# Emailer Microservice

A Rust-based REST API microservice for Gmail integration using service account authentication with domain-wide delegation support. Send emails on behalf of users in your Google Workspace domain.

## Features

- **Gmail API Integration**: Full Gmail API support with service account authentication
- **Domain-Wide Delegation**: Impersonate users within your Google Workspace domain
- **Email Templates**: Pre-built templates for common notifications
- **RESTful API**: Clean REST endpoints for all operations
- **API Key Authentication**: Secure access via `X-API-Key` header
- **Token Caching**: Efficient OAuth token management
- **Docker Support**: Ready for containerized deployment

## Prerequisites

- Rust 1.75+ (for development)
- Docker & Docker Compose (for deployment)
- A Google Cloud project with Gmail API enabled
- A Google service account with domain-wide delegation configured

## Quick Start

### 1. Environment Setup

Copy the example environment file:

```bash
cp .env.example .env
```

Configure your environment variables:

```env
HOST=127.0.0.1
PORT=4000
API_KEY=your-secret-api-key-here
GOOGLE_SERVICE_ACCOUNT_KEY=your-base64-encoded-service-account-key-here
```

### 2. Google Cloud Setup

1. Create a service account in Google Cloud Console
2. Enable the Gmail API for your project
3. Download the service account key JSON file
4. Base64 encode the JSON file: `cat service-account.json | base64 -w 0`
5. Set the encoded value as `GOOGLE_SERVICE_ACCOUNT_KEY` in your `.env` file
6. Configure domain-wide delegation in Google Workspace Admin Console with scope:
   - `https://www.googleapis.com/auth/gmail.send`

### 3. Running the Service

#### Development

```bash
cargo run
```

#### Production (Docker)

```bash
docker-compose up -d
```

The service will be available at `http://localhost:4000` (or port 4500 if using Docker).

## API Documentation

### Authentication

All API endpoints (except `/health`) require authentication via the `X-API-Key` header:

```
X-API-Key: your-secret-api-key-here
```

### User Impersonation

To send emails on behalf of a user, include the `X-Impersonate-User` header:

```
X-Impersonate-User: user@yourdomain.com
```

### Endpoints

#### Health Check

```
GET /health
```

Returns service health status. No authentication required.

#### Send Email

```
POST /api/v1/emails
Content-Type: application/json
X-API-Key: your-api-key
X-Impersonate-User: sender@yourdomain.com

{
  "to": ["recipient@example.com"],
  "cc": ["cc@example.com"],
  "bcc": ["bcc@example.com"],
  "subject": "Email Subject",
  "body": "Email body content",
  "contentType": "text/html"
}
```

#### List Emails

```
GET /api/v1/emails?q=search_query&maxResults=10&pageToken=token
```

Query parameters:
- `q`: Gmail search query
- `maxResults`: Maximum number of results (default: 10)
- `pageToken`: Pagination token
- `labelIds`: Filter by label IDs
- `includeSpamTrash`: Include spam and trash

#### Get Email

```
GET /api/v1/emails/{email_id}
```

#### Delete Email

```
DELETE /api/v1/emails/{email_id}
```

#### Mark as Read/Unread

```
POST /api/v1/emails/{email_id}/read
POST /api/v1/emails/{email_id}/unread
```

### Template Endpoints

#### List Templates

```
GET /api/v1/templates
```

#### Get Template

```
GET /api/v1/templates/{template_id}
```

#### Send Template Email

```
POST /api/v1/templates/send
Content-Type: application/json
X-API-Key: your-api-key
X-Impersonate-User: sender@yourdomain.com

{
  "to": ["recipient@example.com"],
  "template_id": "general_notification",
  "variables": {
    "recipient_name": "John Doe",
    "subject": "Important Update",
    "message": "This is an important notification.",
    "sender_name": "Admin Team"
  }
}
```

## Built-in Templates

### 1. General Notification (`general_notification`)

General purpose notification template.

**Variables:**
- `subject`: Email subject
- `recipient_name`: Recipient name
- `message`: Notification message
- `sender_name`: Sender name/team

### 2. Service Scheduled (`service_scheduled`)

Template for service scheduling notifications.

