At system runtime, it may be necessary for applications to start other applications on demand. Such actions can be executed in reaction to a user request, or they may be needed to perform a specific task.

In order to do so, running applications and services need an established way of discovering installed applications and executing those.

In order to provide a language-independent interface for applications and service to use, AGL includes applaunchd, a system service.

Application launcher service

The purpose of applaunchd is to enumerate applications available on the system and provide a way for other applications to query this list and start those on demand. It is also able to notify clients of the startup and termination of applications it manages.

To that effect, applaunchd provides a gRPC interface which other applications can use in order to execute those actions.

Note: applaunchd will only send notifications for applications it started; it isn't aware of applications started by other means (systemd, direct executable call...), and therefore can't send notifications for those.

Application discovery

Applications are enumerated from systemd's list of available units based on the pattern agl-app*@*.service, and are started and controled using their systemd unit. Please note applaunchd allows only one instance of a given application.

Application identifiers

Each application is identified by a unique Application ID. Although this ID can be any valid string, it is highly recommended to use the "reverse DNS" convention in order to avoid potential name collisions.

gRPC interface

The interface provides methods for the following actions:

Moreover, with the gRPC the client subscribes to a status signal to be notified when an application has successfully started or its execution terminated.

The gRPC protobuf file provides a Request and Response arguments to RPC methods even though in some cases these might be empty in order to allow forward compatibility in case additional fields are required. It is a good standard practice to follow up with these recommendation when developing a new protobuf specification.

Applications list

The ListApplications method allows clients to retrieve the list of available applications.

The ListRequest is an empty message, while ListResponse contains the following:

message AppInfo {
  string id = 1;
  string name = 2;
  string icon_path = 3;

message ListResponse {
  repeated AppInfo apps = 1;

Application startup request

Applications can be started by using the StartApplication method, passing the StartRequest message, defined as:

message StartRequest {
  string id = 1;

In reply, the following StartResponse will be returned:

message StartResponse {
  bool status = 1;
  string message = 2;

The "message" string of StartResponse message will contain an error message in case we couldn't start the application for whatever reason, or if the "id" isn't a known application ID. The "status" string would be boolean set to boolean TRUE otherwise.

If the application is already running, applaunchd won't start another instance, but instead reply with a AppStatus message setting the status string to "started".

Status notifications

The gRPC interface provides clients with a subscription model to receive status events. Client should subscribe to GetStatusEvents method to receive them.

The StatusRequest is empty, while the StatusResponse is defined as following:

message AppStatus {
  string id = 1;
  string status = 2;

message LauncherStatus {

message StatusResponse {
  oneof status {
    AppStatus app = 1;
    LauncherStatus launcher = 2;

As mentioned above, the status string is set to "started" and is also emitted if applaunchd receives a request to start an already running application. This can be useful, for example, when switching between graphical applications: