Firebase iOS Backend with Auth, AWS S3 Files, and Optional FastAPI API (Part 3)

Everything below is designed to be copy/paste-able into a real project, while staying safe by default (no AWS creds in the app, clear authorization boundaries, predictable data model).

---

End-to-end design: Firebase Auth + Firestore metadata + S3 file bytes + FastAPI

This is the pattern I recommend when you want to keep Firebase as your identity + app-data layer, but you already have S3 or you want AWS-native file pipelines.

The core building block is S3 presigned URLs: they grant time-limited upload/download access to a specific object without giving the client AWS credentials. (AWS Docs)

Goals

• iOS client authenticates with Firebase Auth
• iOS client can upload/download file bytes to S3 without AWS keys
• Firestore stores metadata and drives UI lists
• FastAPI handles authorization + presigning + orchestration
• You can later add CloudFront, background processing, virus scanning, etc.

---

Data model: files/{fileId} (Firestore)

I strongly prefer a “file registry” collection in Firestore even if the bytes live in S3.

Collection: files/{fileId}

Suggested fields:

• ownerUid: string (required)
• bucket: string (required)
• key: string (required, e.g. users/<uid>/uploads/<uuid>.jpg)
• contentType: string (required)
• sizeBytes: number (optional until confirmed)
• status: "pending" | "ready" | "failed" (required)
• createdAt: timestamp (server-written if you can)
• updatedAt: timestamp

Opinion:

• Keep status explicit. It makes UI + debugging dramatically easier.
• Prefer server-authoritative metadata writes (details below).

---

API contract (opinionated)

1) Request an upload URL

POST /v1/files/presign-upload

Headers

• Authorization: Bearer <Firebase ID token>
• (Optional but recommended) X-Firebase-AppCheck: <App Check token> (explained later)

Body

{
  "contentType": "image/jpeg",
  "fileExtension": "jpg"
}

Response

{
  "fileId": "01J...ULID",
  "bucket": "my-app-uploads",
  "key": "users/<uid>/uploads/<fileId>.jpg",
  "upload": {
    "method": "PUT",
    "url": "https://s3....",
    "headers": {
      "Content-Type": "image/jpeg"
    },
    "expiresInSeconds": 300
  }
}

2) Mark upload complete (server validates S3 object exists)

POST /v1/files/{fileId}/complete

Response

{
  "fileId": "01J...",
  "status": "ready",
  "sizeBytes": 1234567
}

3) Request a download URL

POST /v1/files/{fileId}/presign-download

Response

{
  "download": {
    "method": "GET",
    "url": "https://s3....",
    "expiresInSeconds": 300
  }
}

---

FastAPI implementation (complete “starter-quality” skeleton)

Project layout I actually like

app/
  main.py
  core/
    config.py
    firebase_admin.py
    auth.py
    appcheck.py
  services/
    s3.py
    firestore.py
  api/
    routes/
      files.py

This keeps “wiring” separate from “routes” and “services,” so it stays maintainable.

---

core/config.py

Use environment variables so secrets never hit git.

from pydantic import BaseModel
import os

class Settings(BaseModel):
    firebase_service_account_path: str = os.environ["FIREBASE_SERVICE_ACCOUNT_PATH"]

    aws_region: str = os.environ["AWS_REGION"]
    s3_bucket: str = os.environ["S3_BUCKET"]
    presign_exp_seconds: int = int(os.environ.get("PRESIGN_EXP_SECONDS", "300"))

settings = Settings()

---

core/firebase_admin.py

Firebase Admin SDK is what lets FastAPI verify Firebase ID tokens. (Firebase)

import firebase_admin
from firebase_admin import credentials

from app.core.config import settings

def init_firebase_admin() -> None:
    if firebase_admin._apps:
        return
    cred = credentials.Certificate(settings.firebase_service_account_path)
    firebase_admin.initialize_app(cred)

---

core/auth.py (verify Firebase ID token)

Firebase’s recommended backend pattern is: client sends ID token (JWT), backend verifies it with Admin SDK. (Firebase)

from fastapi import Depends, HTTPException, status
from fastapi.security import HTTPAuthorizationCredentials, HTTPBearer
from firebase_admin import auth

bearer = HTTPBearer(auto_error=False)

def get_current_user(
    creds: HTTPAuthorizationCredentials | None = Depends(bearer),
) -> dict:
    if creds is None or not creds.credentials:
        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Missing bearer token")

    try:
        decoded = auth.verify_id_token(creds.credentials)
        # decoded contains 'uid' and other claims
        return decoded
    except Exception:
        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid or expired Firebase ID token")

Opinion:

• Always standardize on uid as the server identity.
• Put this in a dependency so every endpoint is consistent.

