# osbuild Comprehensive Top-to-Bottom Analysis

## Overview

osbuild is a pipeline-based build system for operating system artifacts that defines a universal pipeline description and execution engine. It produces artifacts like operating system images through a structured, stage-based approach that emphasizes reproducibility and extensibility. This document provides a complete top-to-bottom analysis of the entire osbuild process, from manifest parsing to final artifact generation.

## Table of Contents

1. [Complete Process Flow](#complete-process-flow)
2. [Core Architecture](#core-architecture)
3. [Pipeline System](#pipeline-system)
4. [Stage System](#stage-system)
5. [Assembler System](#assembler-system)
6. [Build Execution Engine](#build-execution-engine)
7. [Object Store and Caching](#object-store-and-caching)
8. [Security and Isolation](#security-and-isolation)
9. [External Tools and Dependencies](#external-tools-and-dependencies)
10. [Manifest Processing](#manifest-processing)
11. [Integration Points](#integration-points)
12. [Complete Workflow Examples](#complete-workflow-examples)

## Complete Process Flow

### End-to-End Workflow

```
1. Manifest Loading → 2. Schema Validation → 3. Pipeline Construction → 4. Stage Execution → 5. Object Storage → 6. Assembly → 7. Artifact Generation → 8. Cleanup
```

### Detailed Process Steps

#### **Phase 1: Manifest Processing**
1. **File Loading**: Read JSON manifest file from disk or stdin
2. **Schema Validation**: Validate against JSON schemas for stages and assemblers
3. **Pipeline Construction**: Build stage dependency graph and execution order
4. **Source Resolution**: Download and prepare input sources
5. **Configuration**: Set up build environment and options

#### **Phase 2: Build Environment Setup**
1. **BuildRoot Creation**: Initialize isolated build environment
2. **Resource Allocation**: Set up temporary directories and mounts
3. **Capability Management**: Configure process capabilities and security
4. **API Registration**: Set up communication endpoints
5. **Device Management**: Configure loop devices and partitions

#### **Phase 3: Stage Execution**
1. **Dependency Resolution**: Execute stages in dependency order
2. **Input Processing**: Map input objects to stage requirements
3. **Environment Setup**: Mount filesystems and configure devices
4. **Stage Execution**: Run stage scripts in isolated environment
5. **Output Collection**: Store stage results in object store

#### **Phase 4: Assembly and Output**
1. **Object Collection**: Gather all stage outputs
2. **Assembly Execution**: Run assembler to create final artifact
3. **Format Conversion**: Convert to requested output format
4. **Artifact Generation**: Create final image or archive
5. **Cleanup**: Remove temporary files and mounts

### Process Architecture Overview

```
osbuild CLI → Manifest Parser → Pipeline Builder → Stage Executor → Object Store → Assembler → Final Artifact
     ↓              ↓                ↓               ↓            ↓          ↓         ↓
  Main Entry   JSON Schema    Dependency Graph   Stage Runner   Cache      Output Gen  Image/Archive
```

## Core Architecture

### Design Philosophy

osbuild follows a **declarative pipeline architecture** where:
- **Manifests** define the complete build process as JSON
- **Stages** are atomic, composable building blocks
- **Assemblers** create final artifacts from stage outputs
- **Pipelines** orchestrate stage execution and data flow

### Key Components

```
osbuild CLI → Manifest Parser → Pipeline Executor → Stage Runner → Assembler → Artifacts
     ↓              ↓                ↓               ↓            ↓          ↓
  Main Entry   JSON Schema    Pipeline Builder   Stage Exec   Output Gen   Final Files
```

### Architecture Principles

1. **Stages are never broken, only deprecated** - Same manifest always produces same output
2. **Explicit over implicit** - No reliance on tree state
3. **Pipeline independence** - Tree is empty at beginning of each pipeline
4. **Machine-generated manifests** - No convenience functions for manual creation
5. **Confined build environment** - Security against accidental misuse
6. **Distribution compatibility** - Python 3.6+ support

## Pipeline System

### Pipeline Structure

A pipeline is a directed acyclic graph (DAG) of stages:

```json
{
  "pipeline": {
    "build": {
      "stages": [
        {
          "name": "org.osbuild.debian.debootstrap",
          "options": { ... }
        },
        {
          "name": "org.osbuild.apt",
          "options": { ... }
        }
      ]
    },
    "assembler": {
      "name": "org.osbuild.qemu",
      "options": { ... }
    }
  }
}
```

### Pipeline Execution Model

```python
class Pipeline:
    def __init__(self, info, source_options, build, base, options, source_epoch):
        self.info = info
        self.sources = source_options
        self.build = build
        self.base = base
        self.options = options
        self.source_epoch = source_epoch
        self.checkpoint = False
        self.inputs = {}
        self.devices = {}
        self.mounts = {}
```

