Client snowball projection duplicates server simulation (drift risk) #55

New Issue

Closed

opened 2026-05-16 21:43:13 -05:00 by null · 1 comment

null commented

2026-05-16 21:43:13 -05:00

Owner

Severity: LOW 🔵

Affected Files

client/pages/SnowballPage.jsx — computeLiveProjection() lines 37–122
services/snowballService.js — server-side projection

Problem

computeLiveProjection() in SnowballPage.jsx is a client-side reimplementation of the server-side snowball simulation in snowballService.js. A comment acknowledges this ("mirrors server snowballService") but there's no test or assertion to keep them in sync.

If rounding, interest handling, or the rolling-extra logic is fixed on the server, the client projection silently diverges.

Impact

Server and client projections can drift without anyone noticing
Bug fixes in one copy may not be applied to the other
No automated check catches divergence

Fix Options

Preferred: Remove client-side projection entirely; fetch from server on every change (adds latency but guarantees consistency)
Minimal: Add a snapshot test that runs both simulations with the same input and asserts identical output
Compromise: Keep client projection but add a /api/snowball/projection validation endpoint that the client can periodically check against

Acceptance Criteria

Client and server projections produce identical results for the same input
There is a test or mechanism to detect drift

## Severity: LOW 🔵 ## Affected Files - `client/pages/SnowballPage.jsx` — `computeLiveProjection()` lines 37–122 - `services/snowballService.js` — server-side projection ## Problem `computeLiveProjection()` in `SnowballPage.jsx` is a client-side reimplementation of the server-side snowball simulation in `snowballService.js`. A comment acknowledges this ("mirrors server snowballService") but there's no test or assertion to keep them in sync. If rounding, interest handling, or the rolling-extra logic is fixed on the server, the client projection silently diverges. ## Impact - Server and client projections can drift without anyone noticing - Bug fixes in one copy may not be applied to the other - No automated check catches divergence ## Fix Options 1. **Preferred**: Remove client-side projection entirely; fetch from server on every change (adds latency but guarantees consistency) 2. **Minimal**: Add a snapshot test that runs both simulations with the same input and asserts identical output 3. **Compromise**: Keep client projection but add a `/api/snowball/projection` validation endpoint that the client can periodically check against ## Acceptance Criteria - [ ] Client and server projections produce identical results for the same input - [ ] There is a test or mechanism to detect drift

null added the

frontend

priority:nice-to-have

labels 2026-05-16 21:43:13 -05:00

null commented

2026-06-03 21:50:37 -05:00

Author

Owner

Fixed in c353dd9. Deleted computeLiveProjection() client duplicate. Server now accepts ?extra=N for unsaved amount preview. Drift is no longer possible.

Fixed in `c353dd9`. Deleted `computeLiveProjection()` client duplicate. Server now accepts `?extra=N` for unsaved amount preview. Drift is no longer possible.

null closed this issue

2026-06-03 21:50:37 -05:00