Traditional package managers like APT, DNF, and Pacman have served Linux users well for decades, but they come with inherent complexities, dependencies, and compatibility challenges. In response to these limitations, three modern packaging formats have emerged as contenders for the future of Linux application distribution: Flatpak, AppImage, and Snap. If you've been around Linux for the past few years, you've probably heard these terms a lot. Many users adopt one of these package formats because they need it to install an application – or because they use a distro where the format is favored – without a clear understanding of what these formats really are, how they are different from their predecessors, and how they differ from each other. The goal of this article is to shed some light on these recent innovations in package technology.

Each of these formats aims to simplify software distribution, enhance security, and improve application portability, but they do so in different ways. Flatpak is designed with sandboxing and cross-distribution compatibility in mind. AppImage, on the other hand, offers a lightweight, self-contained approach that requires no installation, emphasizing portability and simplicity. Snap, backed by Canonical, integrates deeply with Ubuntu while offering transactional updates and strong security features.

The Problem

For decades, Linux software distribution relied on traditional package managers such as APT (Debian/Ubuntu), DNF/YUM (Fedora/RHEL), and Pacman (Arch Linux). These systems, while effective, came with significant challenges that became increasingly apparent as Linux gained popularity across different distributions and user bases. One of the main issues was dependency management: Software packages often require specific versions of shared libraries, leading to "dependency hell," where installing or updating one application could break another due to conflicting requirements. Furthermore, software packaged for one distribution was rarely compatible with another, forcing developers to either maintain multiple versions of their applications or rely on third-party maintainers, which often caused delays in software availability. Additionally, security concerns arose due to packages running with system-wide privileges, making applications more vulnerable to system-wide exploits.

Updates were also a challenge: Traditional package managers often relied on centralized distribution repositories, meaning users had to wait for their distribution maintainers to package and release updates, sometimes delaying security patches. Moreover, traditional packages were deeply integrated into the system, making rollback and isolation difficult, which became a problem for both security-conscious users and developers testing new software. With the growing need for universal, cross-distro applications that were sandboxed, easy to install, and independent of system libraries, new packaging formats like Flatpak, AppImage, and Snap emerged as solutions to these long-standing problems, offering containerized, self-contained, and cross-platform approaches to software distribution.

You'll experience a classic example of the limitations of traditional package management if you attempt to install a newer version of GNU Image Manipulation Program (Gimp) on an older version of Ubuntu using APT. Suppose a user is running Ubuntu 18.04 LTS, which comes with Gimp 2.8 in its official repositories. Eager to take advantage of the significant improvements in Gimp 2.10, the user quickly realizes that the newer version is not available through the default package manager. At this point, there are a few options, none of which are ideal. One possibility is to manually add a third-party Personal Package Archive (PPA) that provides Gimp 2.10, but this approach carries security risks and might introduce software conflicts because PPAs are not officially maintained by the distribution. Another alternative is to download and compile Gimp from source, a complex and time-consuming process that requires manually resolving dependencies. This option can be particularly frustrating if certain libraries are unavailable or outdated in the system's repositories, forcing the user to upgrade components in a way that could destabilize the system.

A common issue in such scenarios is dependency conflicts. If Gimp 2.10 requires a newer version of GTK (the graphical toolkit it depends on), installing it might require updating system libraries that are also used by other applications, which could lead to a situation where updating one package inadvertently breaks another due to incompatibilities. The more complex the software, the greater the risk of encountering such conflicts, which can quickly turn a simple installation into a troubleshooting nightmare.

The advent of these modern package formats such as Flatpak, AppImage, and Snap was intended to address precisely these types of problems. Instead of relying on system-wide dependencies, Flatpak provides a self-contained package that includes everything needed for an application to run, operating within an isolated environment to avoid conflicts. Similarly, an AppImage version of Gimp can be downloaded and executed immediately without requiring installation, allowing the user to run multiple versions side by side without affecting the system. A Snap package, on the other hand, offers an easy installation process with automatic updates, though at the cost of some additional sandboxing overhead. These approaches ensure that users can access the latest software versions without worrying about dependency issues, conflicting system libraries, or the risk of breaking their operating system. The growing adoption of these new packaging methods demonstrates the need for a more flexible and universal approach to Linux application distribution.

In contrast, Windows applications are typically distributed with all the necessary dependencies bundled together. Most Windows programs install their own required libraries or include them within the installation package, avoiding conflicts with system-wide libraries. However, Windows has its own set of software management issues. One of them is DLL hell, a problem where multiple applications rely on different versions of the same Dynamic Link Library (DLL), leading to compatibility issues when an application overwrites a shared DLL with an incompatible version. This was particularly problematic in older versions of Windows but has been mitigated with features like Side-by-Side (SxS) assemblies and application-specific DLLs stored in the program's own directory.

Flatpak

Flatpak is a contemporary and advanced software packaging framework engineered to deliver cross-distribution compatibility, robust security, and application isolation for Linux-based systems. Developed by Alexander Larsson and maintained under the Freedesktop.org project (www.freedesktop.org), Flatpak achieves this goal by leveraging a sandboxing model and a standardized runtime environment, which decouples applications from the host system's libraries and configurations.