### Pipeline Lifecycle

1. **Initialization**: Load manifest and validate schema
2. **Preparation**: Set up build environment and dependencies
3. **Execution**: Run stages in dependency order
4. **Assembly**: Create final artifacts from stage outputs
5. **Cleanup**: Remove temporary files and resources

## Stage System

### Stage Architecture

Stages are the core building blocks of osbuild:

```python
class Stage:
    def __init__(self, info, source_options, build, base, options, source_epoch):
        self.info = info          # Stage metadata
        self.sources = source_options  # Input sources
        self.build = build        # Build configuration
        self.base = base          # Base tree
        self.options = options    # Stage-specific options
        self.source_epoch = source_epoch  # Source timestamp
        self.checkpoint = False   # Checkpoint flag
        self.inputs = {}          # Input objects
        self.devices = {}         # Device configurations
        self.mounts = {}          # Mount configurations
```

### Stage Types

#### **System Construction Stages**
- `org.osbuild.debian.debootstrap`: Debian base system creation
- `org.osbuild.rpm`: RPM package installation
- `org.osbuild.ostree`: OSTree repository management
- `org.osbuild.apt`: Debian package management

#### **Filesystem Stages**
- `org.osbuild.overlay`: File/directory copying
- `org.osbuild.mkdir`: Directory creation
- `org.osbuild.copy`: File copying operations
- `org.osbuild.symlink`: Symbolic link creation

#### **Configuration Stages**
- `org.osbuild.users`: User account management
- `org.osbuild.fstab`: Filesystem table configuration
- `org.osbuild.locale`: Locale configuration
- `org.osbuild.timezone`: Timezone setup

#### **Bootloader Stages**
- `org.osbuild.grub2`: GRUB2 bootloader configuration
- `org.osbuild.bootupd`: Modern bootloader management
- `org.osbuild.zipl`: S390x bootloader

#### **Image Creation Stages**
- `org.osbuild.image`: Raw image creation
- `org.osbuild.qemu`: QEMU image assembly
- `org.osbuild.tar`: Archive creation
- `org.osbuild.oci-archive`: OCI container images

### Stage Implementation Pattern

Each stage follows a consistent pattern:

```python
#!/usr/bin/python3
import os
import sys
import osbuild.api

def main(tree, options):
    # Stage-specific logic
    # Process options
    # Manipulate filesystem tree
    return 0

if __name__ == '__main__':
    args = osbuild.api.arguments()
    ret = main(args["tree"], args["options"])
    sys.exit(ret)
```

### Key Stages Deep Dive

#### **GRUB2 Stage** (`stages/org.osbuild.grub2.iso`)

**Purpose**: Configure GRUB2 bootloader for ISO images
**Implementation**: Template-based GRUB configuration generation

```python
GRUB2_EFI_CFG_TEMPLATE = """$defaultentry
function load_video {
  insmod efi_gop
  insmod efi_uga
  insmod video_bochs
  insmod video_cirrus
  insmod all_video
}

load_video
set gfxpayload=keep
insmod gzio
insmod part_gpt
insmod ext2

set timeout=${timeout}
search --no-floppy --set=root -l '${isolabel}'

menuentry 'Install ${product} ${version}' --class fedora --class gnu-linux --class gnu --class os {
    linux ${kernelpath} ${root} quiet
    initrd ${initrdpath}
}
"""
```

**External Tools Used**:
- `shim*.efi`: Secure boot components
- `grub*.efi`: GRUB bootloader binaries
- `unicode.pf2`: GRUB font files

#### **bootupd Stage** (`stages/org.osbuild.bootupd.gen-metadata`)

**Purpose**: Generate bootupd update metadata
**Implementation**: Chroot execution of bootupctl

```python
def main(tree):
    with MountGuard() as mounter:
        # Mount essential directories
        for source in ("/dev", "/sys", "/proc"):
            target = os.path.join(tree, source.lstrip("/"))
            os.makedirs(target, exist_ok=True)
            mounter.mount(source, target, permissions=MountPermissions.READ_ONLY)
        
        # Execute bootupctl in chroot
        cmd = ['chroot', tree, '/usr/bin/bootupctl', 'backend', 'generate-update-metadata', '/']
        subprocess.run(cmd, check=True)
    
    return 0
```

**External Tools Used**:
- `chroot`: Filesystem isolation
- `bootupctl`: bootupd control utility
- `mount`: Directory mounting

#### **QEMU Assembler** (`assemblers/org.osbuild.qemu`)

**Purpose**: Create bootable disk images
**Implementation**: Comprehensive disk image creation with bootloader support

```python
def main(tree, options):
    # Create image file
    # Partition using sfdisk
    # Format filesystems
    # Copy tree contents
    # Install bootloader
    # Convert to requested format
```

