Opinionated add-ons: how I’d build this in the real world

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1) My default architecture recommendation

If your goal is to ship a solid iOS app quickly and keep complexity low:

My “default stack”

  • Firebase Auth for identity
  • Cloud Firestore for app data
  • Firebase Storage for files unless you already have S3 or must be on AWS
  • Cloud Functions for small server-side glue
  • FastAPI only when you have real backend needs (see below)

This stack is “Firebase-native,” reduces moving parts, and lets your iOS client be productive with fewer backend concerns.

When I’d bring in FastAPI

I’d add a Python backend when you need at least one of:

  • complex business logic (pricing rules, moderation pipelines, scheduling/queuing)
  • integrations (Stripe webhooks, S3 workflows, internal systems)
  • sensitive operations you don’t want in client code (e.g., generating presigned URLs, AI calls, privileged admin actions)
  • “backend as a product” APIs (rate limits, versioning, multi-client support)

If you’re only adding FastAPI because “apps should have an API,” I’d push back: Firebase already is an API layer for many CRUD and realtime use cases.


2) Strong opinions on authentication UX for iOS

Offer “Sign in with Apple” early (even if you also support email/password)

For iOS, “Sign in with Apple” is often the least-friction option for users. Email/password is still fine, but it can create support burden (password resets, typos, spam signups).

Don’t overbuild roles and permissions on day 1

Start with:

  • “regular user”
  • “admin” (if needed)

When you need more nuance, add custom claims or a “roles” doc that your backend controls.

Treat UID as the canonical identity key everywhere

Do not use email as your primary key in Firestore. Emails change. UIDs do not.


3) Firestore modeling: what works well vs what hurts later

Firestore is amazing when your model matches it. It’s painful when you try to force relational patterns into it.

3.1 The rules I follow almost every time

Avoid unbounded arrays

Example of what not to do:

  • post.likes = [uid1, uid2, uid3, ...] (unbounded growth)

Better:

  • posts/{postId}/likes/{uid} (one doc per like)
  • keep posts/{postId}.likeCount as a counter (denormalized)

Prefer “one entity per document”

Keep docs small and queryable. A document is not a dumping ground.

Denormalize intentionally

It’s normal to store some duplicate fields to make queries fast and cheap:

  • store authorName, authorAvatarUrl on a post
  • update it when user changes profile (via Cloud Function or backend)

This avoids “join-like” fan-out reads in the client.

Put permissions into the path when possible

A great pattern:

  • users/{uid}/…
  • orgs/{orgId}/members/{uid}

Paths make Security Rules simpler and reduce accidents.

3.2 A solid starter schema (common social/app pattern)

users/{uid}
  - displayName
  - photoURL
  - createdAt

posts/{postId}
  - authorUid
  - authorDisplayName (denormalized)
  - text
  - createdAt
  - likeCount

posts/{postId}/likes/{uid}
  - createdAt

users/{uid}/notifications/{notifId}
  - type
  - createdAt
  - readAt (nullable)

This scales surprisingly far if you keep queries simple.


4) Security Rules: my philosophy

Use rules as your “seatbelts,” not as your only security system

  • If clients write directly to Firestore/Storage, rules must be correct.
  • But rules can’t easily enforce complex constraints (e.g., “user can only create 5 items/day” or “must pay before accessing”).

For those, use a backend.

Keep rule logic boring and obvious

Avoid clever, complicated rules. Complexity is where mistakes live.

Make owners explicit

I almost always store an explicit ownerUid in documents that represent user-owned resources. It makes rules easy and audits clear.


5) Firebase Storage vs AWS S3: my blunt recommendation

If you don’t already have S3 content or AWS mandates:

Use Firebase Storage. It’s tightly integrated with Firebase Auth and Security Rules and is “one less system.”

If you already have S3 content, AWS-heavy infra, or want CloudFront pipelines:

Use S3, but do it with a clean pattern (below) so your iOS app stays simple and secure.


6) The S3 pattern I recommend (clean + secure)

Never put AWS credentials in your iOS app

Even “limited” credentials tend to leak. Mobile is hostile territory.

Use presigned URLs generated by trusted server code

This is the most practical pattern for mobile.

