Hc Bff Pattern

hc-bff-pattern

The BFF (frontend/hc-portal/) is the only place in this architecture where data from multiple services can be combined. Backend services never call each other. This skill is the source of truth for how to write a BFF route.

When to use

• "Add a /patient-summary/{id} route that returns demographics + last 3 appointments + active meds"
• "Build the dashboard view that mixes lab + billing data"
• "Fan out to N services concurrently and merge the results"
• "Add a write route to the BFF" (rare — see Write routes below)

When NOT to use

• The user wants to add a single-service route → it belongs in that service, not the BFF. Use hc-obo-auth.
• The user wants the BFF to cache data across requests → don't. Push the join down to UC if it's expensive.
• The user wants the BFF to call its own database → it doesn't have one. The BFF holds no state.

The four BFF rules

1. No persistent state. No DB pool, no Redis, no shared cache. If you find yourself wanting one, stop and reconsider whether the data really belongs in a service.
2. Concurrent fan-out. Use asyncio.gather for independent calls. Sequential awaits are a code review reject.
3. Per-request clients. Construct service clients inside the route handler, not at module scope. Each request gets its own clients with its own bound user token.
4. Forward both headers. Authorization: Bearer <token> and X-Forwarded-Access-Token. The downstream service should be unable to tell whether the user called directly or via the BFF.

Canonical aggregation route

Worked example: the patient-summary route from HEALTHCARE_DATA_MODEL.md.

from __future__ import annotations
import asyncio
from typing import Annotated
from uuid import UUID

from fastapi import APIRouter, Depends, HTTPException
from pydantic import BaseModel

from ..auth import user_token  # same shape as the service-side auth.py
from ..clients import (
    PatientClient,
    ProviderClient,
    AppointmentClient,
    LabClient,
    PrescriptionClient,
    BillingClient,
)

router = APIRouter(tags=["aggregations"])


class PatientSummaryOut(BaseModel):
    patient: dict
    last_appointments: list[dict]
    active_prescriptions: list[dict]
    recent_lab_orders: list[dict]
    outstanding_invoices: list[dict]


@router.get(
    "/patient-summary/{patient_id}",
    response_model=PatientSummaryOut,
    operation_id="getPatientSummary",
)
async def patient_summary(
    patient_id: UUID,
    token: Annotated[str, Depends(user_token)],
):
    async with (
        PatientClient(token) as patient,
        AppointmentClient(token) as appointment,
        LabClient(token) as lab,
        PrescriptionClient(token) as rx,
        BillingClient(token) as billing,
    ):
        # Concurrent fan-out — none of these depends on another
        results = await asyncio.gather(
            patient.get(patient_id),
            appointment.list_for_patient(patient_id, limit=3, order="desc"),
            lab.list_orders_for_patient(patient_id, status="resulted", limit=5),
            rx.list_active_for_patient(patient_id),
            billing.list_outstanding_for_patient(patient_id),
            return_exceptions=True,
        )

    # Partial-failure handling: if the patient lookup itself fails, 404/5xx out.
    # If a peripheral lookup fails, return what we have with a `partial: true` flag.
    p, appts, labs, rxs, bills = results
    if isinstance(p, Exception):
        raise HTTPException(502, "patient-svc unavailable") from p

    def _safe(value, default):
        return default if isinstance(value, Exception) else value

    return PatientSummaryOut(
        patient=p.model_dump(),
        last_appointments=[a.model_dump() for a in _safe(appts, [])],
        active_prescriptions=[r.model_dump() for r in _safe(rxs, [])],
        recent_lab_orders=[l.model_dump() for l in _safe(labs, [])],
        outstanding_invoices=[b.model_dump() for b in _safe(bills, [])],
    )

Notes on the shape:

• return_exceptions=True: a single failing peripheral call doesn't take down the whole page. The patient-load is required (we fail closed).
• No N+1 calls: when you need provider names for the appointments, batch them — provider.list(ids=[...]), not a loop of provider.get(id).
• Pydantic in / Pydantic out: clients return validated objects, the BFF re-serializes for the response. Type errors caught at the boundary.

