stacks
July 20, 2025
System design references and practice.
Premise
With my (currently) limited experience in working on large cloud-based systems or services that scale, I thought it’d be useful (and fun!) to roleplay a bunch of scenarios where I design exactly those.
Problem α: Design a mobile app for notes
Starting with something very simple.
- Build native front-end using Swift or Kotlin and Jetpack Compose. Alternatively Flutter or React Native.
- Connect to an external PostgreSQL server to store notes. Should at least have ID, title, and content, which can be in markdown or plaintext. A relational database is okay because it shouldn’t have to be normalised that much or often.
Problem 1: This doesn’t work when the user is offline
Instead of an external DB, use SQLite with something like Drift on the client’s device. No need to run a server on-device since its the entire database would be stored as one binary file with no need for compute.
This doesn’t scale
Indeed this solution doesn’t scale too well with large datasets. It cannot handle concurrent writes as well, but these aren’t issues with the product where one user will be writing their own notes.
User cannot access notes over the network (sync)
We can maintain the SQLite database to read and write locally for better latency, but can sync to the PostgreSQL server when there is an Internet connection. This should be simple as localDB packages like Drift or Hive can work with backend services smoothly. Auth is trivial. Conflict management could simply be offline priority, or last-write-wins, without having to struggle with something more complicated like CRDTs.