6.1 How I’d structure S3 keys (opinionated)

Use user-scoped prefixes:

  • users/{uid}/uploads/{uuid}.{ext}
  • users/{uid}/exports/{uuid}.zip

Why:

  • easy to authorize (“user can only access their prefix”)
  • easy to lifecycle-manage and debug

6.2 Keep file metadata in Firestore (even if bytes are in S3)

Do this so the app can list content without hitting S3 listing APIs:

files/{fileId}

  • ownerUid
  • bucket
  • key
  • contentType
  • size
  • createdAt
  • status: "pending" | "ready" | "failed"

My favorite workflow

  1. App requests POST /files/presign-upload
  2. Backend verifies Firebase ID token
  3. Backend creates a Firestore files/{fileId} with status="pending"
  4. Backend returns:
    • fileId
    • uploadUrl (presigned PUT)
    • finalBucket, finalKey
  5. App uploads bytes to S3
  6. App calls POST /files/{fileId}/complete
  7. Backend checks S3 head/object exists, sets status="ready"

This makes uploads reliable and debuggable.


7) FastAPI + Firebase: what people get wrong

7.1 The biggest gotcha: Admin SDK bypasses Security Rules

When your server uses Firebase Admin SDK to read/write Firestore, it typically bypasses Security Rules.

That means:

  • You must implement authorization in FastAPI
  • You cannot assume “Firestore rules will protect it” for server-side operations

I strongly recommend a pattern like:

  • get_current_user() dependency (verifies token, returns uid)
  • route-level checks (owner/admin)
  • centralized authorization helpers to avoid copy/paste mistakes

7.2 Don’t create “an API for everything” unless you must

If the client can safely do it with Firestore + Security Rules, letting it do so can be:

  • faster
  • cheaper
  • less code to maintain

Use FastAPI for:

  • presigned URLs
  • payments/webhooks
  • heavy compute
  • strict validation flows
  • cross-service orchestration
  • admin workflows


8) Opinionated iOS client structure that stays sane

Even a small app becomes spaghetti if you dump Firebase calls directly in views.

8.1 Use an “AuthSession” + “Repositories” approach

  • AuthSession: owns login state (user, uid)
  • UserRepository, PostRepository, etc.: Firestore interactions
  • ApiClient: calls FastAPI (attaches Firebase ID token)

Views should call repositories, not Firebase SDK directly.

8.2 Use async/await wrappers where possible

Even if the SDK uses callbacks, wrap them. Your codebase becomes dramatically more readable.

8.3 Treat network errors as normal UX

Plan for:

  • offline
  • timeouts
  • revoked sessions
  • partial failure (upload succeeded but metadata update failed)

A “retry” button and a small local queue for uploads can be worth more than fancy features.


9) Cost and performance: what I’d watch early

Firebase can be very cost-effective—until you accidentally create a “read storm.”

9.1 Common cost traps

  • Too many realtime listeners on high-churn collections
  • Loading large collections without pagination
  • Chatty updates (writing every keystroke)
  • Over-indexing (letting Firestore auto-prompt you into excessive indexes)

9.2 Practical performance habits

  • Pagination everywhere for feeds
  • Load “summary docs” for list screens (small fields)
  • Fetch “detail docs” only when needed
  • Use counters carefully (atomic increments, or backend-controlled aggregates)


10) Testing & environments: the boring stuff that saves you

10.1 Separate Firebase projects per environment

I’d use:

  • myapp-dev
  • myapp-staging
  • myapp-prod

Same for S3 buckets and FastAPI deployments.

On iOS, map these via build configurations / schemes so you cannot accidentally point dev builds at prod.

10.2 Use emulators early (even lightly)

If you test rules only in production, you’ll eventually ship a security bug.

10.3 Contract tests between iOS and FastAPI

Because your iOS app is a deployed client, breaking APIs is expensive. I like:

  • versioned endpoints (/v1/...)
  • consistent error schema
  • typed request/response models (shared docs and clear migrations)


11) My “decision table” for your choices

Firestore vs “API + database” first

  • If your app is CRUD + realtime-ish → Firestore-first
  • If your app is complex workflows, heavy validation, strict invariants → API-first (FastAPI) + Firebase Auth for identity (still fine)

Firebase Storage vs S3

  • If you’re starting fresh → Firebase Storage
  • If you already have S3 assets or need AWS features → S3 with presigned URLs
  • If you need global caching and controlled public distribution → S3 + CloudFront (often best), still gated by your auth layer if private

Cloud Functions vs FastAPI

  • Small glue + Firebase triggers + light endpoints → Cloud Functions
  • Larger APIs, Python ecosystems, heavier compute → FastAPI


12) A concrete “starter plan” I’d actually follow

If you want a practical, low-risk implementation path:

  1. Implement Firebase Auth (Email/Password + Apple when ready)
  2. Build the core Firestore model (users + your main entities)
  3. Write rules early (at least ownership rules)
  4. Ship MVP with Firestore + (optional) Firebase Storage
  5. Only then:
    • Add FastAPI for the first “server-only” need (often S3 presign or payments)
    • Add S3 if you truly need it (or already have it)

This keeps your app moving while leaving a clean seam for growth.

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