**Variables:**
- `recipient_name`: Recipient name
- `customer_name`: Customer name
- `service_date`: Scheduled date
- `service_address`: Service address

### 3. Project Update (`project_update`)

Template for project status updates.

**Variables:**
- `recipient_name`: Recipient name
- `project_name`: Project name
- `project_status`: Current status
- `message`: Additional details

## Usage Examples

### Sending a Simple Email

```bash
curl -X POST http://localhost:4000/api/v1/emails \
  -H "Content-Type: application/json" \
  -H "X-API-Key: your-api-key" \
  -H "X-Impersonate-User: sender@yourdomain.com" \
  -d '{
    "to": ["recipient@example.com"],
    "subject": "Hello from Emailer Service",
    "body": "This is a test email from the emailer service."
  }'
```

### Sending a Template Email

```bash
curl -X POST http://localhost:4000/api/v1/templates/send \
  -H "Content-Type: application/json" \
  -H "X-API-Key: your-api-key" \
  -H "X-Impersonate-User: sender@yourdomain.com" \
  -d '{
    "to": ["team@yourdomain.com"],
    "template_id": "general_notification",
    "variables": {
      "recipient_name": "Team",
      "subject": "System Maintenance",
      "message": "We will be performing maintenance tonight.",
      "sender_name": "IT Team"
    }
  }'
```

## Configuration

### Environment Variables

| Variable | Description | Default |
|----------|-------------|---------|
| `HOST` | Server bind address | `127.0.0.1` |
| `PORT` | Server port | `4000` |
| `API_KEY` | Secret key for API authentication | Required |
| `GOOGLE_SERVICE_ACCOUNT_KEY` | Base64 encoded service account key JSON | Required |

### Service Account Key Formats

The `GOOGLE_SERVICE_ACCOUNT_KEY` can be provided in three formats:

1. **File path**: `/path/to/service_account_key.json`
2. **Base64 encoded**: `cat service-account.json | base64 -w 0`
3. **Raw JSON**: `{"type": "service_account", ...}` (not recommended)

### CORS Configuration

CORS is configured in `src/main.rs`. Modify the allowed origins for your deployment:

```rust
Cors::default()
    .allowed_origin("https://your-app.com")
    // ...
```

## Project Structure

```
├── Cargo.toml
├── Dockerfile
├── docker-compose.yml
├── .env.example
└── src/
    ├── main.rs           # Application entry point
    ├── config.rs         # Configuration management
    ├── handlers/         # HTTP request handlers
    │   ├── emails.rs     # Email operations
    │   ├── templates.rs  # Template operations
    │   └── health.rs     # Health check
    ├── middleware/       # HTTP middleware
    │   └── auth.rs       # API key authentication
    ├── models/           # Data models
    │   ├── email.rs      # Email structures
    │   ├── template.rs   # Template structures
    │   └── error.rs      # Error handling
    └── services/         # Business logic
        └── gmail.rs      # Gmail API client
```

## Error Handling

The service returns standard HTTP status codes:

| Status | Description |
|--------|-------------|
| `200 OK` | Successful operation |
| `201 Created` | Email sent successfully |
| `204 No Content` | Email deleted successfully |
| `400 Bad Request` | Invalid request data |
| `401 Unauthorized` | Invalid or missing API key |
| `404 Not Found` | Resource not found |
| `502 Bad Gateway` | Gmail API error |
| `500 Internal Server Error` | Server error |

Error responses include a JSON body:

```json
{
  "error": "Error type",
  "message": "Detailed error message"
}
```

## Development

### Building from Source

```bash
git clone <repository>
cd emailer-microservice
cargo build --release
```

### Running Tests

```bash
cargo test
```

### Logging

Set the `RUST_LOG` environment variable to control log levels:

```bash
RUST_LOG=info cargo run
RUST_LOG=debug cargo run  # More verbose
```

## Security Considerations

1. **API Key Security**: Keep your API key secure and rotate it regularly
2. **Service Account Security**: Restrict service account permissions to minimum required scope
3. **Network Security**: Use HTTPS in production environments
4. **Domain Verification**: Ensure domain-wide delegation is properly configured
5. **Access Logging**: Monitor access logs for unusual activity

## License

MIT