data volume, and p99 from the running system.
• Find the binding constraint - the one that fails first at the next growth step.
• Design the smallest change that moves it, then re-measure. A rewrite needs a structural constraint the current shape cannot satisfy.

Phase 4 - Deep Dives and Trade-offs

• User picks the 2-3 riskiest components; go deep only there.
• Dispatch each deep dive to specialist-system-architect with the component name, its numbers, and its consistency requirement; keep the session gates in this thread.
• Close with bottlenecks, SPOFs, rejected alternatives plus rejection reasons, and the next scaling step.
• Stage the result: what to build now, the seam that enables the next step, and the metric threshold that triggers it.
• Record one ADR per irreversible decision, each with its reversal trigger - what would make us revisit this. Score the result with system-design-review.

Anti-Patterns

• No architecture before requirements: no diagram until Phase 1 answers exist or defaults are flagged.
• No unjustified components: every box names the constraint it solves.
• No design without the null option: state why doing nothing or buying loses before building.
• No silent assumptions: an unknown input becomes a labeled ASSUMED default, never a hidden guess.
• No full-stack reveal: never dump a finished diagram before incremental agreement.

Red Flags

• Stop if "just give me the architecture": deliver a quick sketch with ASSUMED labels, not fake precision.
• Stop if scale is unknown at Phase 3: return to Phase 2 and estimate from a stated assumption.

References

• Four-Phase Process - per-phase gates, outputs, escalation rules
• Intake Checklist - question bank with defaults