NFC tag in the stampNTAG213 · read by any iPhone XS or newerA sticker-sized chip. Any recent iPhone can read it.
NFC
NTAG213 chip
the-stamp-api…workers.dev/t/kaffeine
01
Stamp serverCloudflare Worker + Cloudflare KVA tiny program at the nearest data centre + 5-minute memory
CloudflareGET /t/kaffeine
→ new random code
--------
→ kept 5 min in KV
→ 302 /?shop=kaffeine&code=Hq7v…
5:00until the code dies
KV · SHORT-TERM MEMORY
Hq7vK2pX
02
The card + sign-inFirebase Authentication · Continue with GoogleGoogle's sign-in, so nobody has to make a password
FirebaseKaffeine & Co
Continue with Google
SSigned in as Sam
1Code taken off the address bar
2Held for this tab only
3Google sign-in, once
03
The checkPOST /api/redeemThe phone asks: "Can I have my stamp?"
Cloudflare1
2
3
4
5
Hq7vK2pX
screenshot of the link
CODE BURNED
✕ RejectedThat code was already spent. No second stamp.
a new tap, 2 min later
✕ Already stampedNext one in 1439 min. Kaffeine & Co allows one a day.
✓Google sign-in is realsignature checked against Google's keys
✓Code exists and is under 5 minutes old
✓Code belongs to this café
✓Cooldown passedKaffeine & Co: 1 per day
✓Code burned, so it can never stamp again
04
The databaseCloud Firestore · Google CloudThe database, kept in Google's cloud
Firestorecardsshops
DOCUMENTNAMESTAMPSUPDATED
kaffeine_8fQ2…Sam3408:42
kaffeine_Lw91…Mia508:31
zaitouna_8fQ2…Sam2Tue
One card per customer per café. Security rules mean a customer can only ever open their own.
05
Apple WalletThe pass, next to their bank cardApple's own Wallet app, already on every iPhone
WalletEveryday Debit•••• 4021
Gym · 24/7MEMBER
Kaffeine & Co
3 of 6
Shown as a preview today. Real signed passes switch on with our Apple Developer certificate, the A$149 in the budget.
06
The café's dashboardStaff console · liveThe owner sees every tap, the moment it happens
DashboardStamps today41
Members128
Rewards given9
LIVE
SSam · free coffee redeemedlater
SSam · stamp 4 of 6just now
MMia · stamp 5 of 608:31
JJay · stamp 2 of 608:14
A FEW VISITS LATER
4 of 6
Staff: hold to redeem
Redeemed. Gone for good, a screenshot can't be held.
Set by the café: reward at 6 stamps · 1 stamp a day · coffee $3 till 11am
07