fetch.diff

diff --git a/src/resourceIndex.js b/src/resourceIndex.js
index 3bcffee4..886f2a74 100644
--- a/src/resourceIndex.js
+++ b/src/resourceIndex.js
@@ -1,6 +1,7 @@
 import fs from 'fs';
 import * as utils from './utils/index.js';
 import zlib from 'zlib';
+import { DateTime } from 'luxon';
 import { promisify } from 'util';
 import { NotFound, BadRequest } from './httpErrors.js';
 import { RESOURCE_INDEX } from './config.js';
@@ -11,7 +12,6 @@ const gunzip = promisify(zlib.gunzip)
 
 const LATEST = "LATEST"
 let resources = {};
-let eTag;
 
 class ResourceVersions {
   constructor(sourceId, resourceType, resourcePath) {
@@ -99,17 +99,24 @@ async function updateResourceVersions() {
     }
     resources = JSON.parse(fileContents);
   } else {
-    const [Bucket, Key] = ['nextstrain-inventories', 'resources.json.gz'];
-    utils.verbose(`Updating available resources index from s3://${Bucket}/${Key}`);
+    const [bucket, key] = ['nextstrain-inventories', 'resources.json.gz'];
+    utils.verbose(`Updating available resources index from s3://${bucket}/${key}`);
+
+    // Generate a signed URL that's stable (and thus cacheable) for up to a day
+    const method = "GET";
+    const issuedAt = DateTime.now().startOf("day").toUnixInteger();
+    const expiresIn = 25 * 60 * 60; // 25 hours in seconds
+    const url = await signedUrl({bucket, key, method, issuedAt, expiresIn});
+
     try {
-      const newETag = (await fetch(await signedUrl({bucket:Bucket, key:Key, method: 'HEAD'}), {method: 'HEAD'}))
-        .headers.get('etag'); // value is enclosed in double quotes, but that doesn't matter for our purposes
-      if (newETag && newETag === eTag) {
-        utils.verbose("Skipping available resource update as eTag hasn't changed");
-        return;
+      // Use no-cache to require revalidation but still allow cached data
+      const response = await fetch(url, {method, cache: 'no-cache'});
+
+      if (response.status !== 200) {
+        throw new Error(`response was: ${response.status} ${response.statusText}`);
       }
-      const newResources = JSON.parse(await gunzip(await (await fetch(await signedUrl({bucket:Bucket, key:Key, method: 'GET'}), {method: 'GET'})).buffer()));
-      [resources, eTag] = [newResources, newETag];
+
+      resources = JSON.parse(await gunzip(await response.buffer()));
     } catch (err) {
       utils.warn(`Resource updating failed: ${err.message}`)
       return;
@@ -123,4 +130,4 @@ export {
   updateResourceVersions,
 
   LATEST,
-}
\ No newline at end of file
+}