The only thing that the proc_lib functions do is set $ancestors. Not a hard requirement for supervision but definitely a good idea.
ityonemo
derek-zhou
True, but there are also some conventions one should follow:
- This MFA shall be named
start_link start_linkshallspawn_linkto another MFA, usually calledinitinitshall callexit(:shutdown)if it ever wants to exit on its own.- a
child_spec/1function will make life easier for integration into a supervision tree.
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ityonemo
#1 is a recommendation so you can declare simplified descriptions: {module, param}.
#s 2 and 3 are not the case for Task, also generally not the case of #4 for Task either.
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derek-zhou
Elixir’s Task will take care of child_spec/1 and spawn_link. Often you want to implement orderly shutdown, like saving some states, I think you still need to do the following:
- trap :EXIT in your process
- handle :EXIT message to do your orderly shutdown
- exit(:shutdown) to terminate the process and notify the supervisor you are done
The supervisor will send you a :EXIT message in shutdown, and wait up to 5 seconds (configurable). It expects you to shutdown yourself and notify the supervisor. If you do not trap, you will crash. If you trap, but do nothing, the supervisor will brutal_kill you after the timeout. If you shutdown but did not call exit(:shutdown) the supervisor will believe you crashed and print a warning.
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ityonemo
LostKobrakai
Besides ˋTaskˋ another good example of processes using ˋ$callersˋ is the phoenix liveview test helpers. Everything runs in separate processes, but still uses e.g. a common ecto sandbox connection.
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ityonemo
I am curious if there is any authorization library that uses the process dictionary and “$callers” to backtrack through to the process handling the web request or websocket connection, as a means to “paint” the scope of any given user’s auth, without having to pass authorization everywhere.
Might be too magical, though.
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ityonemo
yeah this is a good point. Phoenix LiveViews are GenServers, so they don’t get “$callers” by default. It’s my opinion that GenServer should have some way to forward callers from its launching process, this would necessarily be a minor incompatibility with erlang, but then again the GenServer.start_link looks different from :gen_server.start_link anyways.
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benwilson512
Author of Craft GraphQL APIs in Elixir with Absinthe
This would definitely feel too magical to me. Process dictionaries can have value for managing some of the runtime book keeping of process management but are in my opinion a terrible fit for your actual application data. Code that relies on it is harder to test, and may behave in unexpected ways if you call it from a process with an unexpected process dictionary value.
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