Feature Systems Guide

A "system" is a self-contained, domain-driven module that owns everything related to one domain: its API calls, query layer, hooks, components, and public API. Systems live under a systems/<domain>/ directory.

Read references/directory-layout.md for the full directory structure and naming conventions. Read references/patterns.md for annotated implementation patterns per layer.

Quick Reference

Mandatory Companion Skills

Activate alongside this skill — systems span multiple technical domains:

Situation	Activate
Any hook or component	react + tanstack
Data fetching/caching	eng-data-boundaries + tanstack
Mutations	tanstack
XState store	xstate
Utility functions	es-toolkit
Writing/fixing tests	test-antipatterns + vitest
Bug fix	systematic-debugging + no-workarounds

System Directory at a Glance

systems/<domain>/
├── index.ts               # Public API barrel — required for every system
├── types.ts               # TypeScript types for this domain
├── adapters/              # API service layer (HTTP calls, error types)
│   └── <domain>-api.ts
├── lib/                   # Pure utilities, schemas, constants, query keys
│   ├── query-keys.ts      # TanStack Query key factory
│   ├── query-options.ts   # Reusable queryOptions / mutationOptions
│   ├── <domain>-schemas.ts
│   └── constants.ts
├── hooks/                 # React hooks (queries, mutations, view-models)
│   ├── __tests__/
│   ├── use-<action>.ts    # Query hooks
│   ├── use-create-<entity>.ts  # Mutation hooks
│   ├── use-update-<entity>.ts
│   ├── use-delete-<entity>.ts
│   └── use-<domain>-view-model.ts
├── contexts/              # React contexts + providers
│   └── <domain>-context.tsx
├── stores/                # XState stores (complex async state machines)
│   └── <domain>-store.ts
├── components/            # React UI components
│   ├── stories/
│   └── index.ts
└── guards/                # Route guards / access checks

Step-by-Step: Creating a New System

Step 1 — Define types.ts

• Export clean component view-model types; preserve API page envelopes in adapters and query caches, then flatten at the hook/view-model read boundary.
• Derive from the project's API contract types when available.
• Document complex aggregated types with JSDoc explaining derivation rules and invariants.

Step 2 — Build the API service layer

• Create adapters/<domain>-api.ts.
• Use the project's HTTP client for API calls.
• Export a single namespace object: export const <domain>Api = { list, create, update, delete }.
• Export a typed error class: export class <Domain>ApiError extends Error { ... }.
• Accept signal?: AbortSignal on every function to support query cancellation.
• Keep all internal helpers (error extraction, response normalization) private to the module.

Step 3 — Add lib/query-keys.ts

export const <domain>Keys = {
  all: ["<domain>"] as const,
  lists: () => [...<domain>Keys.all, "list"] as const,
  list: (scopeId: string | null) => [...<domain>Keys.lists(), scopeId] as const,
  details: () => [...<domain>Keys.all, "detail"] as const,
  detail: (id: string) => [...<domain>Keys.details(), id] as const,
};

• Use hierarchical key structure for granular invalidation.
• Scope keys with any identifier (userId, orgId, etc.) that isolates the cache correctly.
• Use as const on every key tuple.

Step 4 — Add lib/query-options.ts

import { queryOptions } from "@tanstack/react-query";
import { <domain>Api } from "../adapters/<domain>-api";
import { <domain>Keys } from "./query-keys";

export function <domain>ListOptions(scopeId: string | null) {
  return queryOptions({
    queryKey: <domain>Keys.list(scopeId),
    queryFn: ({ signal }) => <domain>Api.list(scopeId!, signal),
    staleTime: 60_000,
    enabled: Boolean(scopeId),
  });
}

export function <domain>DetailOptions(id: string) {
  return queryOptions({
    queryKey: <domain>Keys.detail(id),
    queryFn: ({ signal }) => <domain>Api.get(id, signal),
    enabled: Boolean(id),
  });
}

• Co-locate queryKey and queryFn via queryOptions for type safety and reuse.
• Export each option factory for hooks, prefetching, route loaders, mutations, and stream writers.
• Keep stable scope/filters in the base key and continuation cursors only in pageParam.
• Always pass signal from the query context through to the API layer.

