Revalidation — the cheap question with the expensive wait

1def handleRequest(url):
2    cell = cache.lookup(url)
3    if cell.cacheControl == 'no-store':
4        return origin.fetch(url)  # always full 200
5    if cell.cacheControl == 'no-cache':
6        return revalidate(cell)   # ask every time
7    # max-age: serve locally while fresh
8    if cell.fresh:
9        return cell.body          # ~2 ms, no RTT
10    return revalidate(cell)       # stale -> ask

1def revalidate(cell):
2    headers = {}
3    if cell.etag:  # If-None-Match: "v7"
4        headers['If-None-Match'] = cell.etag
5    resp = origin.fetch(cell.url, headers)  # full RTT
6    if resp.status == 304:
7        cell.refreshTTL()  # body unchanged
8        return cell.body
9    # 200 OK: origin sent a new body + new ETag
10    cell.body = resp.body
11    cell.etag = resp.headers['ETag']
12    return cell.body

1def handleGET(url, headers):
2    asset = store.load(url)
3    clientEtag = headers.get('If-None-Match')
4    if clientEtag and clientEtag == asset.etag:
5        # short-circuit: empty body, ETag in headers
6        return Response(
7            status = 304,
8            headers = { 'ETag': asset.etag },
9            body = None,
10        )
11    return Response(200, asset.headers, asset.body)