When to resolve names vs leave IDs

The patient-summary example returns appointments with raw provider_id UUIDs. Two options:

The reference defaults to resolve in the BFF for primary aggregation views (patient summary, appointment detail), and leave IDs for list/index views (the "all appointments" page).

Write routes (rare and careful)

The BFF should mostly be read-only aggregation. When a write spans services, prefer:

1. Don't. Reshape the request so it goes to one service. Often this is achievable.
2. If you can't, write a saga-style route: call service A, then service B, with explicit compensation if B fails. Document this loudly.
3. Never wrap multiple service writes in a "transaction" — there are no distributed transactions here.

Concrete example: "book an appointment AND send a notification". Wrong shape — the BFF books, then enqueues a notification job (Lakeflow / job task), with no expectation of atomicity. The notification can fire eventually.

Timeouts and resilience

• Default per-call timeout: connect=2s, read=5s (in _BaseSvcClient).
• A BFF route that fans out to N services should respond within max(per-call) + overhead, not sum(per-call). Always concurrent.
• No retries in v1. A 503 from a service propagates as a 502 from the BFF (or as a partial response). Adding retries hides flakiness — make services boring instead.
• No circuit breakers in v1. Adding pybreaker is a justified follow-up if you observe a service that takes everyone down with it.

How downstream URLs are resolved

The BFF doesn't read per-service URL env vars. clients/_base.py derives them at runtime from two platform-injected vars:

• DATABRICKS_APP_URL — the BFF's own URL, e.g. https://hc-portal-dev-<workspace-id>.aws.databricksapps.com.
• DATABRICKS_WORKSPACE_ID — the workspace this BFF is deployed in.

Sibling services share the same <target>-<workspace_id>.<region>.databricksapps.com tail and follow the <svc>-<target> naming pattern, so the BFF just strips its own hc-portal prefix off the host and prepends the per-service slug. No bundle config.env, no <SVC>_SVC_URL env vars, no apps update step. See clients/_base.py:_own_app_suffix for the canonical implementation.

This was deliberate: DAB has two relevant gaps — ${resources.apps.<key>.url} does not resolve at bundle-deploy time (validate output shows url=null), and bundle deploy doesn't push config.env to the App spec without an extra apps update round-trip. Deriving from DATABRICKS_APP_URL sidesteps both.

Local development

For non-Apps local dev, the BFF derives URLs the same way: set DATABRICKS_APP_URL to a fake hc-portal-*-host value that satisfies the prefix-strip — respx (or your local mocks) can intercept the resulting URLs. See tests/conftest.py:_stub_svc_env for a working stub. Then:

# Make the BFF accept the dev's CLI token as if it were forwarded
LOCAL_DEV_TOKEN_FROM_CLI=true
LOCAL_DEV_TOKEN=<paste from `databricks auth token -p hc-dev | jq -r .access_token`>

apx dev start from frontend/hc-portal/ boots the React app + the BFF.

Anti-patterns

Tests

frontend/hc-portal/tests/integration/test_<route>.py — runs against deployed preview apps in CI:

@pytest.mark.integration
async def test_patient_summary_concurrent(httpx_client, user_token, sample_patient_id):
    import time
    t0 = time.monotonic()
    r = await httpx_client.get(
        f"/api/bff/patient-summary/{sample_patient_id}",
        headers={"X-Forwarded-Access-Token": user_token},
    )
    elapsed = time.monotonic() - t0
    assert r.status_code == 200
    # Concurrent fan-out: should be roughly the slowest single call, not their sum
    # Five 5xx calls in series would be ~25s; concurrent is < ~6s including overhead
    assert elapsed < 6.0, f"BFF took {elapsed:.1f}s — fan-out is probably sequential"

Plus a test_partial_failure that injects a 503 from one service via WireMock or a stub and asserts the response still 200s with partial: true.

Verification

After adding a route:

• apx dev check passes (TS + Python types).
• OpenAPI regenerated; the React Suspense hook for the new route shows up in ui/src/lib/api.ts.
• Integration test exists and passes against the preview app.
• Per-call timeouts set on every client (the base class enforces this).
• No state added to the BFF process.
• No backend service was modified to call another backend service.