I Had Three Sources of Truth

You'll see real project code: a Key-Value Store wrapper, a saveRoutes function that writes to SQLite, mirrors to iCloud, updates sync status, and notifies the widget, plus migration logic that uses "max wins" for streaks and totals.

Transcript

1. I Had Three Sources of Truth 🚶Walk Mate's data persistence journey Emre Degirmenci - @emrdgrmnci - NSSpain XIV
2. 🇹🇷 Izmir 🍇 🫶 Logroño 🇪🇸 Emre Degirmenci - @emrdgrmnci - NSSpain XIV
3. 🇹🇷 Izmir 🍇 🫶 Logroño 🇪🇸 Emre Degirmenci - @emrdgrmnci - NSSpain XIV
4. NSIzmir Emre Degirmenci - @emrdgrmnci - NSSpain XIV
5. Emre Degirmenci • First time at NSSpain 🇪🇸 • iOS Engineer since 2017 • Obsessed with extreme sports 🛹🏂🪁🏄 • Rabbit person 🐰 Emre Degirmenci - @emrdgrmnci - NSSpain XIV LinkedIn
6. A war story, not a brochure • Real shipping app • Real migrations • Real mistakes ME?
7. What is Walk Mate?
8. What was Walk Mate storing? • Random routes on map 🗺 • Favorite routes 🫶
9. What Walk Mate stores now? • Avoided street segments 🚧 • No-go zones 💀⚠
10. What Walk Mate stores now? • Streaks and medals 🥇 • Total distance 🛣 • Achievements 🎉
11. What Walk Mate stores now? • Watch + Widgets + Game Center
12. What users quietly expect? • Follows me to a new phone 📱📲 • Survives reinstall ♻ • Never silently reset my favorites, streak, total walked distance ❌
13. Four tools, four jobs • UserDefaults ⚙ • NSUbiquitousKeyValueStore 🫙 • CloudKit / SQLiteData ☁︎ • Game Center 🎮
14. Era 1: Just UserDefaults 🪨 • Routes and favorites as JSON blobs ☁︎ • Zero sync, zero backup 📲
15. The moment that forced change • New phone 🆕 🤳 • Restore ♻ • Favorites gone 😱
16. 🔪 ☁︎ ☁︎ Era 2: Split iCloud
17. • Big arrays -> iCloud Drive JSON files • Why? The 1 MB KVS limit is real 🔪 • Small scalars -> NSUbiquitousKeyValueStore ☁︎ ☁︎ Era 2: Split iCloud
18. A boring wrapper on purpose 🌯 😱
19. What hurt in Era 2? • Two copies of the same data 🪩 • Which one is newer❓ • Functions named backFilliCloudKVSIfNeeded 🤢 🦨
20. Era 3: One database 🎉 • SQLiteData rows • WalkMateUserStatsRow, Route, Favorite, … ♥ • SyncEngine -> CloudKit private DB ⁉ ☁︎ 🧘
21. Sync as a list of tables
22. Sync as a list of tables
23. Sync as a list of tables
24. Sync as a list of tables
25. Sync as a list of tables
26. Sync as a list of tables
27. Sync as a list of tables
28. Sync as a list of tables
29. Sync as a list of tables
30. Migration is part of the product • Read KVS 📖 • Read Drive JSON • Read old UserDefaults • Write SQLite ✏ • Max wins for streaks and totals 🎉
31. What one save actually does
32. What one save actually does
33. What one save actually does
34. What one save actually does
35. What one save actually does
36. Accidental backup: Game Center • Not designed for storage 🫙 • Leaderboards and achievements encode totals 💥 • Raise if higher ⬆ Never lower ⬇ ’ • User s progress
37. So… three sources of truth 1. SQLiteData + CloudKit 2. iCloud KVS / Drive mirrors 3. Game Center lower bounds
38. Five things I d tell my past self 🗣 1. Pick storage by size and importance 2. UserDefaults is a settings file ⚙ 3. Every mirror is a future merge bug 🪲 4.Migrations are forever 🔁 ’ 5. Redundancy beats elegance when streaks are on the line
39. A simple decision map 🧘 ‣ Tiny per-device setting -> UserDefaults ‣ Tiny synced scalars -> iCloud KVS ‣ Growing user data -> SQLite / CloudKit ‣ Monotonic public totals -> Game Center (as floor only)
40. Recap Era 1: one device, then streaks gone Era 2: two iCloud tools, then merge pain Era 3: one database, then migration forever
41. 🇹🇷 Thank you NSSpain! 🇪🇸 Emre Degirmenci - @emrdgrmnci - @emredeg.bsky.social - in/aemrdgrmnci - NSSpain XIV