Weather Caching

Weather Cache Schema

Database

File: weather_cache.db
Engine: expo-sqlite
Journal mode: WAL

Table: `forecast_points`

Column	Type	Null	Notes
`cell_id`	`TEXT`	`NOT NULL`	Spatial cache bucket key from lat/lon.
`forecast_time`	`TEXT`	`NOT NULL`	Canonical UTC hour key (`toHourKey(...)`).
`valid_to`	`INTEGER`	`NOT NULL`	Expiration timestamp in epoch milliseconds.
`payload`	`TEXT`	`NOT NULL`	JSON-serialized `WeatherPoint`.
`last_accessed`	`INTEGER`	`NOT NULL`	Epoch milliseconds for LRU eviction.

Primary key:

PRIMARY KEY (cell_id, forecast_time)

Data Invariants

forecast_time is normalized by toHourKey(...) for both reads and writes.
payload must decode to WeatherPoint (time, temp, rh, wspd, sky).
Stale rows are rows where Date.now() >= valid_to.

Access Patterns

Exact lookup: getForecastPoint(cellId, forecastTime).
Same-hour candidates: getForecastPointsForHour(forecastTime).
Upsert writes: putForecastPoints(cellId, points, validTo).
LRU eviction: evictLRU(maxRows).
Exact-hit access update: getForecastPoint(...) updates last_accessed after payload validation.
Access-time touch for fallback hits: touchForecastPointAccess(cellId, forecastTime).

Cleanup Behavior

Malformed payload rows are removed on read attempts (best effort).
Nearby-fallback access-time touch failures do not block returning usable weather.