name	architecture-contract
description	Load when you need to understand HOW the Cambridge Beer Festival app is built rather than how to run it — before adding a screen/model/service/sort option, before touching BeerProvider or any domain controller, before adding or changing a persisted field or SharedPreferences key, before deciding where new state or logic belongs, or when a review comment claims a layer boundary was crossed. Triggers — "where does this state live", "can a controller do IO", "is this the right place for this logic", "why is _setAllDrinks the only write path", "do I need a schema migration for this field", "what's the invariant this test is protecting", "why does the provider look like this", "is AGENTS.md's architecture section still accurate". Provides the UI→Provider→controllers→repositories→services→models layer contract, the 11 enforced invariants with file:line and the incident behind each, the storage/persistence contracts (UserDataStore versioning, PreferenceKeys registry, legacy migration), load-bearing design decisions with their rationale, and the known-weak points to stop you from re-discovering them the hard way.

name

architecture-contract

description

Load when you need to understand HOW the Cambridge Beer Festival app is built rather than how to run it — before adding a screen/model/service/sort option, before touching BeerProvider or any domain controller, before adding or changing a persisted field or SharedPreferences key, before deciding where new state or logic belongs, or when a review comment claims a layer boundary was crossed. Triggers — "where does this state live", "can a controller do IO", "is this the right place for this logic", "why is _setAllDrinks the only write path", "do I need a schema migration for this field", "what's the invariant this test is protecting", "why does the provider look like this", "is AGENTS.md's architecture section still accurate". Provides the UI→Provider→controllers→repositories→services→models layer contract, the 11 enforced invariants with file:line and the incident behind each, the storage/persistence contracts (UserDataStore versioning, PreferenceKeys registry, legacy migration), load-bearing design decisions with their rationale, and the known-weak points to stop you from re-discovering them the hard way.

Architecture Contract

This is the map of how the app is built, not how to run it (see run-and-operate) or how to fix a specific bug (see debugging-playbook / failure-archaeology). Read this before adding code to lib/providers/, lib/domain/, lib/services/, or lib/models/, or before trusting a review comment that claims a layer boundary was crossed.

Known doc drift: AGENTS.md's Architecture section still lists FavoritesService, RatingsService, TastingLogService as separate services. These do not exist in the code. They were unified into UserDataStore (lib/services/user_data_store.dart) by issue #391 / PR #395 (f0ed032). lib/services/storage_service.dart now contains only FestivalStorageService. Trust this document and the code over that paragraph of AGENTS.md.

The app is released. Current version is 2026.6.0+2026060500 (pubspec.yaml:4), shipping to Cloudflare Pages and the Google Play Internal track. Every fact below that says "no migration was needed" refers to a pre-release decision. That era is over — see §3.

1. The layer contract

UI (lib/screens/, lib/widgets/)
   context.watch<BeerProvider>() in build(); context.read<BeerProvider>() in callbacks
   ↓ calls provider methods, never touches controllers/repositories/services directly
BeerProvider (lib/providers/beer_provider.dart, ChangeNotifier — 816 lines)
   OWNS: the four UI signals (_isLoading/_isRefreshing/_error/_refreshNotice),
   persistence (calls repos + SharedPreferences), analytics (unawaited),
   notifyListeners(), and orchestration between controllers/repositories.
   ↓ feeds loaded data in, reads derived views out
domain/controllers/  (pure, synchronous, NO Flutter / IO / async / analytics):
   DrinkFilterController, UserDrinkStateController, FestivalController,
   UserPreferencesController (the one exception: talks to SharedPreferences,
   nothing else)
   ↓ use
domain/services/: DrinkFilterService, DrinkSortService (also pure)
   ↓
domain/repositories/ interfaces: DrinkRepository, FestivalRepository
   impls: ApiDrinkRepository, ApiFestivalRepository — THIS is where IO happens
   ↓
services/: BeerApiService (HTTP), DrinkCacheService, UserDataStore
   (SharedPreferencesUserDataStore), FestivalService, FestivalCacheService,
   FestivalStorageService, AnalyticsService, EnvironmentService,
   connectivity_io/web.dart
   ↓
models/: Drink (= Product + Producer), Festival, UserDrinkState,
   MyFestivalEntry

