apt-ostree/docs/.old/apt-ostree-daemon-plan/architecture/database-system.md

🗄️ apt-ostree Database System Architecture

📋 Overview

This document outlines the database system architecture for apt-ostree, based on analysis of how rpm-ostree implements database queries, package diffing, and version tracking. The database system provides access to package information, deployment differences, and system state.

🏗️ Architecture Overview

Component Separation

┌─────────────────┐    ┌─────────────────┐    ┌─────────────────┐
│   CLI Client    │    │   Rust Core     │    │   Rust Daemon   │
│   (apt-ostree)  │◄──►│   (DBus)        │◄──►│   (aptostreed)  │
│                 │    │                 │    │                 │
│ • db list       │    │ • Client Logic  │    │ • APT Database  │
│ • db diff       │    │ • DBus Client   │    │ • Package       │
│ • db version    │    │ • Query Logic   │    │ • Metadata      │
└─────────────────┘    └─────────────────┘    └─────────────────┘

Responsibility Distribution

CLI Client (apt-ostree)

• Command parsing for database subcommands
• User interface and output formatting
• Query parameter handling
• Result display and formatting

Daemon (apt-ostreed)

• APT database access and queries
• Package metadata retrieval
• Deployment comparison and diffing
• Database version management

🔍 rpm-ostree Implementation Analysis

CLI Commands Structure

Based on rpmostree-builtin-db.cxx, rpm-ostree provides these database subcommands:

static RpmOstreeCommand rpm_subcommands[]
    = { { "diff", RPM_OSTREE_BUILTIN_FLAG_LOCAL_CMD, "Show package changes between two commits",
          rpmostree_db_builtin_diff },
        { "list", RPM_OSTREE_BUILTIN_FLAG_LOCAL_CMD, "List packages within commits",
          rpmostree_db_builtin_list },
        { "version", RPM_OSTREE_BUILTIN_FLAG_LOCAL_CMD,
          "Show rpmdb version of packages within the commits", rpmostree_db_builtin_version },
        { NULL, (Rpm_OSTREE_BUILTIN_FLAG_LOCAL_CMD)0, NULL, NULL } };

Key Insights from rpm-ostree

1. Local Commands: All database commands are LOCAL_CMD (don't require daemon)
2. Repository Access: Commands can work with local OSTree repositories
3. RPM Integration: Direct access to RPM database for package information
4. Commit Comparison: Built-in support for comparing different OSTree commits

🚀 apt-ostree Implementation Strategy

1. CLI Command Structure

// src/main.rs - Database command handling
async fn db_commands(args: &[String]) -> AptOstreeResult<()> {
    if args.is_empty() {
        show_db_help();
        return Ok(());
    }
    
    let subcommand = &args[0];
    match subcommand.as_str() {
        "list" => db_list(&args[1..]).await?,
        "diff" => db_diff(&args[1..]).await?,
        "version" => db_version(&args[1..]).await?,
        _ => {
            println!("❌ Unknown db subcommand: {}", subcommand);
            show_db_help();
        }
    }
    Ok(())
}

2. Database Query System

Core Database Manager

// src/database/db_manager.rs
pub struct DatabaseManager {
    ostree_repo: Arc<RwLock<Repo>>,
    apt_manager: Arc<AptManager>,
    cache: Arc<RwLock<DatabaseCache>>,
}

impl DatabaseManager {
    pub async fn list_packages(&self, commit_ref: Option<&str>) -> Result<Vec<PackageInfo>, Error> {
        let commit = match commit_ref {
            Some(ref_name) => self.get_commit(ref_name).await?,
            None => self.get_booted_commit().await?,
        };
        
        // Extract package information from commit
        let packages = self.extract_package_info_from_commit(&commit).await?;
        
        Ok(packages)
    }
    
    pub async fn diff_commits(
        &self,
        from_commit: &str,
        to_commit: &str,
    ) -> Result<DeploymentDiff, Error> {
        // Get package lists for both commits
        let from_packages = self.list_packages(Some(from_commit)).await?;
        let to_packages = self.list_packages(Some(to_commit)).await?;
        
        // Calculate differences
        let diff = self.calculate_package_diff(&from_packages, &to_packages).await?;
        
