03 architecture review

Architecture Review

Purpose

Evaluate architecture, coupling, duplication, integration fit, and whether to keep, adapt, or replace existing structures. Every Keep, Adapt, or Replace decision must be justified with file-level evidence — churn, blast radius, coupling — not intuition. The default bias is toward the smallest change that fits the existing structure; a rewrite is recommended only when the explicit rule below is met, and never by standing up a parallel app or repo.

When to use

• A new feature or refactor touches multiple modules and you need to confirm it follows the existing dependency direction before committing to a plan.
• Circular dependencies, god modules, or competing implementations of the same concern are suspected.
• A pull request or design proposal introduces a second way of solving something already solved (two HTTP clients, two state stores, conflicting conventions).
• Before a larger refactor, to establish which components to Keep, Adapt, or Replace and prove the blast radius is bounded.

When not to use

• The task is unrelated to core operations work.
• The work would require production deploys, destructive data actions, or secret disclosure.
• You first need a structural map or a runnability baseline — use codebase-cartography or repo-diagnostics, then return here.
• A narrower skill or existing project instruction already covers the need.

Procedure

1. Build a module map. List the top-level directories, each one's responsibility, and the dependency direction between them. Locate the boundary between UI, domain or business logic, data access, and infrastructure.
2. Detect cycles and illegal dependencies. Find inversions where lower layers depend on higher ones (UI importing the DB client, domain importing a framework, a shared core importing a routes module).
3. Find duplication. Identify parallel implementations of one concern (two HTTP clients, two state stores, two date utils), copy-pasted logic, and competing patterns for the same job.
4. Assess integration fit for the change. Does the proposed work follow existing data-fetching, error-handling, auth, and state conventions, or introduce a second way of doing the same thing?
5. Decide Keep, Adapt, or Replace for each questioned component using the rule below, justified with evidence (churn, blast radius, coupling).
6. Surface the highest-risk files (high fan-in plus high churn) and state the smallest viable change that fits the existing structure.

Concrete checks

Dependency direction:

• Nothing in domain or core imports from ui, routes, or a framework package.
• Lower layers do not import higher ones; the dependency graph points one way.
• No business rule is embedded directly in a UI component or route handler.

Duplication:

• No two files export the same function name for the same purpose.
• There is one HTTP layer, not an axios wrapper plus bare fetch calls.
• There is one state solution for a given slice of data, not Redux plus Zustand plus Context.

Coupling and fit:

• Changing one feature does not force edits across five or more unrelated files.
• A new module follows the existing data-fetching, error-handling, and config conventions.
• No god module is imported by twenty or more files.
• The proposed change reuses an existing seam rather than introducing a parallel one.

Decision readiness:

• Each questioned component has churn and fan-in evidence attached.
• Any Replace verdict carries a concrete migration path and a rollback.
• The smallest change that fits the structure is identified, not just the cleanest.

Keep / Adapt / Replace rule

• Keep: the pattern is consistent, tested, and the change fits without contortion.
• Adapt: the structure is mostly right but needs a seam — extract an interface, add an adapter, narrow a type. This is the lowest-risk default.
• Replace: only when the existing structure blocks the goal, is duplicated, or is unmaintained, AND a migration path plus rollback are defined. Never replace by standing up a parallel app or repo.

Commands

# --- cycles / orphans ---
# circular deps and orphans (JS/TS)
npx madge --circular --extensions ts,tsx,js,jsx src
npx madge --orphans --extensions ts,tsx src

# --- dependency direction ---
# does domain/core import from ui/routes? (inversion)
rg -n "from ['\"].*(ui|routes|components|pages)" src/domain src/core 2>/dev/null

# boundary leaks: DB calls inside components/handlers
rg -n 'prisma\.|db\.query|sql`' src/components src/app 2>/dev/null

# --- duplication ---
# duplicate exported symbols (parallel implementations)
rg -oN 'export (?:function|const|class) (\w+)' -r '$1' src | sort | uniq -d

# competing HTTP or state libraries in the same codebase
rg -n "from ['\"](axios|node-fetch|redux|zustand|jotai|recoil)" src | sort | uniq -c

# --- god modules / fan-in ---
# most-imported local modules (god-module candidates)
rg -oN "from ['\"](\.[^'\"]+)['\"]" -r '$1' src | sort | uniq -c | sort -rn | head -20

# --- churn / risk ---
# high-change files are refactor risk
git log --since=6.months --name-only --pretty=format: | sort | uniq -c | sort -rn | head -20