**External Tools Used**:
- `truncate`: File size management
- `sfdisk`: Partition table creation
- `mkfs.ext4`, `mkfs.xfs`: Filesystem formatting
- `mount`, `umount`: Partition mounting
- `grub2-mkimage`: GRUB image creation
- `qemu-img`: Image format conversion

## Assembler System

### Assembler Types

#### **Image Assemblers**
- `org.osbuild.qemu`: Bootable disk images (raw, qcow2, vmdk, etc.)
- `org.osbuild.rawfs`: Raw filesystem images
- `org.osbuild.tar`: Compressed archives
- `org.osbuild.oci-archive`: OCI container images

#### **Specialized Assemblers**
- `org.osbuild.ostree.commit`: OSTree repository commits
- `org.osbuild.error`: Error reporting
- `org.osbuild.noop`: No-operation assembler

### Assembler Implementation

```python
class QemuAssembler:
    def __init__(self, options):
        self.options = options
        self.format = options["format"]
        self.filename = options["filename"]
        self.size = options["size"]
        self.bootloader = options.get("bootloader", {})
    
    def assemble(self, tree):
        # Create image file
        # Set up partitions
        # Copy filesystem contents
        # Install bootloader
        # Convert to target format
        return result
```

## External Tools and Dependencies

### Core System Tools

#### **Filesystem Management**
- `parted`: Partition table management
- `sfdisk`: Scriptable partitioning
- `mkfs.ext4`, `mkfs.xfs`, `mkfs.fat`: Filesystem creation
- `mount`, `umount`: Filesystem mounting
- `losetup`: Loop device management
- `truncate`: File size manipulation

#### **Package Management**
- `rpm`: RPM package management
- `yum`, `dnf`: Package manager frontends
- `apt`, `apt-get`: Debian package management
- `pacman`: Arch Linux package management

#### **Bootloader Tools**
- `grub2-install`: GRUB2 installation
- `grub2-mkimage`: GRUB2 image creation
- `bootupctl`: bootupd control utility
- `zipl`: S390x bootloader

#### **Image and Archive Tools**
- `qemu-img`: Image format conversion
- `tar`: Archive creation and extraction
- `gzip`, `bzip2`, `xz`: Compression
- `skopeo`: Container image operations

#### **System Tools**
- `bubblewrap`: Process isolation
- `systemd-nspawn`: Container management
- `chroot`: Filesystem isolation
- `curl`: Network file transfer

### Build System Dependencies

#### **Python Dependencies** (`requirements.txt`)
```
jsonschema
```

#### **System Dependencies**
- `python >= 3.6`: Python runtime
- `bubblewrap >= 0.4.0`: Process isolation
- `bash >= 5.0`: Shell execution
- `coreutils >= 8.31`: Core utilities
- `curl >= 7.68`: Network operations
- `qemu-img >= 4.2.0`: Image manipulation
- `rpm >= 4.15`: RPM package management
- `tar >= 1.32`: Archive operations
- `util-linux >= 235`: System utilities
- `skopeo`: Container operations
- `python3-librepo`: Repository access

### External Tool Integration Points

#### **Command Execution**
```python
def run_command(cmd, cwd=None, env=None):
    """Execute external command with proper environment"""
    result = subprocess.run(
        cmd,
        cwd=cwd,
        env=env,
        capture_output=True,
        text=True,
        check=True
    )
    return result
```

#### **Chroot Execution**
```python
def chroot_execute(tree, cmd):
    """Execute command in chroot environment"""
    chroot_cmd = ['chroot', tree] + cmd
    return run_command(chroot_cmd)
```

#### **Mount Management**
```python
class MountGuard:
    """Context manager for mount operations"""
    def __enter__(self):
        return self
    
    def __exit__(self, exc_type, exc_val, exc_tb):
        self.cleanup()
    
    def mount(self, source, target, permissions=None):
        # Mount with proper permissions
        pass
```

## Manifest Processing

### JSON Schema Validation

osbuild uses JSON Schema for manifest validation:

```python
def validate_manifest(manifest, schema):
    """Validate manifest against JSON schema"""
    validator = jsonschema.Draft7Validator(schema)
    errors = list(validator.iter_errors(manifest))
    return ValidationResult(errors)
```

### Manifest Structure

```json
{
  "version": "2",
  "pipelines": [
    {
      "name": "build",
      "runner": "org.osbuild.linux",
      "stages": [
        {
          "name": "org.osbuild.debian.debootstrap",
          "options": {
            "suite": "bookworm",
            "mirror": "https://deb.debian.org/debian",
            "variant": "minbase"
          }
        }
      ]
    }
  ],
  "assembler": {
    "name": "org.osbuild.qemu",
    "options": {
      "format": "qcow2",
      "filename": "debian.qcow2",
      "size": "10G",
      "ptuuid": "12345678-1234-1234-1234-123456789012"
    }
  }
}
```

