Design Code Architecture

Design Code Architecture

Design the architecture for a new app: get the small number of expensive-to-reverse decisions right and stay aggressively simple everywhere else. This is an interactive, resumable journey of eight phases — the agent asks before every decision and records the outcome in your project's docs/ folder, so you can stop after any phase and resume later. It runs from the most foundational and hardest-to-reverse (boundaries, domain) through the tunable (data, resilience) to the cross-cutting disciplines (complexity, reversibility, scope) you apply throughout. A weekend project uses three phases lightly; a funded team building toward launch wants the whole stack.

Core Principle

Architecture is the set of decisions that are expensive to reverse: make exactly those deliberately, and defer everything cheap. This skill sequences the phases, asks the decision questions, and records every choice in docs/. The constituent skills carry the method — invoke them rather than improvising their frameworks. The whole strategy is to convert expensive decisions into cheap ones by putting a boundary in front of them, so the irreducibly expensive set stays small enough to get right with care.

Journey Map

Operating Rules

1. Resume first. Before anything else, read docs/DESIGN-CODE-ARCHITECTURE-PLAN.md and every artifact in the Journey Map. If the tracker exists, summarize the journey state in 3-5 lines and ask which phase to enter. Done when the user has confirmed an entry point. A journey with a tracker is resumed, never restarted.
2. Intake on first run only. No tracker: run the Intake below, then create docs/DESIGN-CODE-ARCHITECTURE-PLAN.md with every phase statused pending | in-progress | awaiting-evidence | done | deferred: reason | skipped: reason. Done when the tracker exists and the user has confirmed the phase plan.
3. Phase entry. Announce: what the phase does, the decision it forces, the artifact it produces, rough effort. Offer proceed / skip / defer — phases marked GATE may be deferred, never skipped. Mark the phase in-progress on proceed. Done when the user chose.
4. Skill invocation and fallback. Load the phase's skill and use it: each phase's Invoke line names the skill by slug — use that skill to run the phase. If it is not available, offer: npx skills add wondelai/skills/<slug> --global. If the user declines, run the phase from its Brief — the minimum viable method. State which mode you are in.
5. In-phase decisions. Ask every question under "Decide with the user" — with concrete options and your recommendation. Record the choice in the tracker's Key Decisions. A decision made silently is a defect.
6. Phase exit. Present the draft artifact content for sign-off before writing. On approval: write or extend the docs/ files, update the tracker (status, Key Decisions, Next Actions). Done when the files are written and the phase row shows done.
7. Artifact discipline. Read before writing; create a file only if missing, otherwise extend — add or update your sections, preserve everyone else's. Files are UPPERCASE in docs/. Every recommendation lands as a checkbox or a table row with owner and priority. See references/artifact-templates.md when creating a docs/ file for the first time — create it from the full skeleton (all section headings), then fill the sections your phase names.
8. Every expensive-to-reverse decision gets a Decision Log row (decision, why, alternatives rejected) before any code assumes it. Default to a modular monolith: services split only along proven bounded contexts.

Intake

Ask these before creating the tracker:

1. What will the app do, and what is the one feature that is genuinely your competitive advantage? (frames the core subdomain in Phase 2 — where to invest deep modeling versus buy off-the-shelf)
2. What stack are you leaning toward — language, web framework, ORM, database? (gates the Phase 1 boundary and Phase 4 data decisions; treated as a detail, never the skeleton)
3. What load is realistic in year one — rough daily active users and the main actions each takes? (gates Phase 3 sizing: requirements before solutions)
4. What outbound dependencies will it call — payments, email, LLM APIs, shipping, queues? (gates the Phase 6 integration-point audit)
5. Which data is the system of record, and are there second read patterns like search, analytics, or feeds? (gates Phase 4 consistency and derived-data decisions)
6. Has anyone validated that people actually want this app? (if not, route to create-business / create-app first — do not architect an unvalidated idea)
7. How many teams will own this system, and how much of the journey do you want now? (gates the team-topologies optional phase and the Phase 8 appetite)

Phase-skip heuristics: skip Phase 3's scaling machinery and most of Phase 4's replication when year-one load is far below any threshold (a single indexed DB is the answer — record it and move on); skip the team-topologies optional phase for a single-team app. Never skip Phase 1 or Phase 2 — boundaries and the domain model are the additive work that makes every later decision cheap; Phase 6 resilience is not optional once real users and outbound calls exist. Then create the tracker from the template and confirm the plan.