Who is allowed to do what

Layer	May do	May NOT do
`domain/controllers/*`	Hold in-memory state, synchronous derivation (filter/sort/compare), synchronous mutation helpers that return the new value	`async`, network calls, `SharedPreferences` I/O (except `UserPreferencesController`), analytics, `notifyListeners()`
`domain/services/*` (`DrinkFilterService`, `DrinkSortService`)	Pure functions over lists of `Drink`	Any state, any I/O
`BeerProvider`	`await` repository calls, persist preferences, log analytics (always `unawaited`), call `notifyListeners()`, own the four loading/error signals	Contain filter/sort/comparison logic itself (delegate to controllers), write `_allDrinks` anywhere except `_setAllDrinks`
`domain/repositories/Api*Repository`	HTTP calls, cache reads/writes, return persisted state from mutators	Hold UI state, call `notifyListeners()`
`services/*`	Talk to the outside world (HTTP, SharedPreferences, Firebase)	Know about `Drink`/`Festival` business rules beyond parsing

Controllers are pure and synchronous specifically so they can be unit-tested without a Flutter binding or mocks — see the "extracted for testability" rationale in §4.

The `_setAllDrinks` rule — single catalogue write path

_allDrinks (the entire in-memory drinks catalogue) has exactly one place it may be assigned: _setAllDrinks (lib/providers/beer_provider.dart:715-720).

void _setAllDrinks(List<Drink> drinks) {
  _allDrinks = drinks;
  _catalogueRevision++;
  _filter.setSource(drinks);
  _personalState.setSource(drinks);
}

Every call site that loads or clears the catalogue (loadDrinks, setFestival, _refreshDrinksFromNetwork) goes through this method. This guarantees _allDrinks, _filter, and _personalState can never drift out of sync with each other, and _catalogueRevision (used to invalidate the memoised myFestivalEntries, see below) always reflects reality. If you ever see _allDrinks = ... written anywhere else, that is a bug — either route it through _setAllDrinks, or (for a single-drink update) through _replaceDrink (beer_provider.dart:725-736, mutates by id + festivalId, bumps _personalStateRevision, calls _filter.recompute()).

The #410/#447 rule — repositories return persisted state, controllers don't recompute it

Every personal-state mutator (toggleFavorite, setRating, toggleTasted, beer_provider.dart:739-800) follows this exact shape:

final newState = _personalState.apply(
  drink.id,
  await _drinkRepository!.toggleFavorite(currentFestival.id, drink.id),
);
_replaceDrink(drink, drink.copyWith(userState: newState));

The repository (ApiDrinkRepository, lib/domain/repositories/api_drink_repository.dart:128-200) computes the mutation once, calls DateTime.now() once, persists it, and returns exactly what it wrote (or null if the record pruned to empty — see invariant 7). UserDrinkStateController.apply() (user_drink_state_controller.dart:102-108) just stores that value directly — it does not re-derive it.

This exists because of issue #410 (closed by PR #447, fd50fc2): the original code ran the same mutation twice — once in the repository with one DateTime.now(), once in the controller with a second, microseconds later DateTime.now() — so updatedAt diverged between disk and memory. Harmless for booleans; a real bug the moment a feature needs to match a tasting event by its timestamp (multi-tasting, addTasting/removeTasting — tracked in #315). When you add a new personal-state mutator, always follow this shape: repository computes + persists + returns; controller stores what it's given via apply(), never recomputes.

2. The 11 enforced invariants

Every row below is enforced by specific code — if you're touching a nearby line, read the invariant first.