---

Optional but recommended: verify App Check tokens too

App Check helps ensure calls come from your real app, not scripts. (Firebase)

Firebase provides backend verification guidance; it expects the client to send an App Check token and the backend to validate it with Admin SDK. (Firebase)

core/appcheck.py

from fastapi import Header, HTTPException, status
from firebase_admin import app_check

def verify_app_check(
    x_firebase_appcheck: str | None = Header(default=None, alias="X-Firebase-AppCheck"),
) -> dict:
    if not x_firebase_appcheck:
        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Missing App Check token")

    try:
        claims = app_check.verify_token(x_firebase_appcheck)
        return claims
    except Exception:
        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid App Check token")

Opinion:

• I use App Check verification at least on:

  • presign-upload
  • presign-download
  • any “expensive” endpoints

• If you’re in early dev, you can make App Check optional, but plan to enforce it in production.

---

services/s3.py (presigned PUT/GET)

Boto3 can generate presigned URLs with an ExpiresIn in seconds. (Boto3)

import boto3
from app.core.config import settings

_s3 = boto3.client("s3", region_name=settings.aws_region)

def presign_put_object(*, bucket: str, key: str, content_type: str) -> str:
    # generate_presigned_url: valid for ExpiresIn seconds 
    return _s3.generate_presigned_url(
        ClientMethod="put_object",
        Params={"Bucket": bucket, "Key": key, "ContentType": content_type},
        ExpiresIn=settings.presign_exp_seconds,
        HttpMethod="PUT",
    )

def presign_get_object(*, bucket: str, key: str) -> str:
    return _s3.generate_presigned_url(
        ClientMethod="get_object",
        Params={"Bucket": bucket, "Key": key},
        ExpiresIn=settings.presign_exp_seconds,
        HttpMethod="GET",
    )

def head_object(*, bucket: str, key: str) -> dict:
    return _s3.head_object(Bucket=bucket, Key=key)

Opinion:

• Default expiry: 300 seconds (5 minutes) is a good starting point.
• Don’t let the client choose the key. The server constructs it from uid.

---

Advanced option: Presigned POST (stricter constraints)

Presigned POST supports a policy with conditions, including things like content-length-range and Content-Type (good for enforcing max upload size). (Boto3)

Boto3 supports generate_presigned_post() and documents the kinds of conditions you can include. (Boto3)

Opinion:

• Presigned POST is great when you need strong upload constraints.
• It’s more annoying to implement on mobile than PUT (because it’s a form-style upload), so I usually start with PUT + server-side validation at /complete.

---

services/firestore.py (metadata)

Use Admin SDK / server credentials to write metadata. Important: server libraries bypass Firestore Security Rules, so your server must implement authorization correctly. (Firebase)

from google.cloud import firestore

_db = firestore.Client()

def create_file_doc(*, file_id: str, doc: dict) -> None:
    _db.collection("files").document(file_id).set(doc)

def get_file_doc(file_id: str) -> dict | None:
    snap = _db.collection("files").document(file_id).get()
    if not snap.exists:
        return None
    return snap.to_dict()

def update_file_doc(file_id: str, patch: dict) -> None:
    _db.collection("files").document(file_id).update(patch)

Opinion:

• For consistency, I treat Firestore as the “truth” for the UI layer (status, list, ownership).
• S3 is for bytes only.

