Sakura Once-Only Gate
The messenger sometimes delivers the same petal ticket twice. Build a gate program that stamps each new ticket once—and never repeats the same job.
🧠 Before we start
At the castle, you remembered votes before making one shared decision. Today, the gate remembers a ticket's unique token. This makes an action idempotent: repeating the same request still changes the festival only once.
Arrange the gate program
Tap the four visual instruction blocks in the safe order. The gate must check a token before stamping its petal.
Handle three deliveries
Tap every messenger card. Two cards share token P7, so the second P7 must be skipped. P9 is a new job.
📨 DELIVERY LINE
🧠 REMEMBERED TOKENS
Survive a lost reply
Ticket K4 was stamped and remembered, but the messenger never received the “done” reply. The same ticket returns. Choose the safe retry.
Debug the double-stamp gate
The buggy gate stamps first and checks the token afterward. A duplicate has already changed the festival twice. Move the check before the action.
🌟 What you learned
- An idempotent action stays safe when the same request repeats.
- A unique token tells the program whether two deliveries are really one request.
- The program checks remembered tokens before changing anything.
- New tokens act once and are then remembered.
- Repeated tokens skip the action but can still receive a reply.
- Safe retries prevent lost replies from creating duplicate work.