#	Invariant	Enforcing code	Incident it fixed
1	`_error` / `_refreshNotice` are never both non-null	`_refreshDrinksFromNetwork` — success clears both (`beer_provider.dart:482-483`); failure-with-cached-data sets notice, clears error (`497-500`); failure-without-data sets error, clears drinks (`501-503`)	Design invariant from the SWR feature (PR #302); a UI that showed both a banner and a full error screen simultaneously would be a display bug
2	Null vs. empty-`Set` — filter fields are always `{}`, never `null`	`DrinkFilterController` fields initialised to `{}` (`drink_filter_controller.dart:31,35-36`); getters return `Set.unmodifiable(...)`	AGENTS.md-documented convention — empty set means "no filter active", `null` would be an ambiguous third state
3	Multi-toggle filters are `enum` + `Set<EnumValue>`, not parallel bools	`DrinkVisibilityFilter` (availableOnly/notTasted/veganOnly), persisted as `.name` string list via `UserPreferencesController.persistVisibilityFilters` (`user_preferences_controller.dart:73`)	Avoids the N-parallel-booleans anti-pattern; adding a 4th visibility filter is one enum value, not a new field everywhere
4	Analytics calls are always `unawaited(...)`, never logged for trivial values	`beer_provider.dart` e.g. lines 613, 621, 635, 644, 751/754, 775, 796, 458, 510-516; blank search explicitly skipped (`639-645`)	PR #332 (non-blocking analytics), PR #375 (stop logging expected-offline partial failures as errors)
5	Every interactive element has a `Semantics` wrapper with a real label	`overflow_menu.dart:15`, `star_rating.dart:47-49`, `breadcrumb_bar.dart:63,108,125`, `widget_builders.dart:47`, `main.dart` bottom nav	WCAG 2.1 AA / ADA / Section 508 — see skill `ui-and-accessibility` for the full pattern catalog
6	Stable identity for list lookups = `id + festivalId`, never `indexOf`/object identity	`Drink.==`/`hashCode` (`drink.dart:312-323`, empty-id → identity fallback), `_replaceDrink` matches by `d.id == old.id && d.festivalId == old.festivalId` (`beer_provider.dart:727`)	Issue #323 (missing `==`/`hashCode`) + PR #366 (immutable `Drink`, `copyWith`) — after `copyWith` the old instance is a stale snapshot no longer in the list
7	Empty personal-state records are pruned, not stored as empty JSON	`UserDrinkState.isEmpty` (`user_drink_state.dart:69-74`) drives `SharedPreferencesUserDataStore.write` (`user_data_store.dart:73-80`, removes the key rather than writing `{}`); repository mutators return `null` when pruned (`api_drink_repository.dart:138,165,177,199`)	Keeps SharedPreferences from accumulating dead keys for every drink a user ever glanced at
8	The drinks catalogue has exactly one write path	`_setAllDrinks` (`beer_provider.dart:715-720`)	See §1 above
9	Stale network responses are discarded via a monotonic token	`_drinksLoadToken`, checked at `beer_provider.dart:406, 451, 480, 495, 519`	Issue #266, fixed by PR #275 — a slow in-flight `loadDrinks()` from festival A was overwriting festival B's just-loaded data after a rapid switch
10	A persisted-record payload newer than the running build is rejected, not mis-parsed	`SharedPreferencesUserDataStore.migrate` (`user_data_store.dart:213-228`) throws `FormatException` when `version > currentSchemaVersion`; `_decode` catches it and treats the record as absent, data on disk is left untouched	Forward-compatibility fail-safe designed in from the start (no incident yet — this is the "don't create the incident" invariant, see §3)
11	A 404 from a beverage-type endpoint preserves the existing cache instead of wiping it	`BeerApiService.fetchDrinksByType` — a 404 lands in neither `drinksByType` nor `failedTypes` (`beer_api_service.dart:46-48,63-91`); `DrinkCacheService.merge` only overwrites types present in the fresh map (`cache_service.dart:49-62`)	A transient 404 mid-deploy (e.g. `cider.json` momentarily missing) must not blank out yesterday's cached cider list

Also load-bearing but not in the original "11" count, because it's a process invariant rather than a code invariant: PreferenceKeys values are pinned by a test (test/constants/preference_keys_test.dart) — see §3.

