enlace-dev

name

enlace-dev

description

Use when developing or modifying the enlace package itself — adding features, fixing bugs, extending the ASGI composition, adding middleware, or working on any module in the enlace/ source directory. Triggers on: editing enlace source files (base.py, discover.py, compose.py, serve.py, auth.py, stores.py), implementing spec phases 2-4, or when the user says "add X to enlace" or "implement Y in enlace".

Developing enlace

enlace is a multi-app ASGI platform. This skill covers the critical rules, architecture, and patterns needed to modify the enlace codebase safely.

Design Principles

These two principles govern ALL changes. Every feature, fix, and diagnostic suggestion must respect both:

1. Zero coupling — apps don't import enlace

enlace wraps apps from the outside. Apps depend on their own domain libs (FastAPI, numpy, etc.), never on enlace. enlace provides services via:

• ASGI scope injection (auth, store) — not imports
• Environment variables (ENLACE_MANAGED) — optional, apps can ignore it
• Convention (filesystem layout, app.toml) — external to app code

When adding features, NEVER create patterns that require apps to import enlace or depend on enlace-specific APIs.

2. Preserve standalone operation

When enlace (or its diagnostic tool) suggests changes to an app, those changes MUST preserve the app's ability to run independently. The pattern: env-var with current value as default.

# GOOD: works standalone AND under enlace
if not os.environ.get('ENLACE_MANAGED'):
    app.add_middleware(CORSMiddleware, ...)

# BAD: breaks standalone
# (CORSMiddleware deleted entirely)

When writing diagnostic messages, fix suggestions, or documentation:

• Prefer enlace-side solutions (app.toml, build-time env vars) over app changes
• When app changes are needed, always use the env-var-with-default pattern
• Flag any suggestion that would break standalone with breaks_standalone=True

See misc/docs/design_principles__*.md for the full rationale.

Architecture

enlace/
├── base.py        # Pydantic models: AppConfig, PlatformConfig, ConventionsConfig
├── util.py        # Pure helpers: derive_display_name, derive_route_prefix, is_skippable
├── discover.py    # ConventionDiscoverer: walks apps/, detects types, loads TOML
├── compose.py     # build_backend(): mounts sub-apps, cascade_lifespan
├── diagnose.py    # diagnose_app(): scan an app dir for enlace compatibility issues
├── serve.py       # Uvicorn subprocess orchestration, signal forwarding
├── __main__.py    # CLI via argh.dispatch_commands
├── __init__.py    # Public API facade
└── tests/         # Unit tests (test_discover.py, test_compose.py)

Data flow: PlatformConfig.from_toml() → ConventionDiscoverer.discover() → config.check_conflicts() → build_backend(config) → uvicorn --factory

Critical Rules

Never use BaseHTTPMiddleware

It has terminal, unfixable bugs: exception swallowing across mounted sub-apps, ContextVar propagation corruption, synchronous execution of background tasks. The Starlette maintainer has called it unfixable. Always use pure ASGI middleware:

# YES: Pure ASGI middleware
class MyMiddleware:
    def __init__(self, app):
        self.app = app

    async def __call__(self, scope, receive, send):
        if scope["type"] == "http":
            # your logic here
            pass
        await self.app(scope, receive, send)

# NO: BaseHTTPMiddleware — NEVER use this

Mount on FastAPI directly, not APIRouter

app.mount() only works on the top-level FastAPI instance. Mounting on an APIRouter silently fails (FastAPI issues #4194, #10180).

Never silently swallow ImportErrors during discovery

If a module file exists but fails to import (syntax error, missing dep), that is a real error. Propagate it. Only skip directories that have no entry point file. This prevents the Celery autodiscover anti-pattern.

Conflict detection: fail-fast, report ALL

check_conflicts() collects every duplicate route, not just the first. Users need to see all conflicts at once to fix them in one pass.

Starlette does NOT cascade lifespan events

Mounted sub-apps never receive startup/shutdown. The cascade_lifespan context manager in compose.py works around this by iterating Mount routes and entering their lifespan contexts. Any new mounting logic must preserve this.

CORS on parent only

If a sub-app also adds CORS middleware, responses get duplicate headers. All cross-cutting middleware (auth, CORS, logging, store injection) goes on the parent FastAPI app exclusively.

Research Documents

Detailed rationale, code patterns, and pitfalls live in misc/docs/:

Module area	Consult
compose.py, mounting, middleware	asgi_composition__*.md
auth.py, sessions, cookies	auth_cross_cutting__*.md
discover.py, config, show-config	convention_over_configuration__*.md
serve.py, deployment, logging	deployment_observability__*.md
frontend.py, SPA serving	frontend_serving__*.md
stores.py, PrefixedStore, Mall	user_data_persistence__*.md
Design philosophy, coupling, independence	design_principles__*.md

Read the relevant doc before modifying its subsystem.

Implementation Phases (from spec)

Phase 1 (done): Core discovery, composition, CLI, serve Phase 2: frontend.py (SPAStaticFiles, launcher), auth.py (PlatformAuthMiddleware, shared-password login, signed cookies), inject.py (HTML injection), hash-password and create-session-secret CLI commands Phase 3: stores.py (PrefixedStore, Mall, filesystem/SQLite backends), StoreInjectionMiddleware, per-user sessions, CSRF, WebSocket origin validation Phase 4: deploy.py (Caddyfile/systemd generation), structlog middleware, analytics injection, generate CLI namespace

Testing Pattern

pytest enlace/tests/     # Unit tests
pytest tests/            # Integration tests

Tests use tmp_path fixtures to create temporary app directories. Composition tests use starlette.testclient.TestClient against the built app. Always test both the happy path and error cases (import errors, conflicts, missing files).

Adding a New Module

1. Create enlace/{module}.py
2. Add exports to enlace/__init__.py
3. Create enlace/tests/test_{module}.py
4. If it adds CLI commands, register them in enlace/__main__.py via argh.dispatch_commands
5. Run enlace check before and after to verify nothing breaks