/system-design-methodology

name: system-design-methodology
description: "Drives an interactive system design session: classifies depth, elicits scale/SLO/consistency inputs, computes capacity, then reveals components one by one, each justified by a constraint. Use when designing a system or running a design session; diagrams go through `common-architecture-diagramming`."
metadata:
  triggers:
    keywords:
      - system design
      - design a system
      - design session
      - high-level design
      - requirements clarification
      - capacity planning
      - scale this

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. Interview practice runs through `system-design-interview-coaching`: the round on a clock, the rubric after.
• 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](references/intake-checklist.md).
• 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 `common-architecture-diagramming`: a `container` diagram (audience tech) plus `sequence` or `dataflow` for the critical path; every node carries `metric` and `constraint` from its `constraint -> component -> cost` line. Which artifact at which phase: [phase deliverables](references/phase-deliverables.md).

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](references/four-phase-process.md) - per-phase gates, outputs, escalation rules
• [Intake Checklist](references/intake-checklist.md) - question bank with defaults
• [Phase Deliverables](references/phase-deliverables.md) - interview phase to artifact and diagram map
• [Interview Coaching](../system-design-interview-coaching/SKILL.md) - timed mock rounds, rubric, mistakes