### Template Processing

osbuild supports manifest templating through external tools:

```bash
# Example with jq for dynamic manifest generation
jq --arg size "$IMAGE_SIZE" --arg format "$IMAGE_FORMAT" '
  .assembler.options.size = $size |
  .assembler.options.format = $format
' template.json > manifest.json
```

## Build Execution Engine

### Complete Build Execution System

#### **BuildRoot Architecture**
```python
class BuildRoot(contextlib.AbstractContextManager):
    """Build Root
    
    This class implements a context-manager that maintains a root file-system
    for contained environments. When entering the context, the required
    file-system setup is performed, and it is automatically torn down when
    exiting.
    """
    
    def __init__(self, root, runner, libdir, var, *, rundir="/run/osbuild"):
        self._exitstack = None
        self._rootdir = root
        self._rundir = rundir
        self._vardir = var
        self._libdir = libdir
        self._runner = runner
        self._apis = []
        self.dev = None
        self.var = None
        self.proc = None
        self.tmp = None
        self.mount_boot = True
        self.caps = None
```

#### **BuildRoot Setup Process**
```python
def __enter__(self):
    self._exitstack = contextlib.ExitStack()
    with self._exitstack:
        # Create temporary directories
        dev = tempfile.TemporaryDirectory(prefix="osbuild-dev-", dir=self._rundir)
        self.dev = self._exitstack.enter_context(dev)
        
        tmp = tempfile.TemporaryDirectory(prefix="osbuild-tmp-", dir=self._vardir)
        self.tmp = self._exitstack.enter_context(tmp)
        
        # Set up device nodes
        self._mknod(self.dev, "full", 0o666, 1, 7)
        self._mknod(self.dev, "null", 0o666, 1, 3)
        self._mknod(self.dev, "random", 0o666, 1, 8)
        self._mknod(self.dev, "urandom", 0o666, 1, 9)
        self._mknod(self.dev, "tty", 0o666, 5, 0)
        self._mknod(self.dev, "zero", 0o666, 1, 5)
        
        # Mount tmpfs for /dev
        subprocess.run(["mount", "-t", "tmpfs", "-o", "nosuid", "none", self.dev], check=True)
        self._exitstack.callback(lambda: subprocess.run(["umount", "--lazy", self.dev], check=True))
        
        # Prepare all registered API endpoints
        for api in self._apis:
            self._exitstack.enter_context(api)
        
        self._exitstack = self._exitstack.pop_all()
    
    return self
```

#### **Stage Execution Process**
```python
def execute_stage(stage, context):
    """Execute a single stage"""
    try:
        # 1. Prepare stage environment
        stage.setup(context)
        
        # 2. Set up buildroot
        with buildroot.BuildRoot(build_tree, runner.path, libdir, store.tmp) as build_root:
            # 3. Configure capabilities
            build_root.caps = DEFAULT_CAPABILITIES | stage.info.caps
            
            # 4. Set up mounts and devices
            for name, mount in stage.mounts.items():
                mount_data = mount_manager.mount(mount)
                mounts[name] = mount_data
            
            # 5. Prepare arguments
            args = {
                "tree": "/run/osbuild/tree",
                "paths": {
                    "devices": devices_mapped,
                    "inputs": inputs_mapped,
                    "mounts": mounts_mounted,
                },
                "devices": devices,
                "inputs": inputs,
                "mounts": mounts,
            }
            
            # 6. Execute stage
            result = build_root.run([f"/run/osbuild/bin/{stage.name}"],
                                   monitor,
                                   timeout=timeout,
                                   binds=binds,
                                   readonly_binds=ro_binds,
                                   extra_env=extra_env,
                                   debug_shell=debug_shell)
        
        # 7. Process output
        context.store_object(stage.id, result)
        
        return result
        
    except Exception as e:
        # Handle errors
        context.mark_failed(stage.id, str(e))
        raise
```

#### **Command Execution in BuildRoot**
```python
def run(self, argv, monitor, timeout=None, binds=None, readonly_binds=None, extra_env=None, debug_shell=False):
    """Runs a command in the buildroot.
    
    Takes the command and arguments, as well as bind mounts to mirror
    in the build-root for this command.
    """
    
    if not self._exitstack:
        raise RuntimeError("No active context")
    
    stage_name = os.path.basename(argv[0])
    mounts = []
    
    # Import directories from the caller-provided root
    imports = ["usr"]
    if self.mount_boot:
        imports.append("boot")
    
    # Build bubblewrap command
    bwrap_cmd = [
        "bwrap",
        "--dev-bind", "/", "/",
        "--proc", self.proc,
        "--dev", self.dev,
        "--var", self.var,
        "--tmp", self.tmp,
        "--chdir", "/",
    ]
    
    # Add bind mounts
    for bind in binds or []:
        bwrap_cmd.extend(["--bind"] + bind.split(":", 1))
    
    # Add readonly bind mounts
    for bind in readonly_binds or []:
        bwrap_cmd.extend(["--ro-bind"] + bind.split(":", 1))
    
    # Add environment variables
    if extra_env:
        for key, value in extra_env.items():
            bwrap_cmd.extend(["--setenv", key, value])
    
    # Add command
    bwrap_cmd.extend(argv)
    
    # Execute with bubblewrap
    result = subprocess.run(bwrap_cmd,
                           capture_output=True,
                           text=True,
                           timeout=timeout)
    
    return CompletedBuild(result, result.stdout + result.stderr)
```

