Design Instagram (Mobile)

Concepts Utilized: MVVM Architecture, Image Caching (LRU), Pagination, Background Upload, Push Notifications, Camera Integration, Video Compression, Offline Queue, Content Prefetching

This document covers the mobile architecture for a photo-sharing application with an infinite scrolling feed, media upload functionality, and camera integration.

Features

Feature	Description
Feed	Infinite scroll with paginated content loading
Photo/video capture	Native camera integration with filters
Stories	Ephemeral content with 24-hour expiration
Direct messaging	Private conversations between users
Push notifications	Alerts for likes, comments, and follows

Architecture

Loading diagram...

Feed Implementation

Pagination

Cursor-based pagination prevents duplicate content when new posts are added during scrolling.

Implementation approach: The ViewModel maintains a cursor (from the last API response) and a loading flag. The loadMore method checks the loading flag to prevent duplicate requests, sets it to true, then fetches posts using the current cursor. On completion, append new posts to the existing list, update the cursor for the next page, and reset the loading flag.

Image Caching

A two-tier cache (memory and disk) reduces network requests and improves scroll performance.

Implementation approach: Check memory cache first (NSCache or similar). If not found, check disk cache and promote to memory cache if found. If not in disk cache, fetch from network and store in both caches. This hierarchy provides fast access for recently viewed images while persisting images across app sessions.

Photo Upload

Background Upload

Uploads persist across application state changes using background tasks.

Implementation approach:

1. Save the image to local storage first
2. Create a pending upload record in the database with pending status
3. Request background execution time from the OS
4. Start the upload; when complete, end the background task

This pattern ensures uploads continue even if the user navigates away from the app, and pending uploads survive app termination for retry when the app relaunches.

Technical Constraints

Constraint	Impact	Mitigation
Memory limits	Loading high-resolution images can trigger OS termination	Downscale images to display size before rendering
Battery consumption	Background uploads consume significant power	Batch uploads and respect low-power mode
Network variability	Uploads fail on unstable connections	Implement retry logic with exponential backoff
Storage limits	Excessive caching causes user-initiated app deletion	Implement LRU eviction with configurable size limits