# --- convention drift ---
# multiple error-handling shapes across the codebase
rg -n 'try\s*\{|\.catch\(|Result<|Either<|throw new' . | head

# multiple data-fetching patterns for the same concern
rg -n 'useQuery|useSWR|getServerSideProps|loader\(|fetch\(' . | head

# --- layer boundaries ---
# framework types leaking into the domain layer
rg -n "import .*(react|express|next|@nestjs)" src/domain src/core 2>/dev/null

# --- duplication depth ---
# copy-pasted blocks via repeated function signatures
rg -oN 'function (\w+)\(' -r '$1' src | sort | uniq -c | sort -rn | head

Common issues & anti-patterns

• Dependency inversion: a domain module imports a React component or an Express type, so the business logic can no longer be reused or tested in isolation.
• Parallel HTTP layers: half the app uses an axios instance with interceptors and the other half uses bare fetch, so retries, auth headers, and error handling diverge.
• Competing state stores: the same server data lives in both a Redux slice and a Zustand store, and they drift out of sync.
• God util: a helpers.ts imported by thirty files; any change risks an unrelated regression and the file resists testing.
• Replace-by-parallel-app: instead of adapting the existing structure, a second app or package is stood up "to do it cleanly", doubling the maintenance surface and the drift.
• Silent convention fork: a new feature introduces its own error-handling shape, so error responses are now inconsistent across endpoints.
• Unbounded blast radius: a change scoped to one feature quietly edits eight shared files — a signal the seam is in the wrong place.
• Circular dependency between layers: service imports controller and controller imports service, creating load-order fragility and untestable units.
• Premature abstraction: a generic base class or plugin system built for one caller, adding indirection that obscures the single real path through the code.
• Adapter avoided: a new third-party client wired directly into twelve call sites instead of behind one adapter, so swapping or mocking it later means touching all twelve.
• Replace without rollback: a proposal to swap a core module with no migration sequence or fallback, so a mid-migration failure leaves the app in an unshippable state.
• Leaky repository: a data-access layer that returns raw ORM entities up to the UI, so the database schema and the view are coupled and a column rename breaks the frontend.
• Convention by copy-paste: each new endpoint copies the previous one's structure including its quirks, so a single bad pattern propagates instead of a shared helper.
• Hidden temporal coupling: two calls that must happen in a specific order with nothing in the types or signatures enforcing it, so a reordering silently breaks behavior.
• Shotgun surgery: adding one field requires edits in the model, the DTO, the mapper, the validator, and three components, signaling responsibilities are smeared across layers.
• Anemic seam: an interface exists but every implementation is identical and there is only one, so the abstraction adds indirection without buying flexibility.
• Framework lock in the core: business rules written directly against a framework's request or ORM objects, so the logic cannot be unit-tested without booting the framework.
• Duplicated date/money logic: two or three hand-rolled helpers for the same formatting or arithmetic, which drift apart and produce inconsistent values across screens.
• Conditional sprawl: a single function that branches on a type flag everywhere instead of polymorphism, so each new variant edits many call sites rather than adding one implementation.

When a rewrite is genuinely warranted

Recommend Replace only when all three hold: the existing structure provably blocks the goal (not merely inconvenient), the blast radius of adapting it exceeds the cost of replacing it, and a staged migration with a rollback at each step is defined. Even then, replace the component in place behind its existing seam — never by standing up a second app, repo, or framework alongside the first.

Required output

Return: a module and dependency map; coupling and duplication findings with file:line evidence and severity; a Keep, Adapt, or Replace verdict per questioned component with rationale; the high-risk file list (high fan-in plus high churn); and the smallest change that fits the existing architecture. Do not recommend a rewrite unless the Keep/Adapt/Replace rule is met, and when Replace is chosen include the migration path and rollback.

Safety

• Review-only by default; do not begin large refactors without an approved plan and a rollback path.
• Do not introduce a parallel framework, second app, or duplicate repo "to do it cleanly".
• Base every claim on file evidence, not assumption.
• Flag — do not silently make — cross-cutting changes that exceed the task scope.
• Redact any secret-like values surfaced while reading config.

Completion criteria

Done means the architecture is mapped from evidence; coupling and duplication findings are concrete with file:line; each questioned component has a Keep, Adapt, or Replace decision with rationale (and any Replace carries a migration plus rollback path); the high-risk files are listed; and the recommended path is the smallest one that fits the existing structure.