# apt-ostree Architecture

## Overview

apt-ostree follows the same daemon-client architecture as rpm-ostree, providing a robust, secure, and scalable system for atomic package management. The architecture ensures proper privilege separation, transaction management, and system reliability.

## Core Architecture

### Daemon-Client Model

```
┌─────────────────────────────────────────────────────────────┐
│                    apt-ostree (Client)                      │
├─────────────────────────────────────────────────────────────┤
│  ┌─────────────┐  ┌─────────────┐  ┌─────────────┐         │
│  │   CLI       │  │   Parser    │  │   Client    │         │
│  │ Commands    │  │   Options   │  │   Library   │         │
│  └─────────────┘  └─────────────┘  └─────────────┘         │
└─────────────────────────────────────────────────────────────┘
                              │
                              │ D-Bus Communication
                              ▼
┌─────────────────────────────────────────────────────────────┐
│                   apt-ostreed (Daemon)                      │
├─────────────────────────────────────────────────────────────┤
│  ┌─────────────┐  ┌─────────────┐  ┌─────────────┐         │
│  │   D-Bus     │  │ Transaction │  │   System    │         │
│  │ Interface   │  │  Manager    │  │ Operations  │         │
│  └─────────────┘  └─────────────┘  └─────────────┘         │
└─────────────────────────────────────────────────────────────┘
                              │
                              │ System Calls
                              ▼
┌─────────────────────────────────────────────────────────────┐
│                    System Layer                             │
├─────────────────────────────────────────────────────────────┤
│  ┌─────────────┐  ┌─────────────┐  ┌─────────────┐         │
│  │   OSTree    │  │     APT     │  │ Bubblewrap  │         │
│  │ Operations  │  │ Operations  │  │ Sandboxing  │         │
│  └─────────────┘  └─────────────┘  └─────────────┘         │
└─────────────────────────────────────────────────────────────┘
```

## Component Details

### 1. Client (`apt-ostree`)

**Purpose**: User interface and command processing

**Key Responsibilities**:
- Parse command-line arguments
- Validate user input
- Communicate with daemon via D-Bus
- Format output for user consumption
- Handle fallback to client-only operations

**Key Components**:
- `main.rs`: Command-line interface and dispatch
- `daemon_client.rs`: D-Bus communication library
- `system.rs`: Client-only fallback operations

**Architecture Pattern**:
```rust
// Daemon-based command with fallback
let result = call_daemon_with_fallback(
    |client| Box::pin(client.install_packages(packages.clone(), yes, dry_run)),
    || Box::pin(async {
        // Fallback to client-only if daemon unavailable
        let system = AptOstreeSystem::new("debian/stable/x86_64").await?;
        system.install_packages(&packages, yes).await?;
        Ok("Packages installed successfully".to_string())
    })
).await?;
```

### 2. Daemon (`apt-ostreed`)

**Purpose**: Privileged operations and system management

**Key Responsibilities**:
- Handle all privileged operations
- Manage OSTree repository operations
- Execute APT package operations
- Provide transaction management
- Implement security policies

**Key Components**:
- `src/bin/apt-ostreed.rs`: Main daemon process
- D-Bus interface implementation
- Transaction management
- System integration

**Architecture Pattern**:
```rust
#[dbus_interface(name = "org.aptostree.dev.Daemon")]
impl AptOstreeDaemon {
    async fn install_packages(&self, packages: Vec<String>, yes: bool, dry_run: bool) -> zbus::fdo::Result<String> {
        // Privileged package installation logic
        // Transaction management
        // Error handling and rollback
    }
}
```

### 3. D-Bus Communication

**Interface**: `org.aptostree.dev.Daemon`
**Object Path**: `/org/aptostree/dev/Daemon`

**Key Methods**:
- `ping()`: Health check
- `status()`: System status
- `install_packages()`: Package installation
- `remove_packages()`: Package removal
- `upgrade_system()`: System upgrade
- `rollback()`: System rollback

**Communication Pattern**:
```rust
// Client side
let client = DaemonClient::new().await?;
let result = client.install_packages(packages, yes, dry_run).await?;

// Daemon side
async fn install_packages(&self, packages: Vec<String>, yes: bool, dry_run: bool) -> zbus::fdo::Result<String> {
    // Implementation
}
```

## Security Architecture

### Privilege Separation

```
┌─────────────────┐    Unprivileged    ┌─────────────────┐
│   User Space    │ ◄────────────────► │   Daemon Space  │
│   (Client)      │                    │   (Privileged)  │
│                 │                    │                 │
│ • CLI Commands  │                    │ • OSTree Ops    │
│ • Option Parsing│                    │ • APT Ops       │
│ • Output Format │                    │ • File System   │
│ • D-Bus Client  │                    │ • D-Bus Server  │
└─────────────────┘                    └─────────────────┘
```

### Security Features

1. **D-Bus Policy**: Restrictive access control
2. **Systemd Service**: Proper service isolation
3. **Bubblewrap Sandboxing**: Script execution isolation
4. **Capability Management**: Minimal privilege requirements
5. **Transaction Rollback**: Atomic operations

