Self-Checkout¶

Also known as: Scan and Pay | Classification: commerce-pattern

Context¶

A customer wants to complete a purchase in-store without waiting for an associate. The store wants to reduce queue times and labour cost at peak periods while maintaining payment security and promotion accuracy.

Forces¶

Force	Tension
Customer wants speed	Staffed checkout queues during peak periods
Store wants to reduce labour cost	Self-checkout terminals require investment
Promotions must be applied correctly	Customer may not know about active promotions
Payment must be fraud-safe	Self-checkout is a higher-risk fraud environment

Solution¶

The customer scans items at a self-checkout terminal (or via mobile scan-and-go). The basket is evaluated against promotions in real-time. Payment is taken via the payment hub. A receipt is issued digitally. Suspicious weight anomalies or age-restricted items trigger an associate intervention.

Sequence¶

sequenceDiagram
    participant C as Customer
    participant SCO as Self-Checkout Terminal
    participant PromEng as Promotion Engine
    participant PayHub as Payment Hub
    participant OMS

    C->>SCO: Start session (scan loyalty card optional)
    loop For each item
        C->>SCO: Scan barcode
        SCO->>SCO: Add to basket + weight check
    end
    SCO->>PromEng: Evaluate promotions on basket
    PromEng-->>SCO: Applied discounts + loyalty points preview
    SCO-->>C: Show basket total
    C->>SCO: Select payment method
    SCO->>PayHub: Authorise + capture payment
    PayHub-->>SCO: Payment confirmed
    SCO->>OMS: Create + close order
    SCO-->>C: Print/send digital receipt

Business Objects¶

Object	Role	State Change
`basket`	Scanned items accumulation	`active` → `checked-out`
`terminal`	Self-checkout device	`active`, `type: self-checkout`
`order`	Completed sale	`created` → `confirmed` → `closed` (atomic)
`payment`	In-store payment	`authorized` → `captured`
`promotion`	Applied discounts	evaluated at basket level

Capabilities Required¶

Capability	Minimum Level	Why
`basket-management`	🟨 3 — Integrated	Real-time basket with scan input
`checkout`	🟨 3 — Integrated	Consistent checkout including promotions and payment
`payment-processing`	🟨 3 — Integrated	Multiple tender types at terminal
`store-operations`	🟨 3 — Integrated	Terminal management + exception handling
`promotions`	🟧 2 — Fragmented	Real-time promotion evaluation at SCO

Trade-offs¶

BenefitsRisksKnown Issues

Reduces checkout queue time by 30–50% at peak
Reduces cashier labour cost
Consistent promotion application (no human error)
Optional loyalty integration at point of payment

Higher shrinkage rate than staffed checkout (scan avoidance)
Age-restricted items require associate intervention — breaks self-serve flow
Technical failures cause significant queuing if machine density is low

Scan avoidance: typically adds 1–3% shrinkage vs. staffed checkout; requires weight verification and random audits
Minimum viable machine count: customer satisfaction drops sharply if fewer than 1 SCO per 50 customers/hour is available
Accessibility: customers with disabilities may require associate assistance

Process: self-checkout
Validation Rules: self-checkout-requires-active-terminal