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Sends 5 messages

This service sends messages to downstream consumers

Receives 6 messages

This service receives messages from other services

Overview

The Finance service acts as a critical orchestration component within the BookWorm ecosystem, implementing Domain-Driven Design principles to manage financial transactions and state transitions across the order fulfillment process.

Integration Patterns

The Finance service implements a sophisticated event-driven architecture using MassTransit for message-based communication:

• Event Sourcing: Captures all state changes as a sequence of events
• Saga Pattern: Orchestrates long-running business processes across multiple services
• Outbox Pattern: Ensures reliable message delivery with transactional outbox
• Correlation: Links related events using correlation IDs (OrderId)

Technical Implementation

The service is built with modern .NET technologies and patterns:

• MassTransit State Machine: Implements a robust saga pattern for order processing workflow
• Entity Framework Core: Persists saga state and implements outbox pattern
• PostgreSQL: Stores saga state, inbox/outbox messages
• Minimal API: Exposes RESTful endpoints for querying order state
• AsyncAPI: Documents event-driven messaging contracts

State Machine Workflow

The Finance service implements a state machine with the following states:

State	Description
Placed	Initial state when an order is placed via UserCheckedOutIntegrationEvent
Completed	Final state for successfully processed orders
Cancelled	State for orders that have been cancelled
Failed	State for orders that encountered errors during processing

State Transitions

The state machine handles the following transitions:

1. Initially → Placed: When OrderPlaced event is received

   • Publishes PlaceOrderCommand

2. Placed → Failed: When BasketDeletedFailed event is received

   • Publishes DeleteBasketFailedCommand

3. Placed → Completed: When OrderCompleted event is received

   • Publishes CompleteOrderCommand (if email is available)

4. Placed → Cancelled: When OrderCancelled event is received

   • Publishes CancelOrderCommand (if email is available)

Core Features

Feature	Description
Order Processing	Orchestrates the financial aspects of order processing, including payment authorization, capture, and settlement
Basket Cleanup	Manages the deletion of baskets after order placement, with failure handling
Order State Tracking	Maintains the state of orders throughout their lifecycle
Event-Driven Communication	Publishes and subscribes to events for asynchronous processing
State Querying	Provides API endpoints to retrieve current order state

Architecture diagram

Infrastructure

The Finance service is deployed on Microsoft Azure, leveraging Azure Database for PostgreSQL as the primary data store.

Technical Stack

• MassTransit: Distributed application framework for message-based communication
• Entity Framework Core: ORM for data access
• PostgreSQL: Relational database for persistence
• MediatR: In-process messaging with CQRS pattern
• Aspire: Cloud-native application platform
• OpenAPI/AsyncAPI: API documentation