Scene 04

TTL — origin tells the edge how long to trust the copy

max-age applies to every cache; s-maxage overrides it for shared caches like the CDN; TTL is the countdown that flips a cached cell from fresh to stale.

Previously

The user is now reaching a nearby POP — but how long does that POP hold the cached copy before checking origin again? The origin tells it via Cache-Control, and the dial that lets the CDN hold for a day while the browser holds for a minute is s-maxage.

Scene 04

TTL — origin tells the edge how long to trust the copy

Watch
Try it
Predict
Capture

What to watch for

A response just arrived at the POP with Cache-Control: public, max-age=60, s-maxage=3600. The EDGE cell shows 1:00:00 ticking, the BROWSER cell shows 1:00 ticking — both green. Watch the clock advance: the browser will turn gray well before the edge does.

Implementation

Highlighted lines are the ones running in the diagram right now.

Cache.parseCacheControl(header, role)

shared caches override max-age with s-maxage; browsers ignore it

1def parseCacheControl(header, role):
2    d = parseDirectives(header)  # k=v pairs
3    if 'no-store' in d:
4        return Lifetime(ttl=0, store=False)
5    max_age = int(d.get('max-age', 0))
6    s_maxage = d.get('s-maxage')  # may be absent
7    if role == 'shared' and s_maxage is not None:
8        return Lifetime(ttl=int(s_maxage))
9    return Lifetime(ttl=max_age)

Cache.isFresh(cell, now)

the TTL countdown — fresh until age exceeds the lifetime

1def isFresh(cell, now):
2    if cell.lifetime.ttl == 0:
3        return False  # born stale (e.g. max-age=0)
4    age = now - cell.created_at
5    return age < cell.lifetime.ttl

Cache.serve(request, role)

lookup, freshness check, hit-or-revalidate

1def serve(request, role):
2    cell = store.lookup(request.url)
3    if cell is None:
4        return forwardToOrigin(request)  # MISS
5    if isFresh(cell, now()):
6        return cell.response  # HIT
7    # stale: must revalidate before serving
8    return revalidate(cell, request)

Sources

Where this sits in Build a CDN

Scene 04 of 13, in the Freshness act — TTL, revalidation, stale-while-revalidate.. max-age applies to every cache, including the browser; s-maxage overrides it for shared caches like the CDN — and TTL is what flips a cell from fresh to stale.

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

All 13 scenes in Build a CDN · Every curriculum

Designs that use this

Built with Arqly

Every scene in Build a CDN builds on the one before it.All 13 Build a CDN scenes