        Ok(diff)
    }
    
    pub async fn get_database_version(&self, commit_ref: Option<&str>) -> Result<DatabaseVersion, Error> {
        let commit = match commit_ref {
            Some(ref_name) => self.get_commit(ref_name).await?,
            None => self.get_booted_commit().await?,
        };
        
        // Extract database version information
        let version = self.extract_database_version(&commit).await?;
        
        Ok(version)
    }
    
    async fn extract_package_info_from_commit(&self, commit: &str) -> Result<Vec<PackageInfo>, Error> {
        // Extract commit to temporary directory
        let temp_dir = tempfile::tempdir()?;
        let commit_path = temp_dir.path();
        
        self.ostree_repo
            .write()
            .await
            .checkout(commit, commit_path)
            .await?;
        
        // Read package database from commit
        let dpkg_status_path = commit_path.join("var/lib/dpkg/status");
        let packages = self.read_dpkg_status(&dpkg_status_path).await?;
        
        Ok(packages)
    }
    
    async fn read_dpkg_status(&self, status_path: &Path) -> Result<Vec<PackageInfo>, Error> {
        let content = tokio::fs::read_to_string(status_path).await?;
        let packages = self.parse_dpkg_status(&content).await?;
        Ok(packages)
    }
    
    async fn parse_dpkg_status(&self, content: &str) -> Result<Vec<PackageInfo>, Error> {
        let mut packages = Vec::new();
        let mut current_package = None;
        
        for line in content.lines() {
            if line.is_empty() {
                // End of package entry
                if let Some(pkg) = current_package.take() {
                    packages.push(pkg);
                }
            } else if line.starts_with("Package: ") {
                // Start of new package entry
                if let Some(pkg) = current_package.take() {
                    packages.push(pkg);
                }
                let name = line[9..].trim().to_string();
                current_package = Some(PackageInfo::new(name));
            } else if let Some(ref mut pkg) = current_package {
                // Parse package field
                self.parse_package_field(pkg, line).await?;
            }
        }
        
        // Don't forget the last package
        if let Some(pkg) = current_package {
            packages.push(pkg);
        }
        
        Ok(packages)
    }
    
    async fn parse_package_field(&self, package: &mut PackageInfo, line: &str) -> Result<(), Error> {
        if line.starts_with("Version: ") {
            package.version = Some(line[9..].trim().to_string());
        } else if line.starts_with("Architecture: ") {
            package.architecture = Some(line[14..].trim().to_string());
        } else if line.starts_with("Description: ") {
            package.description = Some(line[13..].trim().to_string());
        } else if line.starts_with("Depends: ") {
            package.dependencies = Some(self.parse_dependency_list(&line[9..]).await?);
        } else if line.starts_with("Installed-Size: ") {
            if let Ok(size) = line[16..].trim().parse::<u64>() {
                package.installed_size = Some(size);
            }
        }
        Ok(())
    }
    
    async fn parse_dependency_list(&self, deps_str: &str) -> Result<Vec<Dependency>, Error> {
        let mut dependencies = Vec::new();
        
        for dep_str in deps_str.split(',') {
            let dep_str = dep_str.trim();
            if let Some(dep) = self.parse_single_dependency(dep_str).await? {
                dependencies.push(dep);
            }
        }
        
        Ok(dependencies)
    }
    
    async fn parse_single_dependency(&self, dep_str: &str) -> Result<Option<Dependency>, Error> {
        // Handle complex dependency syntax (e.g., "pkg1 | pkg2", "pkg1 (>= 1.0)")
        if dep_str.contains('|') {
            // Alternative dependencies
            let alternatives: Vec<String> = dep_str
                .split('|')
                .map(|s| s.trim().to_string())
                .collect();
            Ok(Some(Dependency::Alternatives(alternatives)))
        } else if dep_str.contains('(') && dep_str.contains(')') {
            // Versioned dependency
            if let Some((name, version)) = self.parse_versioned_dependency(dep_str).await? {
                Ok(Some(Dependency::Versioned(name, version)))
            } else {
                Ok(None)
            }
        } else {
            // Simple dependency
            Ok(Some(Dependency::Simple(dep_str.to_string())))
        }
    }
}

