The AGL Application Framework is nothing new. However, the implementation used
up until the
lamprey release has been retired starting with the
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
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.
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:
systemd: system services and user session services management
D-Bus: inter-process communication, with `needlefish' release deprecated phase.
gRPC: inter-process communication, new recommmended system-wide IPC, which should be used instead of D-Bus.
Desktop Entry specification: application enumeration and startup, now in deprecated phase, systemd being the one would list out applications and handling start-up.
AGL also provides reference implementations whenever possible and relevant,
located in the meta-agl
meta-app-framework. At the moment, the Application Framework
contains 2 such components:
systemdunit files for user sessions management
applaunchd: application launcher service
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
systemd also provides an efficient first layer of security through its
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
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
User session management
Similarly, user sessions and the services they rely on are also managed by
AGL provides 2
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
$ systemctl enable agl-session@USER.service
By default, AGL enables this service as
running as user
Note: while you can create sessions for as many users as needed, only one
agl-session@.service is allowed per user.
agl-session.target is a user target for managing user services and their
dependencies. It is started by
agl-compositor is part of this target. It is therefore
automatically started for user
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:
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
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:
ConnMan: network connectivity
BlueZ: Bluetooth connectivity
oFono: telephony and modem management
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
for application enumeration and startup, but with the
instead it relies on systemd to provide that functionality, indirectly, by
As no simple reference implementation exists for this part of the
specification, AGL provides an application launcher service named
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.