Done when docs/DESIGN-CODE-ARCHITECTURE-PLAN.md exists with every phase statused and the user has confirmed the plan.

Phases

Phases run in the listed order, from hardest-to-reverse to tunable to cross-cutting — each assumes the previous phase's artifact exists. Any phase can be entered, skipped, or deferred per the Operating Rules; Phases 1-2 are the additive work that makes everything after them cheap to change.

The phases form a dependency chain that mirrors the system: Domain-Driven Design says where the boundaries belong (contexts and aggregate seams); Clean Architecture says which way dependencies cross them; Data-Intensive Apps decides what lives inside them at the persistence layer; System Design says how much infrastructure that actually requires — usually far less than feared. Software Design keeps the modules deep instead of multiplying into shallow ceremony, Release It! hardens the integration points, Pragmatic Programmer supplies the cross-cutting habits that hold the structure over time, and the 37signals Way governs the whole thing by fixing time and cutting scope.

Phase 1 — Draw the boundaries (clean-architecture)

Purpose: Keep business rules independent of the framework, database, and vendors so every later decision stays swappable — the move that buys back all the others.

Brief (fallback): The Dependency Rule — source-code dependencies point inward: Frameworks → Interface Adapters → Use Cases → Entities; nothing inner names anything outer. Database, web, and vendors are details, plugins to your rules. Enforce with Dependency Inversion: a use case owns a repository interface; the Postgres/Stripe implementation lives in an outer adapter. Draw full boundaries only at real volatility (DB, external services, delivery); collapse layers elsewhere — direction matters, not folder count.

Invoke: Use the clean-architecture skill with a concrete first feature and the stack from intake. Ask it to layer that feature (entities, a use case with request/response models, repository + gateway interfaces, the HTTP controller and DB adapter in the outer ring), and to flag which boundaries are ceremony versus earning their cost at real volatility.

Decide with the user: (1) Modular monolith versus services — default to a modular monolith with clean internal boundaries; a microservice with a shared data model is a distributed monolith, strictly worse. (2) Which volatility points get full boundaries with interfaces now versus collapsed layers.

Artifact: Create docs/ARCHITECTURE.md with ## System Context (what it does, integrations), ## Layer Map & Dependency Rule (layers, what depends on what; violation | location | fix | status), and the monolith-versus-services choice in ## Decision Log (date | decision | why | alternatives rejected). Update the tracker.

Done when: the layer map exists, the first feature is layered with framework/ORM types confined to the outer ring, the core is designed to test with no DB/web/framework, the monolith-versus-services decision is a Decision Log row, and Phase 1 shows done.

Phase 2 — Model the domain (domain-driven-design)

Purpose: Put boundaries where the business actually splits and make the code speak the domain — cheapest now, inventing the vocabulary from a blank page.

Brief (fallback): The model is the code — build a Ubiquitous Language so team words are code words. Name after domain concepts (Order.place(), not OrderManager.process()); a name that resists is a design signal, not an annoyance. Bounded contexts: a region where a word means exactly one thing ("Customer" differs in billing versus support) — these are your future service seams. Aggregates: a small root cluster enforcing invariants, immediately consistent inside and eventually consistent outside; reference other aggregates by ID. Push behavior into entities — no anemic data bags.

Invoke: Use the domain-driven-design skill with the domain vocabulary and the Phase 1 layer map. Ask for the bounded-context map built from the words the team actually uses, the core aggregates with their invariants, and a subdomain classification (core / supporting / generic).