3. Package Diffing System

Deployment Comparison

// src/database/diff_engine.rs
pub struct DiffEngine {
    cache: Arc<RwLock<DiffCache>>,
}

impl DiffEngine {
    pub async fn calculate_package_diff(
        &self,
        from_packages: &[PackageInfo],
        to_packages: &[PackageInfo],
    ) -> Result<DeploymentDiff, Error> {
        // Create package maps for efficient lookup
        let from_map: HashMap<String, &PackageInfo> = from_packages
            .iter()
            .map(|p| (p.name.clone(), p))
            .collect();
        
        let to_map: HashMap<String, &PackageInfo> = to_packages
            .iter()
            .map(|p| (p.name.clone(), p))
            .collect();
        
        let mut diff = DeploymentDiff::new();
        
        // Find added packages
        for (name, package) in &to_map {
            if !from_map.contains_key(name) {
                diff.added_packages.push(package.clone());
            }
        }
        
        // Find removed packages
        for (name, package) in &from_map {
            if !to_map.contains_key(name) {
                diff.removed_packages.push(package.clone());
            }
        }
        
        // Find modified packages
        for (name, from_pkg) in &from_map {
            if let Some(to_pkg) = to_map.get(name) {
                if from_pkg != to_pkg {
                    diff.modified_packages.push(PackageModification {
                        name: name.clone(),
                        from: (*from_pkg).clone(),
                        to: (*to_pkg).clone(),
                        changes: self.calculate_package_changes(from_pkg, to_pkg).await?,
                    });
                }
            }
        }
        
        Ok(diff)
    }
    
    async fn calculate_package_changes(
        &self,
        from_pkg: &PackageInfo,
        to_pkg: &PackageInfo,
    ) -> Result<Vec<PackageChange>, Error> {
        let mut changes = Vec::new();
        
        // Version changes
        if from_pkg.version != to_pkg.version {
            changes.push(PackageChange::Version {
                from: from_pkg.version.clone(),
                to: to_pkg.version.clone(),
            });
        }
        
        // Architecture changes
        if from_pkg.architecture != to_pkg.architecture {
            changes.push(PackageChange::Architecture {
                from: from_pkg.architecture.clone(),
                to: to_pkg.architecture.clone(),
            });
        }
        
        // Dependency changes
        if from_pkg.dependencies != to_pkg.dependencies {
            changes.push(PackageChange::Dependencies {
                from: from_pkg.dependencies.clone(),
                to: to_pkg.dependencies.clone(),
            });
        }
        
        // Size changes
        if from_pkg.installed_size != to_pkg.installed_size {
            changes.push(PackageChange::Size {
                from: from_pkg.installed_size,
                to: to_pkg.installed_size,
            });
        }
        
        Ok(changes)
    }
}

4. Database Version Management

Version Information Extraction

// src/database/version_manager.rs
pub struct VersionManager {
    ostree_repo: Arc<RwLock<Repo>>,
}

impl VersionManager {
    pub async fn get_database_version(&self, commit_ref: &str) -> Result<DatabaseVersion, Error> {
        // Extract commit to temporary directory
        let temp_dir = tempfile::tempdir()?;
        let commit_path = temp_dir.path();
        
        self.ostree_repo
            .write()
            .await
            .checkout(commit_ref, commit_path)
            .await?;
        
        // Read version information from various sources
        let dpkg_version = self.get_dpkg_version(&commit_path).await?;
        let apt_version = self.get_apt_version(&commit_path).await?;
        let ostree_version = self.get_ostree_version().await?;
        
        Ok(DatabaseVersion {
            dpkg_version,
            apt_version,
            ostree_version,
            commit_hash: commit_ref.to_string(),
            timestamp: chrono::Utc::now(),
        })
    }
    
    async fn get_dpkg_version(&self, commit_path: &Path) -> Result<String, Error> {
        // Try to read dpkg version from commit
        let dpkg_path = commit_path.join("usr/bin/dpkg");
        if dpkg_path.exists() {
            let output = tokio::process::Command::new(&dpkg_path)
                .arg("--version")
                .output()
                .await?;
            
            if output.status.success() {
                let version = String::from_utf8_lossy(&output.stdout);
                if let Some(ver) = version.lines().next() {
                    return Ok(ver.trim().to_string());
                }
            }
        }
        
        // Fallback: read from package database
        let status_path = commit_path.join("var/lib/dpkg/status");
        if status_path.exists() {
            let content = tokio::fs::read_to_string(&status_path).await?;
            if let Some(version) = self.extract_dpkg_version_from_status(&content).await? {
                return Ok(version);
            }
        }
        
