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diff --git a/docs/3_Developer_Guides/1_Application_Framework/1_Introduction.md b/docs/3_Developer_Guides/1_Application_Framework/1_Introduction.md new file mode 100644 index 0000000..9087254 --- /dev/null +++ b/docs/3_Developer_Guides/1_Application_Framework/1_Introduction.md @@ -0,0 +1,137 @@ +--- +title: Introduction +--- + +# 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. + +# 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 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 and services + management +- [D-Bus](https://www.freedesktop.org/wiki/Software/dbus/): inter-process communication +- [Desktop Entry specification](https://www.freedesktop.org/wiki/Specifications/desktop-entry-spec/): + application enumeration and startup + +AGL also provides reference implementations whenever possible and relevant, located in +the [meta-agl](../6_AGL_Layers/2_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 +- `applaunchd`: application launcher service + +# 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](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. + +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](../2_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 promotes the use of [D-Bus](https://www.freedesktop.org/wiki/Software/dbus/) +as the preferred way for applications to interact with services. + +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 +- [BlueZ](http://www.bluez.org/): Bluetooth connectivity +- [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 + +# Application launcher service + +As mentioned above, the Application Framework follows the guidelines of the +[Desktop Entry specification](https://www.freedesktop.org/wiki/Specifications/desktop-entry-spec/) +for application enumeration and startup. + +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: +- 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](2_Application_Startup/). |