| name | ecommerce-patterns |
| description | E-commerce: cart, checkout, payments (Stripe/Adyen), order state, inventory, promos, tax. Triggers: cart, checkout, SKU, payment, Stripe, Shopify, Medusa, Magento, coupon, refund. |
| effort | medium |
| user-invocable | false |
| allowed-tools | Read |
E-commerce Patterns Skill
Platform Selection
| Scenario | Platform |
|---|
| Enterprise B2B | OroCommerce |
| Enterprise B2C | Magento 2 |
| Mid-market | Shopify Plus |
| Small business | WooCommerce, PrestaShop |
| Custom/Headless | Medusa, Saleor |
Magento 2 Patterns
Module Structure
app/code/Vendor/Module/
├── Api/
│ └── Data/
│ └── EntityInterface.php
├── Block/
├── Controller/
│ └── Index/
│ └── Index.php
├── etc/
│ ├── module.xml
│ ├── di.xml
│ ├── routes.xml
│ └── frontend/
├── Model/
│ └── ResourceModel/
├── Setup/
├── view/
│ └── frontend/
│ ├── layout/
│ └── templates/
├── registration.php
└── composer.json
Dependency Injection
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<preference for="Vendor\Module\Api\ServiceInterface" type="Vendor\Module\Model\Service"/>
<type name="Vendor\Module\Model\Service">
<arguments>
<argument name="logger" xsi:type="object">Psr\Log\LoggerInterface</argument>
</arguments>
</type>
</config>
GraphQL
type Query {
customProducts(
search: String
pageSize: Int = 20
currentPage: Int = 1
): CustomProductOutput @resolver(class: "Vendor\\Module\\Model\\Resolver\\Products")
}
Cart & Checkout Patterns
Cart State Machine
Empty → Has Items → Checkout Started → Payment → Order Placed
↑ ↓
└──── Abandoned ←────┘
Checkout Flow
1. Cart Review
2. Shipping Address
3. Shipping Method
4. Payment Method
5. Order Review
6. Place Order
7. Confirmation
Price Calculation
Base Price
- Catalog Discounts
= Adjusted Price
× Quantity
= Line Total
+ Tax (if exclusive)
= Line Total with Tax
Σ All Lines
= Subtotal
+ Shipping
- Cart Discounts
= Grand Total
Inventory Patterns
Stock Management
class InventoryService:
def reserve_stock(self, product_id: str, qty: int) -> bool:
"""Reserve stock during checkout."""
with transaction.atomic():
stock = Stock.select_for_update().get(product_id=product_id)
if stock.available >= qty:
stock.reserved += qty
stock.available -= qty
stock.save()
return True
return False
def confirm_stock(self, reservation_id: str):
"""Confirm reservation after order placed."""
reservation.status = 'confirmed'
reservation.save()
def release_stock(self, reservation_id: str):
"""Release stock if order cancelled."""
Multi-Warehouse
SELECT
p.sku,
w.name as warehouse,
s.quantity,
s.reserved
FROM stock s
JOIN products p ON s.product_id = p.id
JOIN warehouses w ON s.warehouse_id = w.id
WHERE p.sku = 'SKU-123';
Search & Catalog
Product Indexing
Product → Indexer → Search Index (Elasticsearch/OpenSearch)
→ Price Index
→ Category Index
→ Stock Index
Faceted Search
{
"aggs": {
"categories": {
"terms": { "field": "category_ids" }
},
"price_ranges": {
"range": {
"field": "price",
"ranges": [
{ "to": 50 },
{ "from": 50, "to": 100 },
{ "from": 100 }
]
}
},
"brands": {
"terms"
Performance
Caching Layers
| Layer | Cache |
|---|
| Page | Varnish, CDN |
| Block | Redis |
| Query | MySQL Query Cache |
| Full Page | Redis FPC |
Optimization Checklist
Rules
- MUST model order lifecycle as an explicit state machine (pending → authorized → captured → fulfilled → refunded) — ad-hoc booleans produce inconsistent states on retry
- MUST separate payment capture from inventory decrement — reserve stock on order placement, commit stock on payment success. Coupling them causes stuck-stock bugs during payment retries.
- NEVER store CVV, full PAN, or track data — ever. PCI scope expands to any system that sees them, even "temporarily in memory".
- NEVER use floating-point for money calculations. Use
decimal.Decimal, BigDecimal, or integer minor units (cents). Float rounding produces $0.01 discrepancies that accumulate into angry customer emails.
- CRITICAL: every cart mutation is idempotent via an
Idempotency-Key or natural key — users double-click checkout buttons, networks retry, and duplicate orders are expensive to reconcile.
- MANDATORY: tax and shipping calculations live in isolated services or modules. Inlining them into cart logic prevents caching and creates compliance drift.
Gotchas
- Promotion codes with stackable rules explode combinatorially — "20% off + free shipping + first-time buyer" can produce negative totals if the ordering of applications is not defined. Always normalize to a pipeline with a fixed evaluation order.
- Tax jurisdictions change after the order is placed (shipping address edits). A cart that recalculates on every update but not on address change silently under-collects tax.
- Inventory reservations must expire — otherwise abandoned carts hold stock forever. 15-30 minutes is typical; longer reservations need explicit business approval.
- Currency rounding rules differ by locale. Japanese Yen has 0 decimal places, Kuwaiti Dinar has 3. Defaulting to 2 decimals produces off-by-one-unit bugs in non-USD markets.
- Refunds are not negative orders. Payment processors treat them as separate resources with their own state machine and timing (Stripe refunds can take 5-10 business days to settle). Model accordingly.
- Magento's flat-table option speeds reads but silently stops auto-syncing if a reindex fails — stale product data in production. Monitor reindex job success, not just page latency.
When NOT to Load
- For payment provider protocol specifics (Stripe, Adyen, PayPal) — use the provider's SDK docs; this skill is pattern-level
- For generic API design — use
/api-patterns
- For database schema of orders/inventory — use
/database-patterns
- For promotions and segmentation at scale — use
/data-analyst agent
- For PCI compliance audit — this skill flags the traps but is not a substitute for a QSA audit