        Ok("Unknown".to_string())
    }
    
    async fn get_apt_version(&self, commit_path: &Path) -> Result<String, Error> {
        // Try to read apt version from commit
        let apt_path = commit_path.join("usr/bin/apt");
        if apt_path.exists() {
            let output = tokio::process::Command::new(&apt_path)
                .arg("--version")
                .output()
                .await?;
            
            if output.status.success() {
                let version = String::from_utf8_lossy(&output.stdout);
                if let Some(ver) = version.lines().next() {
                    return Ok(ver.trim().to_string());
                }
            }
        }
        
        // Fallback: read from package database
        let status_path = commit_path.join("var/lib/dpkg/status");
        if status_path.exists() {
            let content = tokio::fs::read_to_string(&status_path).await?;
            if let Some(version) = self.extract_apt_version_from_status(&content).await? {
                return Ok(version);
            }
        }
        
        Ok("Unknown".to_string())
    }
    
    async fn get_ostree_version(&self) -> Result<String, Error> {
        // Get OSTree library version
        let version = ostree::version();
        Ok(version.to_string())
    }
}

5. CLI Command Implementations

List Command

// src/commands/db_list.rs
pub async fn db_list(args: &[String]) -> AptOstreeResult<()> {
    let mut repo_path = None;
    let mut commit_ref = None;
    
    // Parse arguments
    let mut i = 0;
    while i < args.len() {
        match args[i].as_str() {
            "--repo" | "-r" => {
                if i + 1 < args.len() {
                    repo_path = Some(args[i + 1].clone());
                    i += 2;
                } else {
                    return Err(AptOstreeError::InvalidArgument(
                        "--repo requires a path".to_string(),
                    ));
                }
            }
            _ => {
                if commit_ref.is_none() {
                    commit_ref = Some(args[i].clone());
                } else {
                    return Err(AptOstreeError::InvalidArgument(
                        format!("Unexpected argument: {}", args[i]),
                    ));
                }
                i += 1;
            }
        }
    }
    
    // Initialize database manager
    let db_manager = DatabaseManager::new(repo_path.as_deref()).await?;
    
    // List packages
    let packages = db_manager.list_packages(commit_ref.as_deref()).await?;
    
    // Display results
    println!("📦 Packages in {}:", commit_ref.unwrap_or_else(|| "booted deployment".to_string()));
    println!("=====================");
    
    if packages.is_empty() {
        println!("No packages found");
    } else {
        println!("Found {} packages:", packages.len());
        for package in packages {
            println!("  • {} - {}", package.name, package.version.as_deref().unwrap_or("Unknown"));
            if let Some(desc) = &package.description {
                println!("    {}", desc);
            }
        }
    }
    
    Ok(())
}

Diff Command

// src/commands/db_diff.rs
pub async fn db_diff(args: &[String]) -> AptOstreeResult<()> {
    let mut repo_path = None;
    let mut from_commit = None;
    let mut to_commit = None;
    
    // Parse arguments
    let mut i = 0;
    while i < args.len() {
        match args[i].as_str() {
            "--repo" | "-r" => {
                if i + 1 < args.len() {
                    repo_path = Some(args[i + 1].clone());
                    i += 2;
                } else {
                    return Err(AptOstreeError::InvalidArgument(
                        "--repo requires a path".to_string(),
                    ));
                }
            }
            _ => {
                if from_commit.is_none() {
                    from_commit = Some(args[i].clone());
                } else if to_commit.is_none() {
                    to_commit = Some(args[i].clone());
                } else {
                    return Err(AptOstreeError::InvalidArgument(
                        format!("Unexpected argument: {}", args[i]),
                    ));
                }
                i += 1;
            }
        }
    }
    
    // Validate arguments
    let from_commit = from_commit.ok_or_else(|| {
        AptOstreeError::InvalidArgument("FROM_COMMIT is required".to_string())
    })?;
    
    let to_commit = to_commit.ok_or_else(|| {
        AptOstreeError::InvalidArgument("TO_COMMIT is required".to_string())
    })?;
    
    // Initialize database manager
    let db_manager = DatabaseManager::new(repo_path.as_deref()).await?;
    
