# 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.
+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 `needlefish` release, further changes have been added, including a gRPC
+interface, alongside a D-BUs one, as well as using systemd units to start
+applications.
+
# 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.
+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.
+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
+- 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 unneded custom implementation, the
-Application Framework relies mainly on third-party technologies and/or software
-components, most of those being maintained under the
-[freedesktop.org](https://www.freedesktop.org) umbrella. Those include:
+In order to be as simple as possible and avoid any unneded custom
+implementation, the Application Framework relies mainly on third-party
+technologies and/or software components, most of those being maintained under
+the [freedesktop.org](https://www.freedesktop.org) umbrella. Those include:
+
+- [systemd](https://www.freedesktop.org/wiki/Software/systemd/): system
+ services and user session services management
-- [systemd](https://www.freedesktop.org/wiki/Software/systemd/): system services and user session services management
+- [D-Bus](https://www.freedesktop.org/wiki/Software/dbus/): inter-process
+ communication, now in deprecated phase.
-- [D-Bus](https://www.freedesktop.org/wiki/Software/dbus/): inter-process communication
+- [gRPC](https://grpc.io/about): inter-process communication
- [Desktop Entry specification](https://www.freedesktop.org/wiki/Specifications/desktop-entry-spec/):
- application enumeration and startup
+ 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](/04_Developer_Guides/02_AGL_Layers/02_meta_agl/) layer under `meta-app-framework`. At the
-moment, the Application Framework contains 2 such components:
+AGL also provides reference implementations whenever possible and relevant,
+located in the [meta-agl](/04_Developer_Guides/02_AGL_Layers/02_meta_agl/)
+layer under `meta-app-framework`. At the moment, the Application Framework
+contains 2 such components:
- `agl-session`: `systemd` unit files for user sessions management
# 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.
+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](https://www.freedesktop.org/software/systemd/man/systemd.exec.html#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.
+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](/04_Developer_Guides/02_Creating_a_New_Service/)
-document.
+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](/04_Developer_Guides/02_Creating_a_New_Service/) document.
# User session management
-Similarly, user sessions and the services they rely on are also managed by `systemd`.
+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:
+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`.
+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.*
+*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`.
+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:
+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]
# 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 promotes the use of [D-Bus](https://www.freedesktop.org/wiki/Software/dbus/)
-as the preferred way for applications to interact with services.
+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](https://www.freedesktop.org/wiki/Software/dbus/) as the preferred way
+for applications to interact with services. Starting with `needlefish` release
+we instead switched to using [gRPC](https://grpc.io) 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:
+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](https://git.kernel.org/pub/scm/network/connman/connman.git/): network connectivity
+- [ConnMan](https://git.kernel.org/pub/scm/network/connman/connman.git/):
+ network connectivity
- [BlueZ](http://www.bluez.org/): Bluetooth connectivity
-- [oFono](https://git.kernel.org/pub/scm/network/ofono/ofono.git): telephony and modem
- management
+- [oFono](https://git.kernel.org/pub/scm/network/ofono/ofono.git): telephony
+ and modem management
-- [GeoClue](https://gitlab.freedesktop.org/geoclue/geoclue/-/wikis/home): geolocation
+- [GeoClue](https://gitlab.freedesktop.org/geoclue/geoclue/-/wikis/home):
+ geolocation
# Application launcher service
-As mentioned above, the Application Framework follows the guidelines of the
+The Application Framework used to follow the guidelines of the
[Desktop Entry specification](https://www.freedesktop.org/wiki/Specifications/desktop-entry-spec/)
-for application enumeration and startup.
+for application enumeration and startup, but now instead relies on systemd to
+provide that functionality indirectly, 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.
+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
-interface for services and applications to:
+`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` is described with more details in [the following document](../02_Application_Startup/).
+`applaunchd` is described with more details in
+[the following document](../02_Application_Startup/).
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