3. Storage contracts

UserDataStore schema versioning

SharedPreferencesUserDataStore (lib/services/user_data_store.dart) stores one JSON blob per drink-per-festival under key user_state_{festivalId}_{drinkId} (PreferenceKeys.userStatePrefix). Every write stamps a version field (schemaKey, currently currentSchemaVersion = 1). Every read routes through migrate() — a @visibleForTesting static pure function (user_data_store.dart:213-228) — before UserDrinkState.fromJson:

@visibleForTesting
static Map<String, dynamic> migrate(Map<String, dynamic> raw) {
  final version = (raw[schemaKey] as num?)?.toInt() ?? 1;
  if (version > currentSchemaVersion) {
    throw FormatException(...); // newer than this build — fail safe
  }
  return raw; // v1 is current; no transforms yet
}

Two rules this design encodes:

Single upgrade point. When the schema needs to change, the change goes in migrate() as a new if (version < N) branch — never inline at a call site. This is the only place that has ever needed to exist, so far, because v1 is still the only version shipped.
Forward-compat fail-safe. A payload with a version number higher than the running build's currentSchemaVersion cannot be safely downgraded, so migrate throws; _decode (user_data_store.dart:196-204) catches it and treats the record as absent — the stored bytes on disk are never touched, so a user who downgrades the app temporarily loses visibility of that field but never loses the data. This matters now that the app auto-updates across platforms at different cadences (Play staged rollout vs. instant web deploy).

Additive vs. breaking field changes

Additive nullable field, same schema version — no migration needed. Issue #417 (open at time of writing) is the reference pattern: adding wouldRecommend: bool? to UserDrinkState for a future crowd-rating feature. Its own description states the rule precisely:

currentSchemaVersion stays at 1 — this is a purely additive field; fromJson already returns null for absent keys. No migration needed.

This works because UserDrinkState.fromJson (user_drink_state.dart:117-141) already treats every field defensively (json['x'] as T? ?? default), so an old stored record simply parses the new field as null — indistinguishable from "not yet answered." Follow this pattern for any new optional signal field. It stops applying the moment you need to:

change the meaning of an existing field,
make a field non-nullable,
reshape a field's type (e.g. int rating → List<int> ratings),
or remove a field that older-still-installed clients might read. Any of those is a real migration: bump currentSchemaVersion, add a branch in migrate(), and write a test that feeds the old-shape JSON through migrate() and asserts the upgraded shape.

"No users yet, no migration" is retired. The historical unification of FavoritesService/RatingsService/TastingLogService into UserDataStore (#391, #395) deliberately shipped no migration code because the app was pre-release with zero installed users holding old-format data. That justification no longer holds: the app is at 2026.6.0 on the Play Internal track and Cloudflare Pages production. Any schema change from here that isn't purely additive (see above) needs a real migrate() branch and a round-trip test — do not repeat the "no users" shortcut.

PreferenceKeys registry, pinning test, and the "add a preference" checklist

Every SharedPreferences key in the app is a named constant in lib/constants/preference_keys.dart (79 lines, 12 keys) — themeMode, visibilityFilters, hideUnavailableLegacy, excludedAllergens, userStatePrefix, legacyMigrationComplete, favoritesLegacy, ratingsLegacy, tastingLogLegacyPrefix, selectedFestivalId, drinksCachePrefix, festivalsCache. No inline string key literals are permitted — a mistyped key string reads back null silently and the user's data is gone with no error.

test/constants/preference_keys_test.dart pins every value verbatim (expect(PreferenceKeys.themeMode, 'themeMode'), etc.) plus a second test asserting all 12 are pairwise unique. This test failing is not a bug in the test — it means you changed (or collided) an on-disk key, which is a data-loss event for existing installs. Either revert the constant or write a migration that reads the old key before deleting it (see migrateLegacyData() below for the template).

Checklist for adding a new preference (from AGENTS.md, verified against the code):

Add the key to PreferenceKeys and add its expected literal to test/constants/preference_keys_test.dart in the same commit.
Add read/write to the appropriate controller/service (UserPreferencesController for theme/visibility/allergens, SharedPreferencesUserDataStore for per-drink personal fields, or a new service if it's neither).
Load it in BeerProvider.initialize() if it needs to be available at startup.
If you are changing an existing key's value rather than adding a new key, treat it as a migration, not a rename — the pinning test exists to force this to be a deliberate decision.

