FieldNotes: Offline-First Android

Educational Android demo app for learning offline-first system design.

The app is a small Field Notes tool built with Kotlin, Jetpack Compose, Room, Flow, WorkManager, Hilt, and a fake remote API. It is intentionally implemented through micro milestones so each architectural idea can be reviewed in Git history and in docs/learning/.

What The App Demonstrates

• Local database as the source of truth.
• Hilt dependency injection instead of a manual app container.
• Local writes before network sync.
• Visible sync status.
• Pending create, update, and delete operations.
• Manual sync.
• Background sync with WorkManager.
• Auto sync that queues existing pending notes when enabled.
• Tombstones for offline deletes.
• Fake remote API.
• Conflict detection.
• Conflict resolution with keep local, use remote, and merge both.
• Connectivity awareness.
• Dedicated create/edit note screen with a notes list and floating action button.
• Delete confirmation before removing a note.
• Debug sync log.
• Serialized sync with a Kotlin Mutex so manual sync and background sync do not race.
• Fast unit tests for offline-first behavior.

Architecture

flowchart TB
    A["User writes a note"]
    B["Save immediately<br/>to local database"]
    C["Show note from<br/>local database"]
    D["Sync when network<br/>is available"]
    E["Remote server copy"]
    F{"Conflict?"}
    G["Mark note synced"]
    H["Resolve conflict<br/>merge or choose version"]
    I["Save resolved note<br/>locally"]
    J["Show latest note<br/>from local database"]

    A --> B --> C --> D --> E --> F
    F -->|No| G --> J
    F -->|Yes| H --> I --> J

Loading

The key rule is simple:

The app saves first, shows local data immediately, and syncs later when the network is available.

Package Structure

com.fieldnotes.app
+-- data
|   +-- connectivity   # Online/offline observer
|   +-- local          # Room database, DAO, entities
|   +-- remote         # Fake remote API and remote models
|   +-- sync           # WorkManager worker and sync scheduler
+-- di                 # Hilt modules and dependency bindings
+-- domain             # App models, repository contract, sync states
+-- ui
    +-- notes          # Compose screens, route, ViewModel, UI events/state
    +-- theme          # Compose Material theme

Main idea:

• ui renders state and sends user events.
• domain defines stable app concepts.
• data implements storage, fake remote, connectivity, and sync.
• di wires dependencies with Hilt.

Screenshots

Demo Flow

1. Open Notes and tap Create local note or the + floating action button.
2. Save the note from the dedicated editor screen.
3. See it marked as Pending create.
4. Tap Sync pending changes.
5. See it become Synced.
6. Edit the note.
7. See it become Pending update.
8. Open Remote and edit the fake server copy.
9. Open Notes, edit the same local note differently, and save.
10. Open Sync, tap Sync pending changes, and review conflict behavior.
11. Choose Merge both to combine local and remote text, or choose Keep local / Use remote to compare strategies.
12. Enable auto sync and create/edit a pending note. WorkManager queues one-time sync work that waits for network if needed.
13. Delete a synced note, confirm deletion, and observe tombstone-driven sync behavior.

Auto Sync Behavior

Auto sync is not a timer in this project.

• The app schedules one-time WorkManager work.
• The work has a NetworkType.CONNECTED constraint.
• If network is unavailable, WorkManager waits.
• When network is available, Android can run the queued sync.
• If auto sync is turned on while syncable pending notes already exist, the app queues work immediately.
• Conflict notes are not auto-synced until the user resolves the conflict.

Milestone Docs

Learning notes live in docs/learning/.

Each milestone includes:

• Goal.
• What changed.
• Why it matters.
• Possible solutions.
• Advantages and disadvantages.
• Simple diagram.
• Android best practices.
• Verification.
• Junior, mid-level, senior, and architect interview questions.

Verification

Run:

./gradlew testDebugUnitTest