Decide with the user: (1) Where the same word legitimately means different things across contexts — do NOT unify into one omniscient model. (2) Which subdomain is core (invest deep modeling) versus generic (buy or use OSS — auth, email, payments).

Artifact: Extend docs/ARCHITECTURE.md: ## Bounded Contexts & Context Map (contexts, relationships, anti-corruption layers) and ## Domain Glossary (Ubiquitous Language) (term | meaning | code name); record aggregate and core-domain choices in ## Decision Log. Update the tracker.

Done when: contexts are mapped with their relationships, the glossary names the core terms, each aggregate states its invariants and by-ID references, the core subdomain is chosen, and the context boundaries line up with the Phase 1 layer map.

Phase 3 — Size the system honestly (system-design)

Purpose: Prove with numbers how small the system can be, so you skip the machinery you cannot justify.

Brief (fallback): Start with requirements, not solutions. Back-of-envelope: QPS = daily-active-users × actions/day ÷ 86,400, peak 2-5× average; storage = records/day × size × retention. For hundreds-to-thousands of users, a single indexed DB plus a read-path cache carries you a long time. Scale in order: vertical first, then cache-aside (TTL + explicit invalidation), then read replicas, and shard last, only with evidence. Reach for a message queue to decouple slow/spiky work, a CDN for global static assets. Premature sharding and premature service-splitting are named mistakes.

Invoke: Use the system-design skill with the load reality from intake. Ask for average and peak QPS, yearly storage, which component bottlenecks first, and a plain list of the techniques (sharding, replicas, CDN, queues, multi-region) you do NOT need yet.

Decide with the user: Which scaling moves to make now versus defer — tied to the numbers (don't build for 50k users while at 50) — and the first slow workload, if any, to move behind a message queue.

Artifact: Extend docs/ARCHITECTURE.md ## System Context with the load reality and back-of-envelope numbers; record each scaling move (adopt now / defer with trigger) in ## Decision Log. Update the tracker.

Done when: average/peak QPS and yearly storage are written down, the first bottleneck is named, and every scaling technique is either adopted with a reason or deferred with the number that would trigger it.

Phase 4 — Make deliberate data decisions (ddia-systems)

Purpose: Get the layer that outlives the code right — data model, storage engine, and consistency chosen by access pattern, not habit.

Brief (fallback): Data outlives code. Match model to access pattern — relational for many-to-many and ad-hoc queries, document for self-contained aggregates with locality, graph for recursive traversals; storage engines trade reads against writes (LSM write-throughput versus B-tree read-latency). Most databases default to read-committed or snapshot, NOT serializable — naive read-then-write triggers write skew (two buyers taking the last unit). Lock explicitly (SELECT ... FOR UPDATE) or use a serializable transaction where invariants demand it. Single-leader + read replicas is the read-heavy default; replication lag forces deliberate read-your-writes. Separate system-of-record from rebuildable derived data.

Invoke: Use the ddia-systems skill with the workloads implied by the Phase 2 aggregates and the Phase 3 replica plan. Ask for a per-workload model + storage-engine fit, the actual default isolation level and its anomalies, and which read-then-write paths need locking.

Decide with the user: (1) One datastore versus polyglot persistence, per workload fit. (2) Which paths get a lock or serializable transaction versus tolerate eventual consistency; whether a second read pattern (search, analytics) justifies derived data kept in sync by CDC.

Artifact: Extend docs/ARCHITECTURE.md ## Data & Storage Decisions (models, engines, isolation level, locked paths, system-of-record versus derived) and log the reasoning in ## Decision Log. Update the tracker.

Done when: each workload has a model + engine chosen by fit, the default isolation level is documented, every write-skew-prone path is locked or serializable, and any derived data has a defined sync mechanism.

Phase 5 — Keep modules deep (software-design-philosophy)

Purpose: Stop the structure from becoming its own disease — hide machinery behind simple interfaces instead of shattering into shallow classes.

