Designing a mobile podcast app is a deceptively rich system design exercise. Audio streaming with prefetch, offline downloads with smart cleanup, playback position sync across devices, sleep timer with fade-out, smart speed (silence trimming) — all in a constrained-resource mobile environment.
Functional requirements
- Browse and search the podcast catalog
- Subscribe to shows; episodes appear in feed
- Stream or download episodes
- Background audio playback
- Playback position syncs across devices
- Variable playback speed, sleep timer, chapters
Architecture
Three modules: library (subscriptions, episodes), player, downloads.
Subscriptions and feed parsing
RSS feeds are the standard for podcast distribution. Each subscription stores the feed URL. Background job (every 15–60 minutes) refetches feeds, finds new episodes, updates the local DB.
Feed parsing edge cases:
- Feeds can be slow or unavailable — graceful timeout
- Episode dedup by GUID; some feeds reuse GUIDs incorrectly
- Feeds change show titles, artwork — handle gracefully
- Some feeds use chapters (PSC, Podcasting 2.0); render if present
Streaming vs downloading
Streaming: HTTP range requests fetch the audio progressively. Stream the first chunk, start playing, prefetch ahead.
Downloading: full file fetched in advance for offline use. Allow user to set “download new episodes automatically” per podcast.
Smart cleanup: auto-delete episodes after they have been listened to and a configurable age.
Playback engine
iOS: AVAudioEngine with AVAudioPlayerNode for fine-grained control. Android: ExoPlayer (Media3 in 2026).
Variable speed: 1.0x default, configurable per show. Some implementations have smart speed (Overcast) — skip silences in real time, effectively making everything 10–25% faster without pitch change.
Background audio
iOS: declare AVAudioSession category playback, register background audio capability. The player continues when app is suspended; lock screen shows controls.
Android: foreground service with media-style notification. MediaSession integration for system controls and Bluetooth.
Playback position sync
The user listens on phone, then resumes on tablet at the same position. Implementation:
- Periodically (every 30s) push playback position to server
- On app launch, fetch latest positions for the user’s episodes
- Conflict resolution: latest update wins (with HLC timestamps to handle clock skew)
Some apps store the “completed” state explicitly to avoid re-suggesting episodes the user has finished.
Sleep timer
User taps “Sleep in 30 minutes.” App schedules a fade-out at the appointed time. Player gradually reduces volume over 5–10 seconds, then pauses.
Bonus: some apps (Overcast, Pocket Casts) offer “end of episode” or “end of chapter” sleep modes — pause at logical break points.
Battery and data
- Use hardware audio decoder where possible (lower CPU)
- Default to streaming on Wi-Fi only; explicit opt-in for cellular
- Pause downloads when battery is below 20%
- Compress feed XML in transit
Frequently Asked Questions
How do you handle a podcast feed that suddenly has 1000 new episodes?
Cap the feed parser to a reasonable window (last 100 episodes). Some shows publish full archives in their feed.
What about chapter art and show notes?
Both are RSS extensions. Parse and store with the episode. Render in the player UI.
How does background download work on iOS?
URLSession background config — downloads continue when the app is suspended. iOS schedules them based on system state. Best results when the device is on Wi-Fi and charging.