Legacy migration flag (worked example of a real migration)

PreferenceKeys.legacyMigrationComplete ('personal_state_migration_v1') gates SharedPreferencesUserDataStore.migrateLegacyData() (user_data_store.dart:118-189) — the one-time fold of the three pre-#391 key schemes (favorites_{festivalId}, ratings_{festivalId}_{drinkId}, tasting_log_{festivalId}|{drinkId}) into unified UserDrinkState records, run once from BeerProvider.initialize() (beer_provider.dart:260-263) before any repository is constructed. It merges into any record already present rather than overwriting (so a user who somehow has both old- and new-format data doesn't lose either), then deletes the old keys. The flag deliberately does not share the user_state_ prefix, so it cannot collide with a per-drink record key. This is the reference shape for any future real migration: idempotent, merge-not-overwrite, delete the source only after a successful write, gate with a flag so the scan doesn't run on every launch.

4. Load-bearing design decisions (with the why)

Decision	Why (from code/docs, not guesswork)
Pure, synchronous domain controllers; `BeerProvider` owns persistence + notify	Documented in each controller's class doc (e.g. `festival_controller.dart:5-14`, `drink_filter_controller.dart:6-16`): pure logic can be unit-tested without a Flutter binding, mocks, or `async`. The whole `BeerProvider` decomposition (#357→#388→#396→#398→#399→#402→#403) was staged specifically behind this property.
`UserDataStore` is a versioned interface, not a concrete `SharedPreferences` call scattered through the app	Class doc, `user_data_store.dart:9-15`: "today a `SharedPreferencesUserDataStore` (local-first), later a synced store (vision Phase 3) with the local store as the offline cache." The interface boundary is where cloud sync (D1 + v1alpha API) will plug in without touching controllers or the provider.
Personal state (favourites/ratings/tastings) is catalogue-independent (#390)	`DrinkRepository.getPersonalEntries` doc (`drink_repository.dart:56-65`): "the caller can enumerate a user's favourites, ratings, and tasting history purely from the personal-data store, before (or without) the drink catalogue being fetched." Fixed the favourites-flash bug family (#310/#397) as a side effect, because the My Festival list stopped being `_allDrinks.where(...)` (whichever festival happened to be loaded) and became its own festival-scoped query.
Stale-while-revalidate (SWR) with two independent per-type caches	`cache_service.dart:8-19` class doc: render last-good data instantly, refresh in background, keep cache on failure. Per-type (not per-festival) caching specifically so a flaky `cider.json` fetch can't wipe out a good `beer.json` cache — see invariant 11.
The `/` route must have a `builder`, not just a `redirect`	`router.dart:76-83` comment, citing issue #386: a redirect-only route that stays put (because the provider hasn't initialized yet) leaves go_router with an empty `pages` list and no `onGenerateRoute`, which crashes with "Null check operator used on a null value" in release builds only. The minimal `CircularProgressIndicator` builder at `router.dart:84-85` is the fix; removing it reintroduces a release-only crash invisible in debug/tests.
`navigateToRoute()` pushes on every platform (no web/mobile branch since #470)	`navigation_helpers.dart:237-239` is now just `context.push(path)`. It used to branch to `context.go()` on web because `push` from inside a `ShellRoute` didn't update the browser URL bar; enabling `GoRouter.optionURLReflectsImperativeAPIs` (`router.dart`) fixed that, and `go` was disposing the calling screen and losing its scroll position (#470, PR #478). The one-line helper is kept deliberately: it is the only place that rationale is recorded, and the single seam if that flag ever has to come back off. Always use it for drill-down navigation (drink detail, brewery) rather than calling `context.go`/`context.push` directly.
Analytics only fires in production	`AnalyticsService._isAnalyticsEnabled = isProduction()` (`analytics_service.dart:21`); `EnvironmentService.isProduction()`/`isProductionHost()` (`environment_service.dart`). Fixed issue #269: unknown hostnames used to default to "production", polluting real analytics with staging/preview traffic; now unknown → NOT production (under-count is the safe failure direction). `logError` is the one exception — it runs in every environment so Crashlytics still sees staging crashes.
`DefaultFestivals` hard-coded fallback	`models/festival.dart:262+` — four literal `Festival` objects (`cbf2026` active, `cbf2025`, `cbfw2025`, `cbf2024`) used only when both the network and the festival cache are unavailable (`FestivalController.currentFestival` getter, `festival_controller.dart:41-47`, and `BeerProvider.loadDrinks`, `beer_provider.dart:391-398`). This is a last-resort constant, not a data source to keep in sync with `data/festivals.json` — do not add new festivals here expecting them to appear in the switcher; that's the registry's job.