---

api/routes/files.py (endpoints)

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

from app.core.config import settings
from app.core.auth import get_current_user
from app.core.appcheck import verify_app_check
from app.services import s3 as s3svc
from app.services import firestore as fssvc

router = APIRouter(prefix="/v1/files", tags=["files"])

class PresignUploadRequest(BaseModel):
    contentType: str
    fileExtension: str

@router.post("/presign-upload")
def presign_upload(
    body: PresignUploadRequest,
    user=Depends(get_current_user),
    _appcheck=Depends(verify_app_check),  # remove if you want optional
):
    uid = user["uid"]
    file_id = str(ulid.new())
    key = f"users/{uid}/uploads/{file_id}.{body.fileExtension}"

    url = s3svc.presign_put_object(bucket=settings.s3_bucket, key=key, content_type=body.contentType)

    # Server writes metadata as pending
    fssvc.create_file_doc(
        file_id=file_id,
        doc={
            "ownerUid": uid,
            "bucket": settings.s3_bucket,
            "key": key,
            "contentType": body.contentType,
            "status": "pending",
        },
    )

    return {
        "fileId": file_id,
        "bucket": settings.s3_bucket,
        "key": key,
        "upload": {
            "method": "PUT",
            "url": url,
            "headers": {"Content-Type": body.contentType},
            "expiresInSeconds": settings.presign_exp_seconds,
        },
    }

@router.post("/{file_id}/complete")
def complete_upload(
    file_id: str,
    user=Depends(get_current_user),
    _appcheck=Depends(verify_app_check),
):
    uid = user["uid"]
    doc = fssvc.get_file_doc(file_id)
    if doc is None:
        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="File not found")

    if doc.get("ownerUid") != uid:
        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Not your file")

    # Confirm object exists in S3
    head = s3svc.head_object(bucket=doc["bucket"], key=doc["key"])
    size = int(head.get("ContentLength", 0))

    fssvc.update_file_doc(file_id, {"status": "ready", "sizeBytes": size})
    return {"fileId": file_id, "status": "ready", "sizeBytes": size}

@router.post("/{file_id}/presign-download")
def presign_download(
    file_id: str,
    user=Depends(get_current_user),
    _appcheck=Depends(verify_app_check),
):
    uid = user["uid"]
    doc = fssvc.get_file_doc(file_id)
    if doc is None:
        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="File not found")

    if doc.get("ownerUid") != uid:
        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Not your file")

    url = s3svc.presign_get_object(bucket=doc["bucket"], key=doc["key"])
    return {
        "download": {
            "method": "GET",
            "url": url,
            "expiresInSeconds": settings.presign_exp_seconds,
        }
    }

Opinionated notes:

• The server is the only thing deciding bucket/key.
• The client can’t “guess” someone else’s file because it can’t get a URL without passing auth checks.

---

main.py

from fastapi import FastAPI
from app.core.firebase_admin import init_firebase_admin
from app.api.routes.files import router as files_router

def create_app() -> FastAPI:
    init_firebase_admin()
    app = FastAPI()
    app.include_router(files_router)
    return app

app = create_app()

---

iOS client: SwiftUI architecture that stays maintainable

I recommend separating “SDK calls” from “views” early. It’s the difference between a clean app at 10 screens vs a rewrite.

Suggested client structure

• AuthSession (ObservableObject): login state + user
• ApiClient: calls FastAPI, attaches Firebase ID token (+ App Check token if used)
• Repositories: Firestore reads/writes that client is allowed to do
• UploadManager: requests presign → uploads to S3 → completes upload

---

Firebase Auth setup reminder (practical note)

Firebase’s iOS Auth getting-started doc explicitly covers adding email/password sign-in and SDK installation steps. (Firebase)

---

AuthSession (minimal but effective)

import Foundation
import FirebaseAuth

@MainActor
final class AuthSession: ObservableObject {
    @Published var user: User? = nil
    private var handle: AuthStateDidChangeListenerHandle?

    func start() {
        handle = Auth.auth().addStateDidChangeListener { _, user in
            Task { @MainActor in
                self.user = user
            }
        }
    }

    func signIn(email: String, password: String) async throws {
        _ = try await Auth.auth().signIn(withEmail: email, password: password)
    }

    func signUp(email: String, password: String) async throws {
        _ = try await Auth.auth().createUser(withEmail: email, password: password)
    }

    func signOut() throws {
        try Auth.auth().signOut()
    }
}

Opinion:

• You want one source of truth for “who is logged in.”

