feat: more accurate TTL for weather data cache

src/server/cachedFetch.ts

@@ -4,7 +4,7 @@ import { encodeBase64URL } from "./base64url";
 export async function cachedFetch(
   url: string | URL,
   kv: KVNamespace,
-  ttl: number,
+  ttl: number | ((data: unknown) => number),
   fetcher: (url: string | URL) => Promise<Response> = fetch
 ): Promise<unknown> {
   const urlHash = encodeBase64URL(
@@ -26,8 +26,9 @@ export async function cachedFetch(
   }
 
   const networkResult = await response.json();
+  const ttlValue = typeof ttl === "function" ? ttl(networkResult) : ttl;
   await kv.put(cacheKey, JSON.stringify(networkResult), {
-    expirationTtl: ttl,
+    expirationTtl: ttlValue,
   });
 
   return networkResult;

src/server/config.ts

@@ -1,4 +1,6 @@
-export const WEATHER_CACHE_TTL = 5 * 60;
+export const WEATHER_CACHE_TTL_MIN = 2 * 60;
+export const WEATHER_CACHE_TTL_MAX = 14 * 60;
+
 export const REV_GEOCODING_CACHE_TTL = 24 * 60 * 60;
 
 export const BASE_REQUEST_HEADERS = {

src/server/index.ts

@@ -16,11 +16,13 @@ import { cachedFetch } from "./cachedFetch";
 import {
   BASE_REQUEST_HEADERS,
   REV_GEOCODING_CACHE_TTL,
-  WEATHER_CACHE_TTL,
+  WEATHER_CACHE_TTL_MAX,
+  WEATHER_CACHE_TTL_MIN,
 } from "./config";
 import { derivePublicKey, importPrivateKey } from "./cryptoKey";
 import { encryptLocation, parseLocation } from "./location";
 import { locationParamSchema } from "./schemas";
+import { clamp, getNextWeatherDataUpdateTimestamp } from "./utils";
 import { renderWeatherWidget } from "./widgetRenderer";
 
 const NO_CACHE = "private, max-age=0, no-cache, no-store";
@@ -170,7 +172,15 @@ app.get(
     const weatherData = (await cachedFetch(
       forecastURL,
       c.env.KV_FETCH_CACHE,
-      WEATHER_CACHE_TTL,
+      (data) =>
+        clamp(
+          Math.round(
+            (getNextWeatherDataUpdateTimestamp(data as Weather) - Date.now()) /
+              1000
+          ),
+          WEATHER_CACHE_TTL_MIN,
+          WEATHER_CACHE_TTL_MAX
+        ),
       proxiedFetcher
     ).catch(() =>
       Promise.reject(

src/server/utils.ts

@@ -0,0 +1,15 @@
+import type { Weather } from "../types/weather";
+
+export function clamp(value: number, min: number, max: number): number {
+  return Math.min(Math.max(value, min), max);
+}
+
+export function getNextWeatherDataUpdateTimestamp(weather: Weather): number {
+  const localOffsetDate = new Date(`${weather.current.time}:00Z`);
+  const localOffsetTimestamp = localOffsetDate.getTime();
+  return (
+    localOffsetTimestamp -
+    weather.utc_offset_seconds * 1000 +
+    weather.current.interval * 1000
+  );
+}