Dynamic actor spawning

[ddrcode]

Status

It's a heavily expected feature, so there is no debate around whether we should have it or not. But the details are highly undefined at this stage.

feature Ideas

• supervisor allows to call add_actor after start
• there is stop_actor or remove_actor that can be called before calling supervisor.stop();
• actor (ctx?) can spawn another actor

Mental model / example

Think about file processing system. There are some raw files and each raw file produces multiple CSV files (depends on the content). Files are large in size, so to handle them effectively some concurrency is needed. In theory each raw file reader and each csv file writer could be an actor. In such case the writers subscribe to dynamic topics that are file names.

The dynamic actor creation here is needed, as new files may show up during the process. So actors will be created and dropped in the runtime.

In the simple form the coordinator that creates actors could use Maiko's supervisor and orchestrate initial setup and watch for the new files. In the ideal version entire logic could be moved to actors, but that would require an actor to spawn a new actor.

[ddrcode]

The example feels almost like supervision hierarchy in Actix. It starts as a single/root actor - i.e. folder scanner - that creates file reading actors, each creating one or many file writers. As such - actors gain responsibility to spawn new actors (via context). It should be fine , as long as we keep actual supervision away from actors (unlike Actix). So actor's context gives some form of an interface to the supervisor, but it's not stepping into supervisor / broker responsibility.

The solution would require some form of communication between Context / Actor Controller and supervisor and broker (i.e. for channel creation). The same functionality will be also needed by other features like multi-broker architecture. As such, it should be separated from this issue and work as an independent one, while this will remain focused purely on actors management.