Revalidation — the cheap question with the expensive wait

Scene 05 · Revalidation — the cheap question with the expensive wait

When stale, the edge sends a conditional GET with the ETag; origin replies 304 Not Modified — body is empty, but the round trip isn't.

Previously

When the edge's copy goes stale, it does not re-download the whole body — it asks origin a cheap question first.

Scene 05

Revalidation — the cheap question with the expensive wait

Watch

Diagram

On the left, the edge POP holding one cached cell with an **ETag** badge — an opaque version tag the origin attaches to a response so the edge can ask 'is this still v7?'. In the middle, two arrows: the outgoing **conditional GET** carries `If-None-Match: v7`; the incoming reply is either a thin green `304 Not Modified` (no body, but full RTT) or a thick blue `200 OK` (full body). At the bottom, a waterfall strip — the green segment is **wait-for-server** (origin RTT, paid on every revalidation), and the blue segment is body bytes (paid only when the ETag doesn't match).

Sources

The cell starts FRESH. After a moment its TTL expires and the next request triggers a conditional GET — the edge asks origin 'is this still v7?'. Origin returns a thin 304 (no body), the edge serves the stored bytes locally, and the cell goes back to FRESH.

Implementation

Edge.handleRequest

decides: serve local, revalidate, or bypass

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

Edge.revalidate

conditional GET: 304 keeps stored body, 200 replaces it

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

Origin.handleGET

compares If-None-Match; sends 304 (no body) or 200 (full 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)

PreviousTTL — origin tells the edge how long to trust the copy NextStale-while-revalidate — and the bug it ships