Design a Local Delivery Service (DoorDash / Instacart)

#Problem Statement

Design a local delivery service where customers order from local businesses (restaurants, grocery stores), dashers (delivery drivers) pick up and deliver orders, and customers track their delivery in real-time.

📖

Real-world: DoorDash/Instacart blend two patterns you've seen elsewhere — the geospatial matching of Uber (find nearby available dashers) and the state machine of an order moving through placed → accepted → picked-up → delivered, with live location tracking over WebSocket. The interesting wrinkle versus Uber is the two-sided, multi-leg coordination: the dasher, the merchant (is the food ready?), and the customer all need consistent state, and demand is spiky around meal times. Security/integrity angles worth raising: payment fraud and promo abuse (fake accounts farming first-order discounts is a massive cost center for these apps), GPS spoofing by drivers to fake deliveries or game incentive zones, and PII minimization — the customer's address and phone are sensitive, so mature designs mask phone numbers (proxy numbers) and limit how long the dasher can see the address. "How do you stop a driver from spoofing a delivery?" is a fun curveball.

#Requirements

#Functional

• Browse nearby businesses and menus
• Place order → payment → assigned to dasher
• Real-time order tracking (customer tracks dasher)
• Dasher app: accept/decline orders, navigate to pickup + delivery

#Non-Functional

• 10M orders/day → 115 orders/s average; 500/s peak
• Dasher location update: every 5s → 500k active dashers × 0.2/s = 100k location updates/s
• Order assignment latency: < 30 seconds
• Real-time tracking: < 5s location lag

#Core Design

#Dasher Matching

Similar to Uber: geohash-based location index in Redis.

On new order:
  1. Get pickup location geohash
  2. Query Redis for available dashers in geohash cell + neighbors
  3. Filter: unoccupied dasher, correct vehicle type, rating >= threshold
  4. Rank by: proximity + estimated pickup time
  5. Send request to top 3 dashers (first to accept wins)
  6. Timeout 15s: if no accept, expand radius, try next batch

#Order State Machine

PLACED → CONFIRMED (business accepts) → READY_FOR_PICKUP →
PICKED_UP → EN_ROUTE → DELIVERED

State transitions trigger:

• Push notifications to customer
• Business notifications
• Analytics events

#Real-Time Tracking

Dasher app → location update every 5s → Location Service → Redis (TTL 30s)

Customer app subscribes to dasher location for their active order:

• WebSocket: subscribe order:{order_id}:dasher_location
• Server polls Redis for dasher location, pushes updates to all subscribed customers

ETA: distance_to_destination / average_speed_for_area_and_time — or integrate with Google Maps Directions API.

#Architecture

Customer App → Order Service → Payment Service → Order DB (Postgres)
                                                        ↓
                                           Dispatcher → Redis (dasher locations)
                                                        ↓
                                           Notify Dasher App (push notification)
                                                        ↓
                               Dasher Location Service → Redis → WebSocket → Customer
                                                        ↓
                                           Business Notification Service

#Security Considerations

#Interview Tips