# Events with Cosmos DB Example

Demonstrates event-driven architecture patterns using Azure Functions, Service Bus, and Cosmos DB with change feed processing.

## 📋 Overview

This example shows how to:

- Build event-driven systems with Azure Service Bus
- Store and process events in Cosmos DB
- Use Azure Functions for event subscription handling
- Leverage Cosmos DB change feed for event propagation

## 🏗️ Architecture

```mermaid
flowchart TB
    subgraph AppHost["Aspire AppHost"]
        API["Sample.Api<br/>(Events)"] --> ServiceBus["Service Bus<br/>(Topics/Queues)"]
        ServiceBus --> Subs["Sample.Subscriptions"]

        API --> Cosmos
        Subs --> Cosmos

        subgraph Cosmos["Cosmos DB"]
            NewUser["NewUserEvents"]
            Policy["PolicyEvents"]
        end
    end
```

## 🧩 Components

| Project                  | Description                                     |
| ------------------------ | ----------------------------------------------- |
| `Sample`                 | API for creating and publishing events          |
| `Sample.Subscriptions`   | Azure Functions processing Service Bus messages |
| `Sample.Data`            | Cosmos DB models and Entity Framework context   |
| `Sample.AppHost`         | Aspire orchestration with resource wiring       |
| `Sample.Data.Seed`       | Database seeding utility                        |
| `Sample.ServiceBus.Seed` | Service Bus message seeding utility             |

## 🔑 Key Features

- **Event Publishing**: API endpoints for publishing domain events
- **Service Bus Integration**: Topics and subscriptions for event distribution
- **Cosmos DB Storage**: Event storage with partitioning by event type
- **Azure Functions Triggers**: Service Bus triggers for event processing
- **Change Feed**: Cosmos DB change feed for event propagation

## 📊 Event Types

| Event          | Container     | Description                          |
| -------------- | ------------- | ------------------------------------ |
| `NewUserEvent` | NewUserEvents | User registration and profile events |
| `PolicyEvent`  | PolicyEvents  | Insurance policy lifecycle events    |

## 📂 Project Structure

```
foundry/dotnet/events-with-cosmos/
├── Sample.sln
├── Sample.AppHost/
│   ├── AppHost.cs                    # Aspire orchestration
│   └── ResourceBuilders/             # Generated resource builders
├── Sample/
│   └── Endpoints/
│       └── EventEndpoints.cs         # Event publishing API
├── Sample.Subscriptions/
│   ├── PolicyEventSubTrigger.cs      # Policy event handler
│   └── NewUserSubTrigger.cs          # User event handler
├── Sample.Data/
│   ├── Models.cs                     # Event models
│   └── SampleContext.cs              # EF Core context
├── Sample.Data.Seed/                 # DB seeding
└── Sample.ServiceBus.Seed/           # Message seeding
```

## 🚀 Getting Started

### Prerequisites

- .NET 10.0 SDK
- Docker Desktop (for Cosmos DB emulator)
- Azure Service Bus namespace (or emulator)

### Running the Example

```bash
cd foundry/dotnet/events-with-cosmos
dotnet run --project Sample.AppHost
```

## 📡 API Endpoints

| Endpoint         | Method | Description              |
| ---------------- | ------ | ------------------------ |
| `/events/user`   | POST   | Publish a new user event |
| `/events/policy` | POST   | Publish a policy event   |
| `/events`        | GET    | List recent events       |

## 💡 Use Cases

- **Audit Logging**: Immutable event log for compliance
- **Event Sourcing**: Rebuild state from event history
- **CQRS**: Separate read/write models with events
- **Integration Events**: Cross-service communication

## 🔗 Related Documentation

- [Cosmos DB Change Feed](https://docs.microsoft.com/en-us/azure/cosmos-db/change-feed)
- [Service Bus Topics](https://docs.microsoft.com/en-us/azure/service-bus-messaging/service-bus-messaging-overview)

## 📍 Source Code

**Location:** [`foundry/dotnet/events-with-cosmos/`](https://dev.azure.com/SAIFCorporation/Platform/_git/forge?path=/foundry/dotnet/events-with-cosmos)