Foreword

The AGL Application Framework is nothing new. However, the implementation used up until the lamprey release has been retired starting with the marlin release and replaced by a redesigned Application Framework one. However, this new implementation isn't a 1:1 replacement, and as such it doesn't provide all of the features of the previous Application Framework. Some of those will be added back over time, others have been discarded in favor of more modern and/or widely-used alternatives.

With the needlefish release, further changes have been added, including a gRPC IPC, alongside a deprecated D-Bus one, as well as using as using systemd units as opposed on using Desktop Entry specification to list applications, and relies entirely on systemd to start application, rather than spawning them directly.

Once all platforms transitioned to gRPC, the D-Bus functionality will be removed entirely, mentioning it in only in documentation for history purposes.

Introduction

As a provider of an integrated solution to build up on, AGL needs to define a reliable and well-specified method for managing the deployment and integration of applications and services, as well as the way they can interact with the rest of the system.

This is achieved by providing a common set of rules and components, known as the Application Framework. By ensuring conformity to those rules, application developers can have a good understanding of the requirements for creating and packaging applications targeting AGL-based systems. Likewise, system developers and integrators have a clear path for including such applications in AGL-based products.

The Application Framework's scope extends to the following areas: - system services integration and lifecycle management - user session management, including user-level applications and services lifecycle management - inter-process communication

In order to be as simple as possible and avoid any unneeded custom implementation, the Application Framework relies mainly on third-party technologies and/or software components, most of those being maintained under the freedesktop.org umbrella. Those include:

AGL also provides reference implementations whenever possible and relevant, located in the meta-agl layer under meta-app-framework. At the moment, the Application Framework contains 2 such components:

Services management

Both system and user services are managed by systemd, which provides a number of important features, such as dependency management or service monitoring: when starting a service, systemd will ensure any other units this service depends on are available, and otherwise start those dependencies. Similarly, systemd can automatically restart a crashed service, ensuring minimal downtime.

systemd also provides an efficient first layer of security through its sandboxing and other security-related options.

It is also well integrated with D-Bus and can be used for a more fine-grained control over D-Bus activated services: by delegating the actual service startup to systemd, developers can take advantage of some of its advanced features, allowing for improved reliability and security.

Each service should be represented by a systemd unit file installed to the appropriate location. More details can be obtained from the Creating a New Service document.

User session management

Similarly, user sessions and the services they rely on are also managed by systemd.

AGL provides 2 systemd units:

1. agl-session@.service is a template system service for managing user sessions; it takes a username or UID as a parameter, creating a session for the desired user. Instanciating this service can be achieved by enabling agl-session@USER.service, for example by executing the following command on a running system:

$ systemctl enable agl-session@USER.service

By default, AGL enables this service as agl-session@agl-driver.service, running as user agl-driver.

Note: while you can create sessions for as many users as needed, only one instance of agl-session@.service is allowed per user.

2. agl-session.target is a user target for managing user services and their dependencies. It is started by agl-session@.service.

By default, agl-compositor is part of this target. It is therefore automatically started for user agl-driver.

Any other service needed as part of the user session should similarly depend on this target by appending the following lines to their unit file:

[Install]
WantedBy=agl-session.target

Inter-process communication

In order to provide a "standard", language-independent IPC mechanism and avoid the need for maintaining custom bindings for each programming language to be used on top of AGL, the Application Framework used to promote the use of D-Bus as the preferred way for applications to interact with services. Starting with needlefish release, we instead switched to using gRPC for our system-wide IPC, with D-Bus being kept to provide functionality to services and application which haven't transitioned yet to using gRPC.

Most services already included in AGL provide one or several D-Bus interfaces, and can therefore interact with D-Bus capable applications and services without requiring any additional component. Those services include, among others:

Similarly, we're in the phase of expanding various services to expose a gRPC interface.

Application launcher service

The Application Framework used to follow the guidelines of the Desktop Entry specification for application enumeration and startup, but with the needlefish release instead it relies on systemd to provide that functionality, indirectly, by using the applaunchd application.

As no simple reference implementation exists for this part of the specification, AGL provides an application launcher service named applaunchd. This service is part of the default user session, and as such is automatically started on session startup. It can therefore be considered always available.

applaunchd enumerates applications installed on the system and provides a D-bus (deprecated)/gRPC interface for services and applications to: - query the list of available applications - request the startup and/or activation of a specific application - be notified when applications are started or terminated

applaunchd with the D-Bus interface is described with more details in the following document.