5. Known-weak points (state plainly, don't paper over)

These are real, currently-open gaps. Don't rediscover them as "bugs you found" — they're tracked.

Detail routes don't validate the festival ID against the drink's own festival scope. Documented as a known limitation in ADR 0004 (docs/adr/0004-path-based-url-strategy.md:64) and archived todo H3 (docs/todos.md:100-111, historical reference only, do not add new items there). A /wrong-festival/drink/beer/123 link is not currently rejected at the route level.
The festival selector UI doesn't update the URL when switching festivals — ADR 0004 (line 65), archived todo C3 (docs/todos.md:57-68).
No way to navigate back from the /about deep link — archived todo H6 (docs/todos.md:136-147).
URL fragments in the post-init redirect (main.dart's _handlePostInitRedirect) are still dropped — it rebuilds the path from segments + query only. The router's own invalid-festival redirect no longer loses them: _redirectToCurrentFestival (router.dart) carries query and fragment across verbatim, and test/router_test.dart's "URL fragments survive the invalid-festival redirect" pins that. The old lossy-behaviour test and its TODO are gone.
Flutter web renders to <canvas>, so Playwright (the only E2E tool in use, per ADR 0005) can never assert on rendered UI content — a route can return HTTP 200 and still be showing an error state underneath. Accepted tradeoff; see docs/adr/0005-e2e-testing-strategy.md and skill validation-and-qa for what E2E actually covers here.
AGENTS.md architecture doc drift — see the callout at the top of this file. FavoritesService/RatingsService/TastingLogService are gone; UserDataStore is reality.
The screen class is MyFestivalScreen but its route is still /:festivalId/favorites. PR #448 renamed the underlying model (FavoriteDrinkEntry → MyFestivalEntry) and generalised favoriteDrinks → myFestivalEntries, and the class has since been renamed to match its file, but the URL path (/:festivalId/favorites) is unchanged on purpose — URLs are a public contract (see skill change-control's unwritten rule #1). Don't be surprised the class and route disagree; don't rename the class without checking every import, and never rename the route.
The /v1alpha catalogue API is contract-only (issue #432/PR #433): proto + generated OpenAPI + a read-only worker endpoint exist, but there is no production server backing MyFestival sync yet — D1 is unprovisioned and its binding is commented out in cloudflare-worker/wrangler.toml. See run-and-operate for the provisioning gap and api-contract for the proto surface itself.

6. Rules for extending

These restate and ground AGENTS.md's checklists against the actual code — verify against AGENTS.md too, since it may have drifted further by the time you read this.

Adding a screen: lib/screens/new_screen.dart → export from lib/screens/screens.dart → add a GoRoute in lib/router.dart. If it's festival-scoped, give it a redirect: (context, state) => _festivalScopeRedirect(...) like every other /:festivalId/... route (router.dart:93-183) — this is what makes deep links and invalid-festival redirects work uniformly. Every route needs a real builder (see §4's / route rationale) — never a redirect-only route.

Adding a model: lib/models/new_model.dart with fromJson/toJson, defensive parsing per field (variant types — see AGENTS.md's JSON Parsing Pattern, confirmed live in Product.fromJson, drink.dart:95-171), export from lib/models/models.dart, tests covering all type variants + null/ missing/wrong-type.

Adding a service: lib/services/new_service.dart, export from lib/services/services.dart, constructor-injected (no singletons — see BeerProvider's constructor, beer_provider.dart:88-100, which accepts every dependency as an optional named parameter for tests to override).

