X-Git-Url: https://gerrit.automotivelinux.org/gerrit/gitweb?a=blobdiff_plain;f=docs%2F06_Component_Documentation%2FApplication_Framework%2F01_Introduction.md;fp=docs%2F06_Component_Documentation%2FApplication_Framework%2F01_Introduction.md;h=1deaec46febdf3d5de8b90592610cab1e68510d1;hb=ecccce4b62c048272dd48378cc05d7165746b83e;hp=0000000000000000000000000000000000000000;hpb=2887f945590adbb6ceaf590e4fb0f542792c6e04;p=AGL%2Fdocumentation.git diff --git a/docs/06_Component_Documentation/Application_Framework/01_Introduction.md b/docs/06_Component_Documentation/Application_Framework/01_Introduction.md new file mode 100644 index 0000000..1deaec4 --- /dev/null +++ b/docs/06_Component_Documentation/Application_Framework/01_Introduction.md @@ -0,0 +1,186 @@ +--- +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. + +With the `needlefish` release, further changes have been added, including a +[gRPC IPC](https://grpc.io/about), alongside a deprecated D-Bus one, as well as +using as using systemd units as opposed on using +[Desktop Entry specification](https://www.freedesktop.org/wiki/Specifications/desktop-entry-spec/) +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](https://www.freedesktop.org) umbrella. Those include: + + +- [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, with `needlefish' release deprecated phase. + +- [gRPC](https://grpc.io/about): inter-process communication, new recommmended + system-wide IPC, which should be used instead of D-Bus. + + +- [Desktop Entry specification](https://www.freedesktop.org/wiki/Specifications/desktop-entry-spec/): + 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.md) +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](03_Creating_a_New_Service.md) 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](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: + +- [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 + +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](https://www.freedesktop.org/wiki/Specifications/desktop-entry-spec/) +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](02_Application_Startup_Dbus.md).