Brief (fallback): Complexity is the enemy; the test for every decision is whether it makes the whole system simpler. Module depth = functionality ÷ interface complexity — deep modules hide power behind small interfaces; shallow ones (classitis) add interface cost without hiding complexity. Clean layering and deep modules are allies; clean layering and classitis are not. Information leakage — one design decision reflected in many modules — is a top red flag; encapsulate each piece of knowledge once. Strategic over tactical: invest 10-20% to keep the design clean; startup shortcuts compound into debt as the team grows.

Invoke: Use the software-design-philosophy skill with the module set proposed in Phases 1-2. Ask which modules are shallow pass-throughs to consolidate, where knowledge leaks across boundaries, and whether any planned boundary is ceremony rather than depth.

Decide with the user: Which shallow modules to consolidate into deeper ones now, guarding against over-merging genuinely unrelated concerns; the design conventions the team adopts (naming, where behavior lives, one file per piece of knowledge).

Artifact: Extend docs/TECH-DEBT.md ## Smell Inventory (shallow-module / information-leakage entries with the consolidation applied) and record the agreed rules under ## Adopted Conventions. Update the tracker.

Done when: each shallow-module cluster is consolidated or logged with a fix, no single design decision is duplicated across modules, and the design conventions are written down.

Phase 6 — Design for failure (release-it)

Purpose: Make the system degrade gracefully instead of collapsing when a dependency is slow or down — cheapest to design in now, not at 2 a.m.

Brief (fallback): The software that passes QA is not what survives production. Integration points are the number-one killer and a slow response is worse than none — a hanging dependency exhausts threads and pools with nothing in the logs. Non-negotiable: connect + read timeouts on every outbound call; a circuit breaker on critical ones (trips open, fails fast, half-open recovery); bulkheads to isolate pools per dependency; retry with backoff + jitter. Paginate every list endpoint (unbounded result sets crash under real data); schedule steady-state cleanup. Decouple deploy from release with feature flags and backward-compatible expand-contract migrations.

Invoke: Use the release-it skill with the outbound dependencies from intake. Ask for timeout values and breaker thresholds per dependency, bulkhead placement, a graceful-degradation path per integration, and the deep-health-check + RED-metrics + expand-contract-migration essentials.

Decide with the user: Breaker thresholds, which dependencies get dedicated pools, how core flows degrade when a non-critical dependency is down, and the rollback path you trust. Resist chaos engineering / multi-region failover for the first thousand users.

Artifact: Create docs/RELIABILITY.md with ## Integration-Point Audit (dependency | timeout | circuit breaker | bulkhead | retry policy | status), ## Query & Resource Findings, ## Health Checks & Metrics, and ## Deploy vs Release. Update the tracker.

Done when: every planned outbound call has a timeout, critical dependencies have breakers and bulkheads, every list endpoint is paginated, a deep health check + RED metrics + expand-contract migration + trusted rollback are specified, and the audit has no open rows for critical paths.

Phase 7 — Prove the wiring and lock in habits (pragmatic-programmer)

Purpose: Build one thin real slice through every layer to prove the boundaries connect, and set the habits that keep the architecture reversible.

Brief (fallback): Tracer bullet — build one thin but fully real vertical slice (HTTP → use case → repository → DB → back), kept as production code, for end-to-end feedback on day two and proof the boundaries link before you flesh them out. Reversibility: abstract every vendor behind your own interface (forking-road test — could you swap DB or LLM provider in a week?). Orthogonality: a dramatic change to one requirement should touch one module. DRY for knowledge, not coincidence — merge duplicated rules, leave look-alikes alone. Broken Window: fix the first hack or board it up with a tracked ticket.

Invoke: Use the pragmatic-programmer skill with the Phase 1 boundaries. Ask for the thinnest end-to-end tracer bullet that exercises every layer, an adapter interface for each vendor, and an audit of where one change would touch many modules or a vendor API would leak into business logic.