Step 5 — Write hooks

• Query hooks: Wrap useQuery with the queryOptions factories; accept a scope ID + optional { enabled? }.
• Mutation hooks: Use useMutation with proper onMutate / onError / onSettled callbacks for optimistic updates.
• View-model hooks: Compose multiple hooks for a page/shell component; return a flat object.
• Place tests in hooks/__tests__/ or co-locate as use-xxx.test.tsx.

Read references/patterns.md for complete mutation and optimistic update patterns.

Step 6 — (Optional) Add context

Create contexts/<domain>-context.tsx when query data or combined state must be shared across a component subtree without prop-drilling.

// Always nullable context — consumer hook throws if used outside provider
export const <Domain>Context = createContext<<Domain>ContextValue | null>(null);

• Export the context, provider component, and re-export consumer hooks from the same file.
• For performance-sensitive trees, split into Core / UI / Operations sub-contexts.

Step 7 — (Optional) Add an XState store

Create stores/<domain>-store.ts for complex async state machines (multi-step flows, polling, event emission).

export const <domain>Store = createStore({
  context: { ... } as <Domain>Context,
  emits: { ... },
  on: {
    someEvent: (context, event, enqueue) => {
      enqueue.effect(async () => { ... });
      return { ...context, isLoading: true };
    },
  },
});

Step 8 — Wire up index.ts

Organize the barrel with labeled sections and explicit named exports:

// Types
export type { <Domain>Type } from "./types";

// Hooks
export { use<Domain>List, use<Domain>Detail } from "./hooks";
export { useCreate<Domain>, useUpdate<Domain>, useDelete<Domain> } from "./hooks";

// Components
export { <Domain>Component } from "./components";

// Utilities
export { <domain>HelperFn } from "./lib/<domain>-utils";

// Query Keys & Options
export { <domain>Keys } from "./lib/query-keys";
export { <domain>ListOptions, <domain>DetailOptions } from "./lib/query-options";

// API
export { <domain>Api, <Domain>ApiError } from "./adapters/<domain>-api";

Critical Rules

1. Use queryOptions for co-location. Reuse option/key factories across loaders, hooks, mutations, and streams; never redeclare server-query identity at a consumer.
2. Unidirectional dependency flow. adapters -> lib -> hooks -> components. Adapters never import from hooks or components.
3. Scope query keys. Include every stable scope/filter in the base key and keep pagination cursors in pageParam to prevent cross-scope data and cache fragmentation.
4. Typed errors in the API layer. Never throw raw errors from adapters. Use a typed error class so consumers can distinguish error types without inspecting message strings.
5. AbortSignal propagation. Pass signal from the queryFn context through to every API call for proper query cancellation.
6. Follow the owner's mutation contract. Reconcile through canonical keys; invalidate on settle only when the read model requires a server reread.
7. Optimistic updates require rollback. When using cache-based optimistic updates, snapshot previous data in onMutate and restore in onError.
8. Cancel outgoing queries before optimistic updates. Call queryClient.cancelQueries in onMutate to prevent refetches from overwriting optimistic state.
9. Zod schemas in lib/. Place all Zod schemas in lib/<domain>-schemas.ts for runtime validation at API boundaries.

Error Handling

• API layer throws typed error: TanStack Query catches and exposes it via query.error.
• Mutation fails with optimistic update: onError restores the complete cache snapshot; onSettled rereads only when the owner contract requires it.
• Stale cross-scope data: Add the scope ID to the query key and verify that enabled guards check Boolean(scopeId).
• Query cancellation on unmount: TanStack Query automatically cancels in-flight queries via the signal when a component unmounts — ensure signal is propagated to the API layer.

Detailed References

• references/directory-layout.md — Full directory structure, file naming, and barrel conventions
• references/patterns.md — Annotated code patterns for the API layer, query options, hooks, mutations, optimistic updates, contexts, and stores