#### **Process Isolation and Security**
```python
DEFAULT_CAPABILITIES = {
    "CAP_AUDIT_WRITE",
    "CAP_CHOWN",
    "CAP_DAC_OVERRIDE",
    "CAP_DAC_READ_SEARCH",
    "CAP_FOWNER",
    "CAP_FSETID",
    "CAP_IPC_LOCK",
    "CAP_LINUX_IMMUTABLE",
    "CAP_MAC_OVERRIDE",
    "CAP_MKNOD",
    "CAP_NET_BIND_SERVICE",
    "CAP_SETFCAP",
    "CAP_SETGID",
    "CAP_SETPCAP",
    "CAP_SETUID",
    "CAP_SYS_ADMIN",
    "CAP_SYS_CHROOT",
    "CAP_SYS_NICE",
    "CAP_SYS_RESOURCE"
}

def drop_capabilities(caps_to_keep):
    """Drop all capabilities except those specified"""
    import ctypes
    from ctypes import c_int, c_uint
    
    libc = ctypes.CDLL("libc.so.6")
    
    # Get current capabilities
    caps = c_uint()
    libc.cap_get_proc(ctypes.byref(caps))
    
    # Drop unwanted capabilities
    for cap in ALL_CAPABILITIES - caps_to_keep:
        libc.cap_drop(caps, cap)
    
    # Set new capabilities
    libc.cap_set_proc(ctypes.byref(caps))
```

### Build Process Flow

1. **Manifest Loading**: Parse and validate JSON manifest
2. **Pipeline Construction**: Build stage dependency graph
3. **Source Resolution**: Download and prepare input sources
4. **Stage Execution**: Run stages in dependency order
5. **Assembly**: Create final artifacts from stage outputs
6. **Output**: Export requested objects

### Build Environment

```python
class BuildRoot:
    def __init__(self, path, runner):
        self.path = path
        self.runner = runner
        self.mounts = []
        self.devices = []
    
    def setup(self):
        """Set up build environment"""
        # Create build directory
        # Set up isolation
        # Mount required directories
    
    def cleanup(self):
        """Clean up build environment"""
        # Unmount directories
        # Remove temporary files
```

### Stage Execution

```python
def execute_stage(stage, context):
    """Execute a single stage"""
    try:
        # Prepare stage environment
        stage.setup(context)
        
        # Execute stage
        result = stage.run(context)
        
        # Process output
        context.store_object(stage.id, result)
        
        return result
    except Exception as e:
        # Handle errors
        context.mark_failed(stage.id, str(e))
        raise
```

## Object Store and Caching

### Object Store Architecture

```python
class ObjectStore:
    def __init__(self, path):
        self.path = path
        self.objects = {}
    
    def store_object(self, obj_id, obj):
        """Store object in object store"""
        obj_path = os.path.join(self.path, obj_id)
        os.makedirs(obj_path, exist_ok=True)
        
        # Store object metadata and data
        with open(os.path.join(obj_path, "meta.json"), "w") as f:
            json.dump(obj.meta, f)
        
        obj.export(obj_path)
    
    def get_object(self, obj_id):
        """Retrieve object from store"""
        if obj_id in self.objects:
            return self.objects[obj_id]
        
        obj_path = os.path.join(self.path, obj_id)
        if os.path.exists(obj_path):
            obj = self.load_object(obj_path)
            self.objects[obj_id] = obj
            return obj
        
        return None
```

### Caching Strategy

1. **Object-level caching**: Store stage outputs by ID
2. **Dependency tracking**: Reuse objects when dependencies haven't changed
3. **Incremental builds**: Skip stages with unchanged inputs
4. **Checkpoint support**: Save intermediate results for debugging

### Cache Management

```python
def manage_cache(store, max_size=None):
    """Manage object store cache size"""
    if max_size is None:
        return
    
    # Calculate current cache size
    current_size = calculate_cache_size(store.path)
    
    if current_size > max_size:
        # Remove least recently used objects
        remove_lru_objects(store, current_size - max_size)
```

## Security and Isolation

### Process Isolation

osbuild uses multiple isolation mechanisms:

#### **Bubblewrap**
```python
def run_isolated(cmd, cwd=None, env=None):
    """Run command with bubblewrap isolation"""
    bwrap_cmd = [
        "bwrap",
        "--dev-bind", "/", "/",
        "--proc", "/proc",
        "--dev", "/dev",
        "--chdir", cwd or "/"
    ] + cmd
    
    return run_command(bwrap_cmd, env=env)
```