---

ApiClient: attach Firebase ID token to every request

Firebase describes the general pattern: client obtains an ID token and sends it to your backend. (Firebase)

import Foundation
import FirebaseAuth

struct ApiClient {
    let baseURL: URL

    func authedRequest(path: String, method: String) async throws -> URLRequest {
        guard let user = Auth.auth().currentUser else {
            throw URLError(.userAuthenticationRequired)
        }
        let token = try await user.getIDToken()
        var req = URLRequest(url: baseURL.appendingPathComponent(path))
        req.httpMethod = method
        req.setValue("Bearer \(token)", forHTTPHeaderField: "Authorization")
        req.setValue("application/json", forHTTPHeaderField: "Content-Type")
        return req
    }
}

Opinion:

• Centralize this logic so you never forget auth headers in random screens.

---

Upload flow on iOS (presign → PUT to S3 → complete)

Step 1: request presigned upload info

import Foundation

struct PresignUploadResponse: Decodable {
    struct Upload: Decodable {
        let method: String
        let url: String
        let headers: [String:String]
        let expiresInSeconds: Int
    }
    let fileId: String
    let bucket: String
    let key: String
    let upload: Upload
}

extension ApiClient {
    func presignUpload(contentType: String, fileExtension: String) async throws -> PresignUploadResponse {
        var req = try await authedRequest(path: "/v1/files/presign-upload", method: "POST")
        let body = ["contentType": contentType, "fileExtension": fileExtension]
        req.httpBody = try JSONSerialization.data(withJSONObject: body)
        let (data, _) = try await URLSession.shared.data(for: req)
        return try JSONDecoder().decode(PresignUploadResponse.self, from: data)
    }

    func completeUpload(fileId: String) async throws {
        let req = try await authedRequest(path: "/v1/files/\(fileId)/complete", method: "POST")
        _ = try await URLSession.shared.data(for: req)
    }
}

Step 2: upload bytes to S3 using presigned URL (PUT)

import Foundation

struct UploadManager {
    let api: ApiClient

    func uploadFile(data: Data, contentType: String, fileExtension: String) async throws -> String {
        let presign = try await api.presignUpload(contentType: contentType, fileExtension: fileExtension)

        guard let url = URL(string: presign.upload.url) else {
            throw URLError(.badURL)
        }

        var put = URLRequest(url: url)
        put.httpMethod = presign.upload.method
        for (k, v) in presign.upload.headers {
            put.setValue(v, forHTTPHeaderField: k)
        }

        // Upload bytes directly to S3
        _ = try await URLSession.shared.upload(for: put, from: data)

        // Tell backend to validate + mark ready
        try await api.completeUpload(fileId: presign.fileId)

        return presign.fileId
    }
}

Opinion:

• Start with Data uploads for images.
• For large videos, prefer upload(for:fromFile:) so you don’t load the whole file into memory.

---

Firestore Security Rules: production-ready starter patterns

Rules should be:

1. short,
2. obvious,
3. testable.

Firebase’s rules docs emphasize conditions, authentication checks, and incoming-data validation. (Firebase)

Absolute rule #1

Deny by default, then open exactly what you intend:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /{document=**} {
      allow read, write: if false;
    }
  }
}

Then add targeted matches.

---

Helper functions I use constantly

function signedIn() {
  return request.auth != null;
}

function isOwner(ownerUidField) {
  return signedIn() && request.auth.uid == ownerUidField;
}

---

Example: user profiles

match /users/{uid} {
  allow read, write: if signedIn() && request.auth.uid == uid;
}

---

Example: user-owned “files” metadata (server-managed writes)

If your server is writing metadata (recommended), keep client writes closed:

match /files/{fileId} {
  allow read: if signedIn() && resource.data.ownerUid == request.auth.uid;
  allow write: if false; // Only server writes (Admin SDK)
}

Why this is important: server client libraries bypass Firestore Security Rules, which means rules won’t protect server operations—your server must enforce authorization itself. (Firebase)

Opinion:

• This “read-only to clients” pattern is a huge simplifier for security.
• You can still let clients create their own metadata, but validating everything in rules gets complicated fast.

