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-query-best-practices
Data fetching/caching	tanstack-query-best-practices
Mutations	tanstack-query-best-practices
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 domain types; never expose raw API response shapes.
• 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 use in hooks, prefetching, and route loaders.
• 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. Co-locate queryKey and queryFn in reusable option factories. Never scatter the same query key across multiple files.
2. Unidirectional dependency flow. adapters -> lib -> hooks -> components. Adapters never import from hooks or components.
3. Scope query keys. Any query depending on an authenticated scope (user, org, tenant) must include that scope ID in its key to prevent stale cross-scope data.
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. Always invalidate after mutations. Use queryClient.invalidateQueries in onSettled to ensure eventual consistency with the server.
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 callback rolls back the cache to the snapshot from onMutate, then onSettled invalidates to refetch fresh data.
• 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