#### **Systemd-nspawn**
```python
def run_containerized(cmd, tree, env=None):
    """Run command in systemd-nspawn container"""
    nspawn_cmd = [
        "systemd-nspawn",
        "--directory", tree,
        "--bind", "/dev", "/dev",
        "--bind", "/proc", "/proc",
        "--bind", "/sys", "/sys"
    ] + cmd
    
    return run_command(nspawn_cmd, env=env)
```

### Capability Management

```python
DEFAULT_CAPABILITIES = {
    "CAP_AUDIT_WRITE",
    "CAP_CHOWN",
    "CAP_DAC_OVERRIDE",
    "CAP_DAC_READ_SEARCH",
    "CAP_FOWNER",
    "CAP_FSETID",
    "CAP_IPC_LOCK",
    "CAP_LINUX_IMMUTABLE",
    "CAP_MAC_OVERRIDE",
    "CAP_MKNOD",
    "CAP_NET_BIND_SERVICE",
    "CAP_SETFCAP",
    "CAP_SETGID",
    "CAP_SETPCAP",
    "CAP_SETUID",
    "CAP_SYS_ADMIN",
    "CAP_SYS_CHROOT",
    "CAP_SYS_NICE",
    "CAP_SYS_RESOURCE"
}
```

### Security Considerations

1. **Process isolation**: Prevent host system contamination
2. **Capability dropping**: Limit process privileges
3. **Resource limits**: Prevent resource exhaustion
4. **Input validation**: Validate all external inputs
5. **Output sanitization**: Ensure safe output generation

## Integration Points

### CLI Interface

#### **Main Entry Point** (`main_cli.py`)
```python
def osbuild_cli():
    """Main CLI entry point"""
    args = parse_arguments(sys.argv[1:])
    
    # Load manifest
    manifest = parse_manifest(args.manifest_path)
    
    # Validate manifest
    result = validate_manifest(manifest)
    if not result:
        show_validation(result, args.manifest_path)
        return 1
    
    # Execute build
    store = ObjectStore(args.cache)
    result = build_manifest(manifest, store)
    
    # Export results
    if args.export:
        for export_id in args.export:
            export(export_id, args.output_directory, store, manifest)
    
    return 0
```

#### **Command Line Options**
```bash
osbuild [OPTIONS] MANIFEST

Options:
  --cache DIR              Cache directory (default: .osbuild)
  --libdir DIR             Library directory (default: /usr/lib/osbuild)
  --cache-max-size SIZE    Maximum cache size
  --checkpoint ID          Stage to checkpoint
  --export ID              Object to export
  --output-directory DIR   Output directory
  --monitor NAME           Monitor to use
  --stage-timeout SECONDS  Stage timeout
```

### API Interface

#### **Python API** (`api.py`)
```python
def build_manifest(manifest, store, libdir=None):
    """Build manifest using object store"""
    # Load stages and assemblers
    # Execute pipeline
    # Return build result
```

#### **REST API** (Future)
```python
@app.route('/api/v1/build', methods=['POST'])
def build_manifest_api():
    """REST API for manifest building"""
    manifest = request.json
    result = build_manifest(manifest, store)
    return jsonify(result)
```

### External Tool Integration

#### **Container Integration**
```bash
# Docker
docker run --rm -v $(pwd):/workspace osbuild/osbuild manifest.json

# Podman
podman run --rm -v $(pwd):/workspace osbuild/osbuild manifest.json
```

#### **CI/CD Integration**
```yaml
# GitHub Actions example
- name: Build OS Image
  run: |
    osbuild \
      --cache .osbuild \
      --output-directory outputs \
      manifest.json
```

#### **Monitoring Integration**
```python
class Monitor:
    def __init__(self, name):
        self.name = name
    
    def stage_started(self, stage):
        """Called when stage starts"""
        pass
    
    def stage_completed(self, stage, result):
        """Called when stage completes"""
        pass
    
    def stage_failed(self, stage, error):
        """Called when stage fails"""
        pass
```

## Advanced Features

### Multi-Architecture Support

osbuild supports multiple architectures through stage variants:

```json
{
  "stages": [
    {
      "name": "org.osbuild.debian.debootstrap",
      "options": {
        "suite": "bookworm",
        "arch": "arm64"
      }
    }
  ]
}
```

### Parallel Execution

Stages can execute in parallel when dependencies allow:

```python
def execute_parallel(stages, context):
    """Execute independent stages in parallel"""
    import concurrent.futures
    
    with concurrent.futures.ThreadPoolExecutor() as executor:
        futures = {
            executor.submit(execute_stage, stage, context): stage
            for stage in stages
        }
        
        for future in concurrent.futures.as_completed(futures):
            stage = futures[future]
            try:
                result = future.result()
                context.store_object(stage.id, result)
            except Exception as e:
                context.mark_failed(stage.id, str(e))
```