    // Calculate diff
    let diff = db_manager.diff_commits(&from_commit, &to_commit).await?;
    
    // Display results
    println!("📊 Package differences between {} and {}:", from_commit, to_commit);
    println!("===============================================");
    
    if diff.is_empty() {
        println!("No differences found");
    } else {
        // Show added packages
        if !diff.added_packages.is_empty() {
            println!("\n➕ Added packages ({}):", diff.added_packages.len());
            for package in &diff.added_packages {
                println!("  • {} - {}", package.name, package.version.as_deref().unwrap_or("Unknown"));
            }
        }
        
        // Show removed packages
        if !diff.removed_packages.is_empty() {
            println!("\n➖ Removed packages ({}):", diff.removed_packages.len());
            for package in &diff.removed_packages {
                println!("  • {} - {}", package.name, package.version.as_deref().unwrap_or("Unknown"));
            }
        }
        
        // Show modified packages
        if !diff.modified_packages.is_empty() {
            println!("\n🔄 Modified packages ({}):", diff.modified_packages.len());
            for modification in &diff.modified_packages {
                println!("  • {}: {} → {}", 
                    modification.name,
                    modification.from.version.as_deref().unwrap_or("Unknown"),
                    modification.to.version.as_deref().unwrap_or("Unknown")
                );
                
                for change in &modification.changes {
                    match change {
                        PackageChange::Version { from, to } => {
                            println!("    Version: {} → {}", 
                                from.as_deref().unwrap_or("Unknown"),
                                to.as_deref().unwrap_or("Unknown")
                            );
                        }
                        PackageChange::Architecture { from, to } => {
                            println!("    Architecture: {} → {}", 
                                from.as_deref().unwrap_or("Unknown"),
                                to.as_deref().unwrap_or("Unknown")
                            );
                        }
                        _ => {}
                    }
                }
            }
        }
    }
    
    Ok(())
}

Version Command

// src/commands/db_version.rs
pub async fn db_version(args: &[String]) -> AptOstreeResult<()> {
    let mut repo_path = None;
    let mut commit_ref = None;
    
    // Parse arguments
    let mut i = 0;
    while i < args.len() {
        match args[i].as_str() {
            "--repo" | "-r" => {
                if i + 1 < args.len() {
                    repo_path = Some(args[i + 1].clone());
                    i += 2;
                } else {
                    return Err(AptOstreeError::InvalidArgument(
                        "--repo requires a path".to_string(),
                    ));
                }
            }
            _ => {
                if commit_ref.is_none() {
                    commit_ref = Some(args[i].clone());
                } else {
                    return Err(AptOstreeError::InvalidArgument(
                        format!("Unexpected argument: {}", args[i]),
                    ));
                }
                i += 1;
            }
        }
    }
    
    // Initialize version manager
    let version_manager = VersionManager::new(repo_path.as_deref()).await?;
    
    // Get version information
    let commit_ref = commit_ref.unwrap_or_else(|| "booted deployment".to_string());
    let version_info = version_manager.get_database_version(&commit_ref).await?;
    
    // Display results
    println!("📋 Database version information for {}:", commit_ref);
    println!("=========================================");
    println!("DPKG Version: {}", version_info.dpkg_version);
    println!("APT Version: {}", version_info.apt_version);
    println!("OSTree Version: {}", version_info.ostree_version);
    println!("Commit Hash: {}", version_info.commit_hash);
    println!("Timestamp: {}", version_info.timestamp.format("%Y-%m-%d %H:%M:%S UTC"));
    
    Ok(())
}

🔐 Security and Privileges

1. Repository Access Control

// Security checks for database access
impl DatabaseManager {
    pub async fn check_repository_access(&self, repo_path: Option<&Path>) -> Result<(), SecurityError> {
        let repo_path = repo_path.unwrap_or_else(|| Path::new("/sysroot/ostree/repo"));
        
        // Check if user has read access to repository
        if !self.security_manager.can_read_repository(repo_path).await? {
            return Err(SecurityError::RepositoryAccessDenied(
                repo_path.to_string_lossy().to_string(),
            ));
        }
        
