feat: implement etag on fetch entities actions

[tomrndom]

What kind of change does this PR introduce?

• Add ETag headers to fetch entities actions
• Function to fetch with ETag and manage 304 errors

Does this PR introduce a breaking change?

No

What needs to be documented once your changes are merged?

Nothing

ref: https://app.clickup.com/t/86b70mfce

Signed-off-by: Tomás Castillo tcastilloboireau@gmail.com

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[smarcet]

(See inline comments below)

[smarcet]

etagCache grows unbounded with no eviction strategy. In long,running sessions (conference attendees keeping tabs open for hours), this could consume significant memory.

Consider using a Map with a size limit:

const etagCache = new Map();
const MAX_CACHE_SIZE = 100;

// Before inserting:
if (etagCache.size >= MAX_CACHE_SIZE) {
  const oldest = etagCache.keys().next().value;
  etagCache.delete(oldest);
}

[smarcet]

These helper functions (byLowerCase, toLowerCase, getKeys, getHeaderCaseInsensitive) are unnecessary. The fetch Headers object is not a plain object, so Object.keys(headers) won't iterate it correctly.

Headers.get() is already case,insensitive by spec. You can replace all of this with:

const responseETAG = res.headers.get('etag');

This is simpler and actually correct.

[smarcet]

This ETag cache may conflict with the real,time updates system. When Ably pushes an update and the sync strategies call these fetch functions to get fresh data, the cached ETag could cause a 304 response, returning stale data instead of the updated entity.

Scenario:

1. User loads summit → cached with ETag
2. Backend updates summit → Ably message triggers synchEntityData
3. fetchSummitById sends If,None,Match with old ETag
4. Server responds 304 → stale cached data returned

Consider exporting a clearEtagCache function to invalidate entries when real,time updates are received:

export const clearEtagCacheForUrl = (urlPattern) => {
  for (const key of Object.keys(etagCache)) {
    if (key.includes(urlPattern)) {
      delete etagCache[key];
    }
  }
};

Then call it from sync strategies before re,fetching.

[smarcet]

Non,200/304 status codes are silently swallowed here. If the server returns 401, 403, 404, or 500, the function just returns null with no indication of what went wrong.

Consider adding error logging:

if (!res.ok) {
  console.error(`fetchWithEtag failed (${res.status}):`, url);
}

return null;

[smarcet]

Minor: cacheKey is always identical to apiUrlWithParams (same pattern repeats in every function below). Consider removing the cacheKey parameter from fetchWithEtag and just using the URL directly as the cache key inside the function. This would simplify all call sites from:

const cacheKey = apiUrlWithParams;
return fetchWithEtag(apiUrlWithParams, cacheKey);

To just:

return fetchWithEtag(apiUrlWithParams);

[smarcet]

@tomrndom please review