Flatpak's architecture is built around two fundamental components: runtimes and applications. Runtimes are shared, pre-packaged collections of libraries and frameworks (e.g., Freedesktop SDK, Gnome, or KDE runtimes) that provide a common base for multiple Flatpak applications. By utilizing runtimes, Flatpak minimizes redundancy and storage overhead, as applications can share these foundational libraries rather than bundling them individually. Applications, on the other hand, are packaged with their specific dependencies and metadata, ensuring they remain self-contained while still benefiting from the shared runtime environment. This modular approach allows developers to target a single, universal packaging format, eliminating the need to maintain multiple distribution-specific versions of their software.

A defining feature of Flatpak is its sandboxing mechanism, which isolates applications from the host system and restricts their access to system resources unless explicitly permitted. This is enforced through a combination of Linux namespaces, cgroups, and seccomp filters, which create a secure execution environment. Permissions are managed via portals, which provide controlled access to system features such as filesystems, network interfaces, and hardware devices. This sandboxing not only enhances security by preventing malicious or misconfigured applications from compromising the system but also improves stability by limiting the scope of potential conflicts.

Flatpak applications are distributed and managed through repositories, with Flathub (Figure 1) [1] being the most prominent and widely used centralized repository. Flathub hosts a vast array of applications, ranging from open source projects to proprietary software, providing users with a streamlined and unified source for software discovery and installation. Developers can also host their own repositories, enabling decentralized distribution.

Figure 1: Flathub is the best known Flatpak repository.

Although Flatpak offers significant advantages in terms of portability, security, and ease of distribution, it is not without tradeoffs. The inclusion of dependencies within Flatpak packages can result in larger file sizes compared to traditional native packages and increased resource usage. Additionally, the sandboxing mechanism, while beneficial for security, can introduce a slight performance overhead due to the additional layers of abstraction and isolation. Furthermore, integrating Flatpak applications with system components, such as themes, fonts, or hardware drivers, can sometimes require additional configuration or permissions, making it more complex than traditional package management in certain scenarios. Finally, because Flathub is the most popular repository for Flatpak applications, the ecosystem is still relatively centralized, which could raise concerns about vendor control and dependency on a single source for software distribution.

AppImage

AppImage leverages a self-contained, monolithic architecture that encapsulates an application, all its required dependencies, libraries, and runtime components into a single compressed executable file. This file is typically structured using the SquashFS filesystem, a lightweight, read-only compression format that allows the contents to be efficiently mounted and accessed at runtime. The AppImage format adheres to the principle of "one app = one file," ensuring that the entire application ecosystem is bundled into a single, distributable unit. This design eliminates the need for traditional installation processes, dependency resolution, or system-wide modifications, making it inherently distribution-agnostic and compatible with most Linux distributions, including Ubuntu, Fedora, Arch Linux, and others.

The execution of an AppImage is facilitated by a small, embedded runtime component that is included within the file itself. When the AppImage is executed, this runtime handles the mounting of the SquashFS filesystem, typically to a temporary directory (e.g., /tmp/.mount_<AppImageName>) and launches the application from within this isolated environment. This approach ensures that the application runs in a sandboxed manner, without interfering with the host system's libraries, packages, or configurations. Once the application is closed, the runtime automatically unmounts the filesystem and cleans up the temporary files, leaving no residual traces on the system. To use an AppImage, users simply download the file, set the executable permission using a command like chmod +x filename.AppImage, and run it directly. No root privileges or installation steps are required, making it highly user-friendly and accessible. For enhanced usability, AppImage supports optional desktop integration, allowing users to register the application with their desktop environment. This can be achieved using tools like appimaged or manually by creating a .desktop file, which enables the application to appear in system menus, launchers, and application dashboards. Additionally, AppImage supports digital signing to ensure the integrity and authenticity of the software, as well as delta updates for efficient and bandwidth-saving version upgrades. Tools like appimagetool enable developers to create AppImages from their applications, while AppImageUpdate provides a mechanism for seamless and automated updates.

One of the key advantages of AppImage is its decentralization. Unlike traditional package managers that rely on centralized repositories, AppImage allows developers to distribute their software directly to users via websites, cloud storage, or other channels. This feature reduces reliance on distribution maintainers and enables faster, more flexible software delivery. Furthermore, AppImage's design aligns with the principles of user autonomy and system cleanliness, as it does not require installation or modification of system files, making it ideal for use cases such as portable applications, testing, or running software on restricted systems.

One of AppImage's most significant limitations is its lack of built-in sandboxing. By default, AppImages run with unrestricted access to the host system, which can pose security risks, particularly when running untrusted applications. Although tools like Firejail can add sandboxing, this step requires manual configuration and is not integrated into the AppImage format itself. Additionally, AppImage does not support dependency sharing, meaning each AppImage must bundle all its dependencies, leading to larger file sizes and potential redundancy when multiple AppImages include the same libraries. This can result in increased storage requirements and inefficiency. Another limitation is the absence of a centralized update mechanism. Although tools like AppImageUpdate exist, they are not universally adopted, and users often need to manually download new versions of AppImages. Furthermore, AppImage lacks native desktop integration, requiring users to manually create .desktop files or use helper tools for proper menu integration, which can be cumbersome for less technical users.