## Transaction Management

### Transaction Lifecycle

```
┌─────────────┐    ┌─────────────┐    ┌─────────────┐
│   Start     │───►│   Execute   │───►│   Commit    │
│ Transaction │    │ Operations  │    │ or Rollback │
└─────────────┘    └─────────────┘    └─────────────┘
       │                   │                   │
       ▼                   ▼                   ▼
┌─────────────┐    ┌─────────────┐    ┌─────────────┐
│ Validate    │    │ Monitor     │    │ Update      │
│ Input       │    │ Progress    │    │ System      │
└─────────────┘    └─────────────┘    └─────────────┘
```

### Atomic Operations

1. **Pre-transaction**: Validate system state
2. **Transaction**: Execute operations atomically
3. **Post-transaction**: Update system state
4. **Rollback**: Restore previous state on failure

## Filesystem Architecture

### OSTree Integration

```
/ostree/
├── repo/                    # OSTree repository
│   ├── objects/            # Content-addressed objects
│   ├── refs/               # Branch references
│   └── config              # Repository configuration
├── deploy/                 # Deployments
│   ├── debian/stable/x86_64/  # State root
│   │   ├── deploy/         # Deployment directories
│   │   └── var/            # Persistent state
└── boot/                   # Boot configuration
    ├── loader/             # Bootloader configuration
    └── ostree/             # OSTree boot data
```

### Package Layering

```
┌─────────────────────────────────────────┐
│              User Packages              │
│  (Layered on top of base deployment)   │
├─────────────────────────────────────────┤
│              Base Deployment            │
│  (Immutable base system)               │
├─────────────────────────────────────────┤
│              OSTree Repository          │
│  (Content-addressed storage)           │
└─────────────────────────────────────────┘
```

## Bubblewrap Integration

### Sandboxing Architecture

```
┌─────────────────────────────────────────┐
│           Bubblewrap Sandbox            │
├─────────────────────────────────────────┤
│  ┌─────────────┐  ┌─────────────┐      │
│  │   Script    │  │   Package   │      │
│  │ Execution   │  │  Extraction │      │
│  └─────────────┘  └─────────────┘      │
├─────────────────────────────────────────┤
│  Namespace Isolation                    │
│  • User namespace                       │
│  • PID namespace                        │
│  • Mount namespace                      │
│  • Network namespace                    │
└─────────────────────────────────────────┘
```

### Security Controls

1. **Namespace Isolation**: Complete process isolation
2. **Bind Mounts**: Read-only system directories
3. **Capability Dropping**: Minimal capabilities
4. **Environment Control**: Controlled execution environment

## Error Handling

### Error Hierarchy

```
AptOstreeError
├── PermissionDenied
├── DaemonError
│   ├── ConnectionFailed
│   ├── MethodCallFailed
│   └── TransactionFailed
├── PackageError
│   ├── DownloadFailed
│   ├── InstallationFailed
│   └── DependencyError
├── OSTreeError
│   ├── RepositoryError
│   ├── CommitError
│   └── DeploymentError
└── SystemError
    ├── FilesystemError
    ├── NetworkError
    └── ConfigurationError
```

### Recovery Mechanisms

1. **Automatic Rollback**: Transaction rollback on failure
2. **Fallback Operations**: Client-only fallback when daemon unavailable
3. **Graceful Degradation**: Continue operation with reduced functionality
4. **Error Reporting**: Clear error messages and recovery suggestions

## Performance Considerations

### Optimization Strategies

1. **Parallel Operations**: Concurrent package downloads
2. **Caching**: Package metadata and content caching
3. **Hardlink Optimization**: OSTree hardlink deduplication
4. **Streaming**: Large file streaming operations
5. **Memory Management**: Efficient memory usage patterns

### Monitoring

1. **Transaction Monitoring**: Real-time progress reporting
2. **Resource Usage**: Memory and CPU monitoring
3. **Performance Metrics**: Operation timing and throughput
4. **Error Tracking**: Error rate and type monitoring

## Deployment Architecture

### System Integration

```
┌─────────────────────────────────────────┐
│              System Boot                │
├─────────────────────────────────────────┤
│  ┌─────────────┐  ┌─────────────┐      │
│  │   GRUB      │  │  systemd    │      │
│  │ Bootloader  │  │   Boot      │      │
│  └─────────────┘  └─────────────┘      │
├─────────────────────────────────────────┤
│              OSTree Deployment          │
├─────────────────────────────────────────┤
│  ┌─────────────┐  ┌─────────────┐      │
│  │   apt-ostree│  │ apt-ostreed │      │
│  │   Client    │  │   Daemon    │      │
│  └─────────────┘  └─────────────┘      │
└─────────────────────────────────────────┘
```

### Service Management

1. **systemd Integration**: Proper service lifecycle management
2. **D-Bus Activation**: Automatic daemon startup
3. **Service Dependencies**: Proper dependency management
4. **Health Monitoring**: Service health checks

## Future Architecture Considerations

### Scalability

1. **Multi-Client Support**: Multiple concurrent clients
2. **Distributed Operations**: Network-based operations
3. **Caching Layer**: Distributed caching
4. **Load Balancing**: Operation distribution

### Extensibility

1. **Plugin Architecture**: Extensible functionality
2. **API Evolution**: Backward-compatible API changes
3. **Feature Flags**: Optional feature enablement
4. **Configuration Management**: Flexible configuration

### Security Enhancements

1. **AppArmor Integration**: Mandatory access control
2. **SELinux Support**: Security context management
3. **Audit Integration**: Comprehensive audit logging
4. **Cryptographic Verification**: Package integrity verification