Designing a microservices-based system for auto-debiting payments in an asset leasing product involves several considerations to ensure scalability, fault tolerance, and maintainability. Let's outline the architecture and the design choices for this system:

High-Level Architecture

The system can be divided into the following microservices:

1. Mandate Management Service
2. Payment Processing Service
3. Receipt Management Service
4. Reversal Service

Microservices Overview

1. Mandate Management Service:
   • Responsibilities: Handles the registration, update, and lifecycle of mandates. Manages the association of mandates with payment schedules and contracts.
   • Endpoints:
     • POST /mandates: Register a new mandate.
     • GET /mandates/{id}: Retrieve mandate details.
     • PUT /mandates/{id}: Update mandate details.
     • DELETE /mandates/{id}: Deactivate a mandate.
2. Payment Processing Service:
   • Responsibilities: Processes payments based on the payment schedule, either via file-based processing or using GoCardLess.
   • Endpoints:
     • POST /payments: Initiate a payment.
     • GET /payments/{id}: Retrieve payment status.
3. Receipt Management Service:
   • Responsibilities: Creates and manages receipts for successful payments.
   • Endpoints:
     • POST /receipts: Create a new receipt.
     • GET /receipts/{id}: Retrieve receipt details.
4. Reversal Service:
   • Responsibilities: Handles reversal of receipts in case of failed payments.
   • Endpoints:
     • POST /reversals: Initiate a reversal.
     • GET /reversals/{id}: Retrieve reversal status.

Design Considerations

Scalability

• Horizontal Scaling: Each microservice can be scaled independently based on load. For example, the Payment Processing Service might need more instances during peak payment times.
• Load Balancing: Use load balancers to distribute requests evenly across instances of each service.
• Asynchronous Processing: Use message queues (e.g., Azure Service Bus) for asynchronous tasks like payment processing and receipt creation to decouple services and improve responsiveness.

Fault Tolerance

• Retry Mechanisms: Implement retries for transient failures in external dependencies (e.g., GoCardLess API).
• Circuit Breakers: Use circuit breaker patterns to prevent cascading failures by stopping attempts to access a failing service.