ADVANCED⚙ Go Backend5-6hMilan

Payment Settlement Service (DDD)

Model a payment settlement service using Domain-Driven Design in Go. Handle the full lifecycle: authorization → clearing → settlement, with proper aggregate roots, domain events, and repository patterns. This directly mirrors Gnosis Pay's card payment backend.

Tech Stack

GoDDD patternsPostgreSQLDomain eventsCQRS (read/write separation)

What You'll Learn

→DDD tactical patterns: Aggregates, Value Objects, Domain Events
→Payment lifecycle: auth → capture → clear → settle
→Event sourcing basics for audit trails
→Repository pattern with PostgreSQL
→Idempotency keys for payment processing

Milestones (0/5)

M1Domain model — aggregates & value objects

▾

M2Repository & persistence

▾

M3Settlement batch processor

▾

M4CQRS read model & API

▾

M5Testing & error scenarios

▾

Architecture Hints

Architecture (DDD Layers)

┌─────────────────────────────────────┐
│           API Layer (HTTP)          │
│  POST /payments/authorize           │
│  POST /payments/:id/capture         │
│  GET  /payments (read model)        │
└────────────────┬────────────────────┘
                 │
┌────────────────▼────────────────────┐
│       Application Services          │
│  PaymentService.Authorize()         │
│  SettlementService.RunBatch()       │
│  (orchestration, no business logic) │
└────────────────┬────────────────────┘
                 │
┌────────────────▼────────────────────┐
│          Domain Layer               │
│  Payment (Aggregate Root)           │
│  Money, CardHolder (Value Objects)  │
│  PaymentAuthorized (Domain Event)   │
│  PaymentRepository (interface)      │
└────────────────┬────────────────────┘
                 │
┌────────────────▼────────────────────┐
│      Infrastructure Layer           │
│  PostgresPaymentRepository          │
│  EventPublisher (Kafka/channel)     │
│  Read Model Projector               │
└─────────────────────────────────────┘

Maps to Gnosis Pay: - Payment aggregate = Visa card transaction lifecycle - Idempotency = Visa network may retry authorizations - Settlement batching = daily Visa settlement process - Domain events = audit trail for compliance

Interview Talking Points

▸Walk through the payment lifecycle and how each state maps to Visa's authorization/clearing/settlement flow
▸Explain why DDD is appropriate for payment processing — complex business rules, invariant enforcement
▸Discuss idempotency key pattern and why it's critical for financial transactions
▸Talk about CQRS trade-offs — eventual consistency in read models vs strong consistency in writes
▸Explain how domain events enable audit trails required for financial compliance