### Checkpoint and Resume

```python
def checkpoint_stage(stage, context):
    """Checkpoint stage execution"""
    if stage.checkpoint:
        # Save stage state
        checkpoint_path = os.path.join(context.store.path, f"{stage.id}.checkpoint")
        stage.save_checkpoint(checkpoint_path)
        
        # Store checkpoint metadata
        context.store.store_object(f"{stage.id}.checkpoint", {
            "type": "checkpoint",
            "stage_id": stage.id,
            "timestamp": time.time()
        })
```

### Remote Execution

```python
class RemoteExecutor:
    def __init__(self, host, user=None, key_file=None):
        self.host = host
        self.user = user
        self.key_file = key_file
    
    def execute_stage(self, stage, context):
        """Execute stage on remote host"""
        # Copy stage to remote host
        # Execute remotely
        # Retrieve results
        pass
```

## Performance Characteristics

### Build Time Optimization

1. **Parallel execution**: Independent stages run concurrently
2. **Object caching**: Reuse unchanged stage outputs
3. **Incremental builds**: Skip stages with unchanged inputs
4. **Resource allocation**: Optimize memory and CPU usage

### Resource Usage

```python
def optimize_resources(stages, available_memory, available_cpus):
    """Optimize resource allocation for stages"""
    # Calculate stage resource requirements
    # Allocate resources optimally
    # Prevent resource contention
```

### Benchmarking

```python
def benchmark_build(manifest, iterations=5):
    """Benchmark build performance"""
    times = []
    
    for i in range(iterations):
        start_time = time.time()
        result = build_manifest(manifest, store)
        end_time = time.time()
        
        times.append(end_time - start_time)
    
    return {
        "mean": statistics.mean(times),
        "median": statistics.median(times),
        "std": statistics.stdev(times),
        "min": min(times),
        "max": max(times)
    }
```

## Conclusion

osbuild represents a sophisticated, production-ready build system for operating system artifacts. Its architecture emphasizes:

1. **Reproducibility**: Consistent results through declarative manifests
2. **Extensibility**: Pluggable stages and assemblers
3. **Performance**: Optimized execution and caching
4. **Security**: Process isolation and capability management
5. **Integration**: Easy integration with existing toolchains

### Key Strengths

- **Structured approach**: Clear separation of concerns
- **Extensible architecture**: Easy to add new stages and assemblers
- **Performance optimization**: Efficient caching and parallel execution
- **Security focus**: Built-in isolation and capability management
- **Distribution support**: Works across multiple Linux distributions

### Areas for Enhancement

- **Bootloader integration**: Limited built-in bootloader support
- **Package management**: Focus on RPM-based systems
- **Image formats**: Limited output format support
- **Validation**: Basic manifest validation capabilities

### Use Cases

1. **Distribution building**: Creating official distribution images
2. **Custom images**: Building specialized OS images
3. **CI/CD pipelines**: Automated image building
4. **Development**: Testing and development environments
5. **Production deployment**: Creating production-ready images

## Complete Workflow Examples

### Example 1: Basic Debian System Image

#### **Manifest Definition**
```json
{
  "version": "2",
  "pipelines": [
    {
      "name": "build",
      "runner": "org.osbuild.linux",
      "stages": [
        {
          "name": "org.osbuild.debian.debootstrap",
          "options": {
            "suite": "bookworm",
            "mirror": "https://deb.debian.org/debian",
            "variant": "minbase"
          }
        },
        {
          "name": "org.osbuild.apt",
          "options": {
            "packages": ["sudo", "openssh-server", "systemd-sysv"]
          }
        },
        {
          "name": "org.osbuild.users",
          "options": {
            "users": {
              "debian": {
                "password": "$6$rounds=656000$...",
                "shell": "/bin/bash",
                "groups": ["sudo"]
              }
            }
          }
        }
      ]
    }
  ],
  "assembler": {
    "name": "org.osbuild.tar",
    "options": {
      "filename": "debian-basic.tar.gz",
      "compression": "gzip"
    }
  }
}
```

#### **Complete Execution Flow**
1. **Manifest Loading**: Parse JSON manifest and validate schema
2. **Pipeline Construction**: Build dependency graph for 3 stages
3. **Source Resolution**: Download Debian packages and sources
4. **Stage Execution**:
   - `debootstrap`: Create base Debian filesystem
   - `apt`: Install packages and dependencies
   - `users`: Create user accounts and groups
5. **Assembly**: Create compressed tar archive
6. **Output**: Generate `debian-basic.tar.gz`

