Snap

Snap is a containerized software packaging and deployment system developed by Canonical. At the core of Snap's architecture is the snapd daemon, a background service that manages the installation, updating, and execution of snaps. Snapd interacts with the Snap Store (Figure 2) [2], a centralized repository hosted by Canonical, to fetch and install snaps, though it also supports the use of third-party or private repositories. Each snap is packaged as a compressed, read-only filesystem (typically using SquashFS) that is mounted at runtime. This filesystem contains the application, its dependencies, and metadata defining its behavior, permissions, and integration with the host system. The read-only nature of snaps ensures immutability, reducing the risk of corruption or tampering and simplifying updates.

Figure 2: The Snap Store is a centralized repository maintained by Canonical.

One of Snap's most notable features is its strong isolation and security model, which is enforced through a combination of Linux kernel technologies such as AppArmor, seccomp, and cgroups. Each snap runs in a confined environment, or sandbox, with restricted access to the host system's resources. Permissions are explicitly defined in the snap's manifest and can include access to hardware devices, network interfaces, or specific directories. Users can modify these permissions using the snap connect command, granting or revoking access as needed. This sandboxing mechanism not only enhances security by preventing unauthorized access to system resources but also improves stability by isolating applications from one another and from the underlying system.

Snap also uses the concept of channels, which allow users to choose between different release tracks (e.g., stable, candidate, beta, or edge) for a given snap. Channels enable developers to provide multiple versions of their software simultaneously, catering to different user needs, such as stability versus cutting-edge features. Additionally, snaps support automatic updates, ensuring that users always have the latest version of an application without manual intervention. These updates are transactional, meaning they are applied atomically: If an update fails, the system reverts to the previous version, minimizing downtime and ensuring reliability.

Another key advantage of Snap is its modular runtime system. Similar to Flatpak's runtimes, Snap uses a shared base snap (e.g., "core" or "core18") that provides common libraries and frameworks, reducing redundancy and storage requirements. Applications can extend these base snaps with additional dependencies as needed, maintaining a balance between self-containment and efficiency.

Despite its many benefits, Snap has faced criticism for certain design choices. For example, the Snap Store's centralized nature and snapd's proprietary back end have raised concerns about vendor lock-in and control. Additionally, the use of compressed filesystems and sandboxing can result in slightly longer startup times and increased resource usage compared to traditional packages. Snap's automatic update feature, while convenient, can sometimes lead to unexpected behavior or compatibility issues, particularly if an update introduces bugs or breaks existing functionality. The transactional update system, which ensures reliability by reverting to the previous version in case of failure, mitigates some of these concerns but does not eliminate them.

Comparison

AppImage, Flatpak, and Snap are three prominent software packaging and distribution solutions designed to address the challenges of dependency management, cross-distribution compatibility, and security in the Linux ecosystem. Although they share common goals, their architectures, design philosophies, and implementation details differ significantly, with each offering unique advantages and tradeoffs.

AppImage is a self-contained, portable packaging format that bundles an application along with all its dependencies and a lightweight runtime into a single compressed executable file, typically using the SquashFS filesystem. This monolithic design ensures that the application runs consistently across different Linux distributions without requiring installation or system-wide modifications. AppImages are executed directly after setting the appropriate executable permission (chmod +x), and they operate in a temporary, isolated environment that is cleaned up after the application exits. This approach eliminates dependency conflicts and simplifies distribution, as AppImages can be shared directly by developers via websites or other channels without relying on centralized repositories. However, AppImage lacks built-in dependency sharing, which can lead to larger file sizes, and it does not inherently support sandboxing or advanced security features, although optional tools like Firejail can add isolation.

Flatpak, on the other hand, is a more structured and modular packaging system that emphasizes sandboxing, security, and dependency sharing. It uses a runtime-based architecture, where shared runtimes (e.g., Freedesktop SDK, Gnome, or KDE runtimes) provide common libraries and frameworks that multiple applications can use, reducing redundancy and storage overhead. Applications are packaged with their specific dependencies and metadata, ensuring self-containment while still benefiting from shared runtimes. Flatpak enforces strong isolation through Linux namespaces, cgroups, and seccomp filters, restricting applications' access to system resources unless explicitly permitted via portals. This sandboxing mechanism enhances security and stability, making Flatpak particularly suitable for desktop applications. Flatpak applications are distributed through repositories, with Flathub being the most popular centralized source, though developers can also host their own repositories. Although Flatpak offers robust security and portability, its reliance on runtimes and sandboxing can introduce slight performance overhead and complexity in system integration.

Snap is another containerized packaging system that shares some similarities with Flatpak but differs in its implementation and focus. Like Flatpak, Snap packages (called "snaps") are self-contained and include all necessary dependencies, ensuring cross-distribution compatibility. However, Snap uses a more centralized model, with the Snap Store serving as the primary repository for distributing snaps. The snapd daemon manages the installation, updating, and execution of snaps, which are packaged as compressed, read-only SquashFS filesystems. Snap also employs a strong isolation model, leveraging AppArmor, seccomp, and cgroups to confine applications within a sandboxed environment. Permissions are explicitly defined and can be adjusted using the snap connect command. Snap uses the concept of channels, allowing users to choose between different release tracks (e.g., stable, beta, edge), and supports automatic, transactional updates that ensure reliability. Although Snap's centralized nature and proprietary back end have drawn criticism, its robust security features, ease of use, and support for a wide range of applications make it a popular choice, particularly in Ubuntu and IoT environments.

AppImage, Flatpak, and Snap were developed to address the fragmentation in the Linux ecosystem, but, in solving one form of fragmentation, they have inadvertently introduced another. Each format operates independently, with its own runtime systems, distribution mechanisms, and management tools. For example, a developer might need to package their application as an AppImage, a Flatpak, and a Snap to reach all users, effectively recreating the problem of multiple packaging formats. Additionally, users might encounter inconsistencies in availability, performance, or integration depending on which format they use. Although these technologies have made significant strides in unifying Linux software distribution, their coexistence has created a new layer of fragmentation, highlighting the ongoing challenge of achieving true standardization in the open source ecosystem.

Conclusion

AppImage, Flatpak, and Snap each offer innovative solutions to Linux software distribution, addressing dependency management, cross-distribution compatibility, and security. AppImage excels in simplicity and portability, Flatpak provides robust sandboxing and dependency sharing, and Snap delivers centralized distribution and strong isolation. However, each has limitations: AppImage lacks built-in sandboxing, Flatpak introduces performance overhead, and Snap's centralized model raises concerns about vendor control.