System Design Session Guide

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.
• 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.
• Select views only when they answer a named question, per common-architecture-diagramming: a context/container, sequence, dataflow, deployment, or state view may be used when useful; prose or a table is sufficient otherwise. Carry metric and constraint only when the design states them; never invent a number to populate a node. See phase deliverables.

HLD, LLD, and Low-Level Design Routing

• HLD answers audience-level boundaries, shaping constraints, ownership, failure domains, and the decision to make. Use context/container or prose only when that is enough; no diagram is mandatory.
• LLD (the same lane as “low-level design”) answers one component or critical flow: data/state ownership, API or event contracts, ordering, idempotency, failure behavior, and verification. Use sequence, dataflow, or state only when that view resolves a named question.
• Trace every handoff as requirement -> HLD decision -> component -> LLD contract -> verification. Give each link a stable ID and carry unresolved assumptions forward; an LLD must not silently change the HLD invariant.
• Each selected view declares audience, question, decision, scenario, invariant, scope, status, evidence, and omissions. Lifecycle is proposed|implemented|retired; evidence is a citation, not confidence. Keep evidence_kind and evidence_confidence separate per the renderer-owned diagram spec and view manifest; never infer deployment from a code/document citation.
• Views are evidence for a question, not a completeness checklist. Prefer a precise paragraph or table over a diagram that adds no decision value.

Specialist Deep-Dive Contract

• Have the user pick the 2-3 riskiest components, then send one specialist brief per component with profile, audience/question, workload/SLO/team/budget, invariant, scope, evidence status, and current HLD decision. The specialist does not re-run intake, add neighboring components, or invent numbers.
• Require options with rejection reasons, the recommended LLD contract, failure timeline/recovery, verification hooks, and any ADR reversal trigger. Merge the result back into the HLD-to-LLD trace before scoring.

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

• Stage what to build now, the enabling seam and metric threshold; record one ADR per irreversible decision with its reversal trigger, then score with system-design-review.

Design-to-Delivery Gate

• Once HLD/LLD is fixed, list bounded docs/diagram slices: exact files, evidence, acceptance, verification, integrator. Route production to the cheapest qualified configured executor if available; lead owns decisions and final review.
• If still defective after one focused correction, use the configured fallback or report BLOCKED. Log executor/model, corrections, exceptions and fallback reason; report actual usage/cost or unavailable, never assumed savings.

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
• Phase Deliverables - interview phase to artifact and diagram map
• Interview Coaching - timed mock rounds, rubric, mistakes