Decide with the user: Which slice is the tracer bullet (one authenticated core action, minimal functionality); the broken-windows policy and debt budget per iteration; which vendors get an owned interface first.

Artifact: Extend docs/TESTING.md ## Test Strategy, ## Safety Net Map (the tracer-bullet path as the first end-to-end test), and ## CI Gates; extend docs/TECH-DEBT.md ## Debt Budget & Broken-Windows Policy and ## Adopted Conventions (reversibility, orthogonality). Update the tracker.

Done when: the tracer-bullet slice runs end-to-end through every layer and is pinned as the first CI gate, each vendor sits behind an owned interface, and the broken-windows policy and debt budget are written down.

Phase 8 — Cut scope to the essential (37signals-way)

Purpose: Decide whether any of this ships — fix time, flex scope, and delete speculative abstraction before it becomes complexity you carry.

Brief (fallback): Build less — the best products do fewer things well; half a product beats a half-assed one. Fix an appetite (the time this work is genuinely worth) and cut scope to fit, rather than estimating an open-ended architecture that balloons. YAGNI: every speculative abstraction (generic plugin system, event sourcing, configurable multi-tenancy for zero users) is a decision deferred to an imaginary future at the cost of present complexity. Make tiny reversible decisions; say no by default so the great decisions breathe. Never cut the small set of expensive-to-reverse decisions.

Invoke: Use the 37signals-way skill with the full architecture plan from Phases 1-7. Ask it to shape the work into a fixed appetite, separate essential-for-launch from gold-plating, name the rabbit holes, and list the speculative abstractions to delete or replace with the simplest thing that could work.

Decide with the user: The appetite for v1 architecture work; which abstractions to cut now, defer with a revisit trigger, or replace with the simplest thing; confirm no expensive-to-reverse decision is being cut just to save time.

Artifact: Extend docs/ARCHITECTURE.md ## Decision Log with what is deliberately NOT built for v1; extend docs/TECH-DEBT.md ## Debt Ledger (deferred abstractions as deliberately-taken debt, each with the trigger that would revisit it). Update the tracker.

Done when: v1 scope is fixed to an appetite, every cut or deferred abstraction is a Decision Log or Debt Ledger row with a revisit trigger, and no expensive-to-reverse decision was cut for time.

Optional Phases

Optional phases follow the same operating rules — load and use each listed skill exactly as a core phase would; insert where the Add-when condition first becomes true — here, right after Phase 2, once the bounded contexts that team boundaries must mirror exist.

Common Mistakes

Completing the Journey

Match the dose to the project: a weekend build leans on the Phase 1 Dependency Rule, a quick Ubiquitous Language, timeouts on outbound calls, and the Phase 8 instinct to cut scope — a few hours that save weeks. A funded team building toward launch works the whole stack, pulling the data and resilience phases in as real bottlenecks and integration points appear.

Exit checklist — every box tied to an artifact:

• Each expensive-to-reverse decision (boundaries, contexts, data/consistency) is a Decision Log row with alternatives rejected (ARCHITECTURE.md).
• Core business rules are designed to run with no DB, web, or framework (ARCHITECTURE.md Layer Map, no inward-pointing violations).
• Every outbound call has a timeout, critical ones have breakers, and every list is paginated (RELIABILITY.md Integration-Point Audit clear).
• A tracer-bullet slice proves the boundaries connect end-to-end and is the first CI gate (TESTING.md).
• v1 scope is fixed to an appetite with speculative abstractions cut or deferred with a trigger (TECH-DEBT.md Debt Ledger).

Close the tracker: every phase done or skipped: reason, with remaining Next Actions carried into the ARCHITECTURE.md Decision Log and TECH-DEBT.md so nothing is lost. Then route forward: when the architecture serves a product that still needs validating and building, continue with the create-app skill; when an existing prototype must be brought up to this structure, continue with the improve-code-quality skill.