Adding a sort option: enum value in DrinkSort (lib/domain/models/drink_sort.dart) → case in DrinkSortService (lib/domain/services/drink_sort_service.dart) → dropdown entry in drinks_screen.dart. Sorting is pure and lives entirely in the domain service — never add sort logic to BeerProvider or a widget.

Where does a new personal-state field go? UserDrinkState (lib/models/user_drink_state.dart) — add the field to the constructor, copyWith (use the _sentinel pattern if the field is nullable and needs explicit-clear semantics, see rating/notes at lines 83-93), toJson/ fromJson, isEmpty (decide whether this field alone should keep a record alive — most new optional signal fields should not count toward isEmpty if they represent low-commitment interactions, but this is a per-field product decision, not a mechanical rule), ==/hashCode, and toString. Then read §3's additive-field rule to confirm you don't need a schema bump. Do not add a new top-level PreferenceKeys entry for it — it lives inside the existing per-drink JSON blob under userStatePrefix.

Adding a new drink-visibility toggle: add a value to DrinkVisibilityFilter, not a new boolean field — see invariant 3.

Adding a new mutator that touches personal state (e.g. a future addTasting/removeTasting, tracked in #315): follow the §1 #410/#447 shape exactly — repository computes, persists, and returns the value; UserDrinkStateController.apply() just stores it. Do not call DateTime.now() in more than one layer for the same conceptual mutation.

When NOT to use this skill

Running, building, deploying, or provisioning anything (dev server, web/Android builds, Cloudflare Pages/Worker deploys, D1 setup, festival data updates) → skill run-and-operate.
"Why did this break" / bug investigation / historical incidents → skill debugging-playbook (symptom → triage) or failure-archaeology (the full chronicle with root causes and rejected fixes). This skill states current invariants and why they exist; those skills cover the process of finding a new bug or confirming an old one is really fixed.
UI/widget patterns, semantics label wording, golden test workflow, festival-flash guard usage → skill ui-and-accessibility. This skill only states that invariant 5 (Semantics) and the festival-flash pattern exist and where they're enforced, not the full pattern catalog.
Change classification, CI gates, the Do-Not-Modify list, unwritten discipline rules (URL contract, free-tier-only, festival freeze) → skill change-control.
Proto/AIP API design questions, worker implementation patterns → skill api-contract.

Provenance and maintenance

Written 2026-07-02. Verified against the working tree at commit 517e613 (the tip at time of writing) by direct file reads. Every file:line citation above was opened and read in full; issue numbers #390, #410, #417 were confirmed live via the GitHub API (title, body, state). Line counts: beer_provider.dart 816, drink.dart 348, festival.dart 354, user_drink_state.dart 175, drink_filter_controller.dart 241, festival_controller.dart 206, user_drink_state_controller.dart 144 — all confirmed with wc -l against the actual files.

Re-verification commands (run these if this document feels stale):

# Confirm the doc-drift claim and single-write-path invariant still hold
grep -n "class FestivalStorageService" lib/services/storage_service.dart
grep -n "_setAllDrinks" lib/providers/beer_provider.dart

# Confirm the schema version and migrate() shape haven't changed
grep -n "currentSchemaVersion\|static Map<String, dynamic> migrate" lib/services/user_data_store.dart

# Confirm PreferenceKeys count and the pinning test still agree
grep -c "static const" lib/constants/preference_keys.dart
grep -c "expect(PreferenceKeys" test/constants/preference_keys_test.dart

# Confirm the release version (invalidates "no users, no migration" reasoning)
grep "^version:" pubspec.yaml

# Confirm known-weak-points are still open (re-check state, not just existence)
gh issue view 432 --json state,title 2>/dev/null || echo "use mcp__github__issue_read method=get issue_number=432 instead"

# Confirm the router redirect still preserves query + fragment
grep -n "hasFragment\|hasQuery" lib/router.dart

If any of these disagree with the text above, the code has moved on — update this file, don't patch around the discrepancy elsewhere.

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