# DESIGN — F-004 Typed config and feature flags

## Layering
- `src/infrastructure/config/config.ts` — pure config loader. Owns env parsing and
  validation. No side effects on `process.env`; takes an env object so tests are
  deterministic. Throws a single `ConfigError` listing every problem found.
- `src/modules/flags/` — the feature-flag module. Business-facing, boundary-checked.
  Exposes only `index.ts`. Contains the `FeatureFlagProvider` interface and a simple
  in-memory store. Does NOT depend on infrastructure/config (kept independent so it
  can later be backed by a DB/external service without touching config).

## Config loader (infrastructure)
```ts
export interface AppConfig {
  nodeEnv: 'development' | 'test' | 'production';
  port: number;
  host: string;
  logLevel: string;
  databaseUrl: string;      // required
  redisUrl?: string;        // optional
  flags: Record<string, boolean>; // parsed FLAG_* vars (initial seed)
}
export class ConfigError extends Error { problems: string[] }
export function loadConfig(env: StringRecord): AppConfig
```
- `DATABASE_URL` required: missing -> ConfigError naming the var.
- `PORT` parsed to int, validated 1..65535.
- `NODE_ENV` whitelisted, default `development`.
- `FLAG_<NAME>` vars: value `true`/`false` (case-insensitive) -> boolean; other values
  -> ConfigError naming the var. Names lowercased after stripping `FLAG_` prefix.
- Accumulate all problems, then throw once (fail fast AND clear).

## Feature flag module
```ts
export interface FeatureFlagProvider {
  isEnabled(name: string): boolean;
}
export class InMemoryFeatureFlagStore implements FeatureFlagProvider {
  constructor(initial?: Record<string, boolean>)
  isEnabled(name): boolean        // unknown flag -> false (fail-safe default)
  setEnabled(name, enabled): void // runtime mutation, no redeploy needed
}
export function createFlagStore(initial?): InMemoryFeatureFlagStore
```
- Unknown flags are OFF by default (fail-safe: risky paths stay skipped).
- `setEnabled` mutates live state: deployment ≠ activation.
- Guarded-path helper kept trivial; consumers do `if (flags.isEnabled('x')) { ... }`.

## Wiring (composition root)
- `src/app/build-app.ts` stays DB-free for now; but `buildApp` accepts an optional
  `flags` dep so routes can guard paths. Default: empty store (all flags off).
- `src/infrastructure/http/server.ts` calls `loadConfig(process.env)`, uses typed
  `port`/`host`, and passes `config.flags` into the app. On ConfigError it logs the
  problems and exits non-zero.
- `pool.ts` keeps `createPoolFromEnv` but gains an optional typed overload note; no
  breaking change needed this ticket (config owns DATABASE_URL validation at startup).

## Boundary notes
- `flags` module: only imports from `shared` (currently nothing needed). It must not
  import `infrastructure/config`. Config parses FLAG_* into a plain record and hands it
  to the module at composition time — inversion keeps the module clean.
- Boundary checker will enforce this automatically.

## Files
| File | Role |
|---|---|
| src/infrastructure/config/config.ts | typed loader + ConfigError |
| src/infrastructure/config/tests/config.test.ts | loader unit tests |
| src/modules/flags/index.ts | module public API |
| src/modules/flags/domain/feature-flag-store.ts | interface + store |
| src/modules/flags/tests/feature-flag-store.test.ts | store unit tests |
| src/app/build-app.ts | optional flags dep injection |
| src/infrastructure/http/server.ts | loadConfig + typed listen + ConfigError exit |

## Risks / mitigations
- Silent flag default: unknown -> false, documented; tested.
- ConfigError leaking secrets: messages name var NAMES only, never values.
- Boundary violation (flags importing config): prevented by composition-time injection + checker.