---

Example: posts + likes (good “anti-unbounded-array” approach)

match /posts/{postId} {
  allow read: if true;

  allow create: if signedIn()
    && request.resource.data.authorUid == request.auth.uid
    && request.resource.data.keys().hasOnly(["authorUid","text","createdAt"])
    && request.resource.data.text is string
    && request.resource.data.text.size() <= 2000;

  allow update, delete: if signedIn()
    && resource.data.authorUid == request.auth.uid;
}

match /posts/{postId}/likes/{uid} {
  allow create, delete: if signedIn() && request.auth.uid == uid;
  allow read: if true;
}

Opinion:

• keys().hasOnly([...]) is one of the best tools for preventing “surprise fields.”
• Keep validation modest in rules; do complex validation in your backend.

---

Testing rules and auth locally (Emulator Suite)

I recommend adopting emulators early so you don’t “test security in production.”

Firebase documents connecting your app to the Authentication Emulator, and notes that when Auth emulator + other emulators are running, they can work together for testing auth + rules. (Firebase)

Opinion:

• I use emulators for:

  • rules unit tests
  • “can user A read user B?” regressions
  • CI sanity checks

---

Cost & performance: concrete patterns that prevent pain

Most Firebase cost surprises come from reads, not writes.

Use pagination by default for feeds

Firestore supports pagination using query cursors like startAfter() combined with limit(). (Firebase)

Opinion:

• Never load “all posts” in production UI.
• Always page.

Prefer “get once” over realtime listeners for non-live screens

Firestore queries can be used with either “get once” or realtime listeners. (Firebase)

Opinion:

• Realtime listeners are awesome for chat and live dashboards.
• They’re wasteful for screens that users open briefly (settings, profile editor, static lists).

Use listeners sparingly and detach them

Opinion:

• Attach listeners when a screen appears.
• Detach when it disappears.
• Don’t leave 10 listeners running in background because it “felt convenient.”

Denormalize for list screens

Opinion:

• Put the fields you need for a list cell directly on the list documents.
• Avoid N+1 reads (fetch list → fetch each author doc).

---

S3 “real-world” hardening checklist (opinionated)

Even if you do presigned URLs correctly, production needs a few extra guardrails:

Keep expirations short

Boto3’s presigned URL generation is explicitly time-bound via ExpiresIn. (Boto3)

Opinion:

• 5 minutes is usually plenty.
• If uploads are large, increase expiry but add /complete validation.

Restrict what the presign can do

Opinion:

• Presign only put_object and get_object.
• Do not expose list operations.

Consider size constraints (if abuse risk is high)

Presigned POST supports conditions like content-length-range. (Boto3)

Opinion:

• If you’re seeing abuse, switch uploads to presigned POST or enforce strict post-upload checks and delete oversized objects.

---

Optional: App Check end-to-end (client → FastAPI)

App Check tokens are cached client-side and are meant to be attached to requests to protected services. (Firebase) Firebase documents verifying App Check tokens on custom backends and shows server-side verification flow. (Firebase)

Opinion:

• App Check is worth it if your app has:

  • public signup
  • expensive endpoints (presign, AI, export jobs)
  • abuse risk

• I’d roll it out gradually:

  1. instrument + monitor
  2. enforce on sensitive endpoints
  3. enforce more broadly once stable

---

Final opinionated roadmap (if you want the “least regret” path)

1. Firebase Auth + Firestore core data model
2. Security Rules with an “ownerUid everywhere” discipline
3. Emulator Suite tests for auth + rules

4. If/when needed:

   • FastAPI for S3 presigning + privileged operations
   • App Check enforcement on FastAPI endpoints

5. Scale optimization:

   • pagination everywhere
   • reduce listeners
   • denormalize for list views

---

If you want, I can add one more “fully integrated” section with:

• a complete Dockerfile + deployment notes (Cloud Run or AWS ECS/Fargate),
• an example IAM policy for the presigning service account/user,
• and Firestore index planning for common feed queries.