The Qt Application Manager
The Qt Application Manager is a daemon that helps you to create embedded Linux systems that have a highly complex UI setup, which you can optionally split into a multi-process setup to increase flexibility and stability.
Qt Application Manager Architecture
The main building blocks of the application manager are:
- Wayland Window Compositor
- Application Launcher
- User Input Management
- Application Installation
Combining these building blocks has certain advantages, as described below.
To support multiple UI processes on an embedded Linux system, you need a central window compositor: a Wayland compositor is the state-of-the-art solution for this. Consequently, the application manager incorporates a compositor that is fully-compliant with the Wayland protocol, based on the QtWayland module.
The window compositing part is project-specific, that you can write using QtQuick elements, giving you all of QML's capabilities to implement rich and fluid animations in your compositor.
In addition to the Qt Wayland Compositor, the application manager also provides an interface to which you can attach arbitrary meta-data to any Wayland window. This interface is particularly useful on custom embedded UI systems, where the top-level UI elements may not follow the classic desktop-centric Application Window, Modal Dialog, and Popup Menu window classification approach.
The launcher part is the central component for application life-cycle management: it starts and stops applications (internal or third-party) either on an explicit request or as an automatic reaction to external triggers. For example, in low-memory situations, you want to gracefully terminate applications that the user hasn't interacted with in a while.
Since UI applications typically have to be stopped and restarted multiple times during the up-time of an embedded system, and given the fact that most customers have tight constraints on the startup times of UI applications, the application manager implements some tricks to improve the startup performance: any application type that relies on a runtime component, such as QML or HTML, can be quick-launched.
The actual implementation depends on the type of runtime. The application manager ships with a QML quick-launcher that you can configure to keep at least one process with a fully-initialized QML engine available in the background (dependent on actual CPU load). In this case, starting an application only requires you to load the application's QML files into the existing engine.
Support for other runtimes can be added via an external, independent runtime launcher binary.
In addition to the runtime abstraction, the application manager is also able to run any application inside a container instead of just an external Unix process. Support for these container-based solutions must be provided by the customer and could range from full container solutions like KVM or XEN to LXC or even down to mandatory access control frameworks like AppArmor or SELinux. The application manager includes an example integration of Pelagicore's SoftwareContainers, which are based on Linux Containers (LXC).
Since most embedded UI applications rely on a virtual keyboard, the Qt Virtual Keyboard module can be integrated into the System UI and the Compositor. Through the Wayland protocol, this virtual keyboard component can then be transparently used from any Qt/QML application for full internationalized text input, without any special provisions on the application side. In contrast, non-Qt applications need to provide support for the required, open Wayland text input protocol.
The application manager acts as a freedesktop.org standards-compliant notification server on the D-Bus. For QtQuick applications, a QML component is provided which encapsulates the client side of the freedesktop.org notification protocol.
Both, the client and server sides, also come with Qt-like APIs to extend any notification request with additional meta-data using standard-compliant mechanism.
In addition to built-in applications, that are part of the System UI or the base installation, the application manager also supports third-party applications, that are dynamically installed, updated, and uninstalled.
Developing a built-in application compared to a third-party application is not much different, except for the additional packaging step for third-party applications. The application manager comes with a simple package format as well as a tool to create and digitally sign these packages.
The application manager's installer component can install and update these packages in a background thread using either plain HTTP(S) transfers or by receiving data from a customer-specific local socket connection. To support devices with limited disk space, all package installations and updates take place as they are downloaded to the device. This is more efficient than downloading the entire package first, and then proceeding with the installation or update.
Using a custom package format instead of a standard Unix format is deliberate for two reasons: they either use libraries that support legacy formats or they include hooks for arbitrary shell scripts; these hooks could be used to circumvent security measures.
Instead, the application manager uses a TAR archive with further restrictions on the type of content and some named YAML metadata files, that are application manager-specific. These packages are parsed via the BSD standard
libarchive library, which is also the basis for the actual
Additionally, these packages can be cryptographically signed by both the developer and the production app-store server. The devices' "developer mode" allows using only a developer signature or disabling this signature verification completely.
Aggregating all these building blocks into a single daemon enables them to work together much more efficiently:
- The compositor can perform security checks when an application requests to show a window, as it has access to the process IDs of all applications started by the application manager. By default, windows from unknown processes are not shown on the screen.
- The application manager enforces policies on the usage of its internal IPC mechanism as well as provides a D-Bus interface to the system's middleware to allow other process and libraries to authenticate resource usage of applications started by the application manager. This is possible because of the application's capabilities, that are stored in digitally signed manifests, and the application manager's knowledge of the application-to-PID mappings.
The application manager's ability to run the same System UI and QML applications in both single- and multi-process mode also has quite some advantages - both during the development phase as well as for product scaling. Typically, this does not require any changes to the System UI or the QML applications themselves.
- As a developer you can choose which desktop OS you want to develop on. For single-process testing you can choose from Windows, macOS, or Linux; without requiring Wayland. For multi-process, you can choose between Linux or macOS. On Linux, the application manager uses nested Wayland. On macOS, Wayland support is experimental.
- Both modes can be mixed, for example, third–party applications could be run in separate processes (or even in a secure container), while built-in QML applications could be loaded in-process. This configuration results in quicker startup times.
- The UI code that runs on the target system is the same code that runs on the developers' machines.
- The application manager gives you the possibility to scale down your product to lower-end hardware by saving on system and graphics memory, as well as startup times. You can do this by moving either all or just a few critical applications from multi- to single-process mode.
Be aware that any non-QML application, such as a native C++ compiled executable, will break this setup for developers on machines without Wayland support.
Still, there are projects that require applications using multiple UI technologies like QML, HTML, or native OpenGL rendering. In this scenario, the application manager's ability to support various runtimes makes it possible to composite all these applications seamlessly into a consistent user experience.
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