Mobile App Backend Guide
A mobile app backend has a few non-negotiable pieces regardless of provider: an authentication system that issues and refreshes tokens correctly across app restarts and device changes, an API that mobile clients can call efficiently over unreliable networks, a push notification pipeline, and file storage for user-generated content. The mistake most teams make is building all four from scratch when managed services exist specifically for this shape of problem.
Decide early whether you want a Backend-as-a-Service (Firebase, Supabase, AWS Amplify) that bundles auth, database, and storage, or a custom API you deploy yourself with separate managed pieces for each concern. BaaS gets you to a working app faster; a custom backend gives you more control as the app business logic grows more particular. Most teams should start with BaaS and only peel off custom services when a specific piece (usually the API business logic) outgrows the managed offering.
Authentication
For BaaS-first apps, Firebase Authentication or Supabase Auth handle email/password, OAuth (Google, Apple, Facebook), phone/SMS OTP, and session/JWT management out of the box — critical for mobile since Sign in with Apple is an App Store requirement if you offer any other third-party login. For custom backends, Auth0, AWS Cognito, or Clerk provide the same primitives as a drop-in service. Whichever you choose, use short-lived access tokens (15–60 min) with refresh tokens stored in secure device storage (Keychain on iOS, Keystore on Android) — never store long-lived tokens in plain app storage.
API design for mobile clients
Mobile networks are slow and flaky, so design the API to minimize round trips: prefer a few chunky endpoints over many chatty ones, support field selection or use GraphQL (Apollo Server, or AWS AppSync if you are Amazon-native) if screens need very different data shapes from the same resources. Version your API from day one (/v1/ in the path or an API-version header) since mobile clients cannot be forced to update instantly — old app versions may hit your API for months after a new release ships. Deploy the API on Cloud Run, Fargate, DigitalOcean App Platform, or Render for a traditional REST/GraphQL server; use Cloudflare Workers if your endpoints are simple and benefit from edge placement for global latency.
Push notifications
You will almost always route through Firebase Cloud Messaging (FCM) for Android and increasingly for cross-platform delivery, and Apple Push Notification service (APNs) for iOS — FCM can actually proxy to APNs so many teams integrate FCM once and cover both platforms. AWS SNS Mobile Push and OneSignal are common alternatives if you want a single API across FCM/APNs with added features like scheduling and segmentation. Store device push tokens tied to user ID in your database and handle token refresh/invalidation — tokens rotate when apps reinstall or update, and sending to a stale token silently fails.
File storage for user content
Use object storage, not your database, for photos/videos/attachments: AWS S3, GCP Cloud Storage, or Cloudflare R2 (which has no egress fees, useful if your app serves a lot of media to a wide user base) are the standard choices. Have the mobile client upload directly to object storage via a signed/presigned URL generated by your backend, rather than proxying large file uploads through your API server — this avoids tying up server resources and timeout limits on mobile-network uploads. Run images through a resizing/CDN layer (Cloudflare Images, Imgix, or a Cloud Function resizer) so you are not sending full-resolution originals to every device.
Offline support and sync
Mobile apps need to tolerate connectivity gaps: use a local database (SQLite via a wrapper like WatermelonDB or Realm, or Core Data/Room natively) to cache data and queue writes, then sync when connectivity returns. Firebase Firestore and Supabase both offer built-in offline persistence and realtime sync if you are using their client SDKs directly, which removes a substantial amount of custom sync logic you would otherwise have to write and test yourself.
Frequently asked questions
Should I use Firebase or build a custom backend?
Start with Firebase (or Supabase) unless you already know your business logic needs are unusual — both bundle auth, database, storage, and often push notifications, getting you to a working app much faster than building each piece yourself. Migrate specific pieces to custom services only when you hit a real limitation.
How do I handle push notifications for both iOS and Android?
Integrate Firebase Cloud Messaging (FCM) once — it delivers natively to Android and can proxy to Apple Push Notification service (APNs) for iOS, so you typically do not need to integrate APNs separately unless you need APNs-specific features.
Where should users photos and videos be stored?
In object storage (S3, GCP Cloud Storage, or Cloudflare R2), never in your primary database. Generate presigned upload URLs so the mobile client uploads directly to storage, and serve images through a CDN/resizing layer rather than the originals.
Do I need to version my mobile API?
Yes, from the first release. Unlike a website where you control what code runs, mobile clients update on their own schedule (or never), so your API needs to keep serving older app versions correctly for months after you ship API changes.