### Example 2: Bootable QEMU Disk Image

#### **Manifest Definition**
```json
{
  "version": "2",
  "pipelines": [
    {
      "name": "build",
      "runner": "org.osbuild.linux",
      "stages": [
        {
          "name": "org.osbuild.debian.debootstrap",
          "options": {
            "suite": "bookworm",
            "variant": "minbase"
          }
        },
        {
          "name": "org.osbuild.apt",
          "options": {
            "packages": ["grub2-efi-amd64", "efibootmgr", "linux-image-amd64"]
          }
        },
        {
          "name": "org.osbuild.grub2",
          "options": {
            "root_fs_uuid": "6e4ff95f-f662-45ee-a82a-bdf44a2d0b75",
            "uefi": {
              "vendor": "debian",
              "unified": true
            }
          }
        }
      ]
    }
  ],
  "assembler": {
    "name": "org.osbuild.qemu",
    "options": {
      "format": "qcow2",
      "filename": "debian-bootable.qcow2",
      "size": "10G",
      "ptuuid": "12345678-1234-1234-1234-123456789012",
      "partitions": [
        {
          "name": "esp",
          "start": 1048576,
          "size": 268435456,
          "type": "fat32",
          "mountpoint": "/boot/efi"
        },
        {
          "name": "root",
          "start": 269484032,
          "size": 10485760000,
          "type": "ext4",
          "mountpoint": "/"
        }
      ]
    }
  }
}
```

#### **Complete Execution Flow**
1. **Manifest Loading**: Parse JSON manifest and validate schema
2. **Pipeline Construction**: Build dependency graph for 3 stages
3. **Source Resolution**: Download Debian packages and GRUB components
4. **Stage Execution**:
   - `debootstrap`: Create base Debian filesystem
   - `apt`: Install GRUB and kernel packages
   - `grub2`: Configure GRUB bootloader
5. **Assembly**: Create QCOW2 disk image with partitions
6. **Output**: Generate `debian-bootable.qcow2`

### Example 3: OSTree-Based System

#### **Manifest Definition**
```json
{
  "version": "2",
  "pipelines": [
    {
      "name": "build",
      "runner": "org.osbuild.linux",
      "stages": [
        {
          "name": "org.osbuild.debian.debootstrap",
          "options": {
            "suite": "bookworm",
            "variant": "minbase"
          }
        },
        {
          "name": "org.osbuild.apt",
          "options": {
            "packages": ["ostree", "systemd", "systemd-sysv"]
          }
        },
        {
          "name": "org.osbuild.ostree",
          "options": {
            "repository": "/var/lib/ostree/repo",
            "branch": "debian/bookworm/x86_64/standard"
          }
        }
      ]
    }
  ],
  "assembler": {
    "name": "org.osbuild.ostree.commit",
    "options": {
      "repository": "debian-ostree",
      "branch": "debian/bookworm/x86_64/standard"
    }
  }
}
```

#### **Complete Execution Flow**
1. **Manifest Loading**: Parse JSON manifest and validate schema
2. **Pipeline Construction**: Build dependency graph for 3 stages
3. **Source Resolution**: Download Debian packages and OSTree
4. **Stage Execution**:
   - `debootstrap`: Create base Debian filesystem
   - `apt`: Install OSTree and systemd packages
   - `ostree`: Configure OSTree repository
5. **Assembly**: Create OSTree commit
6. **Output**: Generate OSTree repository with commit

## Conclusion

osbuild provides a solid foundation for building operating system images with a focus on reproducibility, performance, and extensibility. Its stage-based architecture makes it easy to customize and extend while maintaining consistency and reliability.

### Key Strengths

- **Structured approach**: Clear separation of concerns with stages and assemblers
- **Extensible architecture**: Easy to add new stages and assemblers
- **Performance optimization**: Efficient caching and parallel execution
- **Security focus**: Built-in isolation and capability management
- **Distribution support**: Works across multiple Linux distributions
- **Declarative manifests**: JSON-based configuration with schema validation
- **Process isolation**: Bubblewrap and systemd-nspawn integration
- **Object caching**: Intelligent caching of stage outputs

### Areas for Enhancement

- **Bootloader integration**: Limited built-in bootloader support
- **Package management**: Focus on RPM-based systems
- **Image formats**: Limited output format support
- **Validation**: Basic manifest validation capabilities
- **Template support**: No built-in templating system
- **Cross-architecture**: Limited architecture support

### Complete Process Summary

osbuild implements a **complete end-to-end image building pipeline** that:

1. **Processes Manifests**: JSON with schema validation
2. **Manages Stages**: Atomic, composable building blocks
3. **Executes Builds**: Isolated execution with security controls
4. **Handles Objects**: Intelligent caching and storage
5. **Manages Devices**: Loop devices and partition management
6. **Provides Assembly**: Multiple output format support
7. **Ensures Security**: Process isolation and capability dropping
8. **Generates Artifacts**: Images, archives, and repositories

The system's architecture emphasizes **reproducibility**, **security**, and **extensibility** while maintaining **performance** through intelligent caching and isolated execution environments.