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Using component groups

Seeing the group order in the log

Let's assume we have a handwritten component with the following run method:

component.cpp
sen::kernel::FuncResult run(sen::kernel::RunApi& api) override
{
  std::cout << "MyComponent: running\n";
  auto result = api.execLoop(sen::Duration::fromHertz(1.0));
  std::cout << "MyComponent: finished running\n";
  return result;
}

Now modify your configuration file to enable kernel logs:

config.yaml
# you can configure the kernel that will
# host your components. Here we only show
# how to set the global log level for the kernel.
kernel:
  logConfig:
    level: debug

load:
  - name: term
    group: 1

  - name: my_component
    group: 2

➜ ./sen run config.yaml
[2023-03-07 10:02:21.866] [sen.kernel] [debug] advancing to group 1
[2023-03-07 10:02:21.866] [sen.kernel] [debug] loading shell
[2023-03-07 10:02:21.867] [sen.kernel] [debug] loading kernel
[2023-03-07 10:02:21.867] [sen.kernel] [debug] initializing shell
    _________  __
   / __/ __/ |/ /          compiler GNU-11.3.0 [mode: debug]
  _\ \/ __/    /           revision 5c937e6e5d344315b8c07c0af7e14cb682a837dc
 /___/___/_/|_/   0.0.1    branch   refs/heads/master [modified]
▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄

[2023-03-07 10:02:21.867] [sen.kernel] [debug] initializing kernel
[2023-03-07 10:02:21.867] [sen.kernel] [debug] running shell
[2023-03-07 10:02:21.867] [sen.kernel] [debug] running kernel
[2023-03-07 10:02:21.867] [sen.kernel] [debug] reached group 1
[2023-03-07 10:02:21.867] [sen.kernel] [debug] advancing to group 2
[2023-03-07 10:02:21.867] [sen.kernel] [debug] loading my_component
[2023-03-07 10:02:21.867] [sen.kernel] [debug] initializing my_component
[2023-03-07 10:02:21.867] [sen.kernel] [debug] running my_component
[2023-03-07 10:02:21.867] [sen.kernel] [debug] reached group 2
[2023-03-07 10:02:21.867] [sen.kernel] [debug] running
MyComponent: running
❯ shutdown
  Shutting down...
[2023-03-07 10:03:26.618] [sen.kernel] [debug] stopping my_component
MyComponent: finished running
[2023-03-07 10:03:26.868] [sen.kernel] [debug] unloading my_component
[2023-03-07 10:03:26.868] [sen.kernel] [debug] reached group 1
[2023-03-07 10:03:26.868] [sen.kernel] [debug] stopping term
[2023-03-07 10:03:26.889] [sen.kernel] [debug] stopping kernel
[2023-03-07 10:03:26.903] [sen.kernel] [debug] unloading shell
[2023-03-07 10:03:26.903] [sen.kernel] [debug] unloading kernel
[2023-03-07 10:03:26.903] [sen.kernel] [debug] reached group 0
bye ☺

You can see the progression of the initialization logic:

  1. The kernel starts at group 0, and it advances to group 1.
  2. In group 1, the kernel finds the shell and kernel components, so it: (1) loads them, (2) initializes them (this makes the term print the banner), and (3) runs them.
  3. In group 2, the kernel finds my_component, so it (1) loads it, (2) initializes it and (3) runs it (this is what prints the message).
  4. The kernel does not find any other group, so it reaches the "running" state.

At this point we command the kernel to shut-down (using the shell), so the kernel does the same process, but in reverse.

  1. In group 2, the kernel finds my_component, so it stops it and then unloads it.
  2. In group 1, the kernel finds the shell and kernel components, so it stops them and then unloads them.
  3. The kernel does not find any other group, so it reaches the "stopped" state, and finishes the execution.

You see that during start-up the kernel proceeds by groups, and within each group it loads all the components, then initializes all of them, then runs all of them. During shut-down it works through the groups in reverse, stopping all the components in a group before unloading any of them.

Group numbers are used as written. The kernel counts up from 1 and skips any number nothing is assigned to, so leaving gaps costs nothing and only the relative order matters.