        Ok(())
    }
}

2. Package Information Sanitization

// Sanitize package information for display
impl PackageInfo {
    pub fn sanitize_for_display(&self) -> SanitizedPackageInfo {
        SanitizedPackageInfo {
            name: self.name.clone(),
            version: self.version.clone(),
            architecture: self.architecture.clone(),
            description: self.description.as_ref()
                .map(|d| self.sanitize_description(d)),
            // Don't expose sensitive dependency information
            dependencies: None,
            installed_size: self.installed_size,
        }
    }
    
    fn sanitize_description(&self, description: &str) -> String {
        // Remove potentially sensitive information
        description
            .lines()
            .filter(|line| !line.contains("password") && !line.contains("secret"))
            .collect::<Vec<_>>()
            .join("\n")
    }
}

📊 Performance Optimization

1. Caching Strategy

// Database query caching
impl DatabaseManager {
    pub async fn get_cached_package_list(&self, commit_ref: &str) -> Result<Vec<PackageInfo>, Error> {
        // Check cache first
        if let Some(cached) = self.cache.read().await.get_packages(commit_ref) {
            return Ok(cached.clone());
        }
        
        // Fetch from repository
        let packages = self.list_packages(Some(commit_ref)).await?;
        
        // Cache the result
        self.cache.write().await.cache_packages(commit_ref, &packages);
        
        Ok(packages)
    }
}

2. Parallel Processing

// Parallel package information extraction
impl DatabaseManager {
    pub async fn extract_package_info_parallel(
        &self,
        commits: &[String],
    ) -> Result<HashMap<String, Vec<PackageInfo>>, Error> {
        let mut tasks = JoinSet::new();
        
        // Spawn parallel extraction tasks
        for commit in commits {
            let commit = commit.clone();
            let db_manager = self.clone();
            
            tasks.spawn(async move {
                let packages = db_manager.list_packages(Some(&commit)).await?;
                Ok::<_, Error>((commit, packages))
            });
        }
        
        // Collect results
        let mut results = HashMap::new();
        while let Some(result) = tasks.join_next().await {
            let (commit, packages) = result??;
            results.insert(commit, packages);
        }
        
        Ok(results)
    }
}

🧪 Testing Strategy

1. Unit Tests

#[cfg(test)]
mod tests {
    use super::*;
    
    #[tokio::test]
    async fn test_package_listing() {
        let db_manager = DatabaseManager::new(None).await.unwrap();
        let packages = db_manager.list_packages(None).await.unwrap();
        assert!(!packages.is_empty());
    }
    
    #[tokio::test]
    async fn test_package_diffing() {
        let diff_engine = DiffEngine::new();
        let from_packages = vec![
            PackageInfo::new("vim".to_string()),
            PackageInfo::new("git".to_string()),
        ];
        let to_packages = vec![
            PackageInfo::new("vim".to_string()),
            PackageInfo::new("git".to_string()),
            PackageInfo::new("curl".to_string()),
        ];
        
        let diff = diff_engine.calculate_package_diff(&from_packages, &to_packages).await.unwrap();
        assert_eq!(diff.added_packages.len(), 1);
        assert_eq!(diff.removed_packages.len(), 0);
    }
}

2. Integration Tests

#[tokio::test]
async fn test_full_database_workflow() {
    // Set up test repository
    let test_repo = create_test_repository().await?;
    
    // Initialize database manager
    let db_manager = DatabaseManager::new(Some(&test_repo.path())).await?;
    
    // Test package listing
    let packages = db_manager.list_packages(None).await?;
    assert!(!packages.is_empty());
    
    // Test version information
    let version = db_manager.get_database_version("test-ref").await?;
    assert!(!version.dpkg_version.is_empty());
    
    // Test diffing
    let diff = db_manager.diff_commits("from-ref", "to-ref").await?;
    assert!(diff.is_valid());
}

🚀 Future Enhancements

1. Advanced Query Features

• Package search with regex and filters
• Dependency analysis and visualization
• Package conflict detection
• Security vulnerability scanning

2. Performance Improvements

• Incremental updates for large repositories
• Background indexing and caching
• Query optimization and parallelization
• Memory-efficient processing for large datasets

3. Integration Features

• External database integration (e.g., CVE databases)
• Package metadata enrichment from external sources
• Automated reporting and monitoring
• API endpoints for programmatic access

This architecture provides a solid foundation for implementing production-ready database queries in apt-ostree, maintaining compatibility with the rpm-ostree ecosystem while leveraging the strengths of the Debian/Ubuntu package management system.