What does each arrow carry?
Separate the tray’s movement from the pickup notice.
The question
You and the interviewer review a fictional ceramics studio’s tray pickup. The sketch has one set of arrows.
- Unlabeled path
- Maker → Shelf → Collector
- Tray
- The maker puts it on the shelf. The collector takes it.
- Notice
- The maker sends it when the tray is on the shelf.
“How would you redraw the movement and the message?”
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A strong answer, step by step
Open each step in turn. This is one strong answer, not the only one.
Step 1: What needs a label?
What would you do here? Open the move you’d make.
Use one unlabeled arrow for everything
Tempting, but it misses something. The interviewer cannot tell what moves or who sends a notice.
Replace the shelf with a notice icon
Tempting, but it misses something. The shelf is where the physical tray waits.
Identify which links move an object and which send information
The strong move. This lets the interviewer trace each route separately.
See how a strong candidate says it
“I’ll label the links that move the tray and use a separate link for the message.”
Why it works: The tray and notice have different routes even though they both reach the collector.
Step 2: How do both paths run?
What would you do here? Open the move you’d make.
Draw a tray route and a separate labeled notice
The strong move. Each arrow has one job and the placement triggers the notice.
Draw the notice from Shelf to Collector
Tempting, but it misses something. In the opening description the maker sends it.
Draw Collector moving the tray to Shelf
Tempting, but it misses something. That reverses the physical movement described in the packet.
See how a strong candidate says it
“Tray: Maker to Shelf to Collector. Notice: Maker to Collector, after the tray goes on the shelf.”
Why it works: The notice has its own sender, recipient and trigger.
Step 3: What triggers the notice now?
The desk volunteer now sends the notice. They scan the shelf tag after the tray is in place and send the notice after the scan.
What would you do here? Open the move you’d make.
Keep a notice from the maker after placement
Tempting, but it misses something. The revised process gives that action to the desk volunteer after scanning.
Replace the notice edge with Scan tag, then Send notice
The strong move. The revised path shows the volunteer and the condition for sending.
Show the scan moving the tray to the collector
Tempting, but it misses something. The scan triggers a notice. It does not move the tray.
See how a strong candidate says it
“I’ll keep the tray route. On the notice path I’ll show the volunteer scanning the shelf tag, then sending the notice to the collector.”
Why it works: The change is to the sender and trigger on the message path. The tray still moves through the shelf.
Step 4: Trace the unscanned tray.
What would you do here? Open the move you’d make.
Say the notice went out when the tray arrived
Tempting, but it misses something. The revised trigger is the volunteer’s scan.
Move the tray back to Maker on the board
Tempting, but it misses something. A pending notice does not change its physical location.
Show the tray on the shelf and the notice pending
The strong move. The tray path reached the shelf while the message path awaits the scan.
See how a strong candidate says it
“The tray is on the shelf. The tag has not been scanned, so the volunteer has not sent the notice. This is where the paths differ.”
Why it works: The physical state can advance while the message waits for its trigger.
The answer, in short
- Decision
- Label two routes and move the notice trigger to the volunteer’s scan.
- Because
- Tray placement and a sent notice are different events.
- What would change it
- If someone changes how the tray moves, update the tray route separately.
Common traps
- Giving one arrow two meanings
- Treating an unsent message as a missing object
Check your answer
- What moves on each path?
- Who sends the notice?
- What triggers it now?
- Where is the tray while the notice waits?
Last reviewed Nov 11, 2026