Task.Supervisor: start_child vs async_nolink

[Po Chen]

Hi all,

so Task.Supervisor has start_child and async_nolink, both of them DON'T link to the caller, when should you use which?

@benwilson512 kindly replied to me on Slack:

benwilson512 11:07 AM
If you plan on awaiting or yielding

11:07

Use async no link.

11:07

If you want a fire and forget

11:08

Use start child

However, I have some more questions:

Does one cover the functionality of the other?
If you want a fire and forget, you could still use async no link though, couldn't you?
Is there any scenario that you should absolutely use start_child instead of async_nolink?


Thanks,
Po

[Ben Wilson]

The code itself provides the main differences.

start_child simply spawns a process under a supervisor: https://github.com/elixir-lang/elixir/blob/574c6db3f8566a941b55c66499078d61661d08c6/lib/elixir/lib/task/supervisor.ex#L135

async_nolink on the other hand spawns the process under a supervisor, monitors it, and sends it a message.

https://github.com/elixir-lang/elixir/blob/574c6db3f8566a941b55c66499078d61661d08c6/lib/elixir/lib/task/supervisor.ex#L158

My previous point still stands though, because while an extra monitor and a couple of extra messages sent aren't a big deal, the larger point is semantic. start_child makes it clear that you don't care about the result. async_nolink implies that you probably do, or at a minimum care about the life of task as the monitor will tell you when when it dies.

[Po Chen]

Hi Ben,

Thank you very much for the detailed answer :) that helps a lot.