Stale-while-revalidate — and the bug it ships

1def handleRequest(url):
2    cell = cache.lookup(url)
3    age = now() - cell.storedAt
4    if age < cell.maxAge:
5        return cell.body  # FRESH hit, ~2 ms
6    if age < cell.maxAge + cell.swr:
7        spawn revalidate(url, cell)  # async
8        return cell.body  # STALE, served instantly
9    # past TTL+SWR — blocking origin fetch
10    fresh = origin.fetch(url)
11    cache.store(url, fresh)
12    return fresh.body

1def revalidate(url, oldCell):
2    # runs in the background; user has already been served
3    fresh = origin.fetch(url)  # may return v6 or v7
4    cache.store(url, {
5        body: fresh.body,
6        storedAt: now(),
7        maxAge: fresh.cacheControl.maxAge,
8        swr: fresh.cacheControl.swr,
9    })
10    # next request lands in the new FRESH window

1def purgeUrl(url):
2    # explicit invalidation at the edge
3    cell = cache.lookup(url)
4    cache.evict(url)  # next request misses, fetches v7
5    # without this call, the edge keeps serving the cached
6    # copy until storedAt + maxAge + swr — even after
7    # origin has been deploying the fix the whole time

1def recordServe(servedVersion, t):
2    if deployTimeSec is None:
3        return  # no deploy yet; nothing to count
4    if t < deployTimeSec:
5        return  # served before the fix shipped
6    if servedVersion == PRE_DEPLOY_VERSION:
7        bugCounter += 1  # user saw the bug post-fix