Phillipp
Hey,
let’s say I have a DynamicSupervisor and I want to start children on it. The children itself consist of a Supervisor with a few children on it, which act like one unit under the DynamicSupervisor.
Something like this:
DynamicSupervisor
-- Supervisor
------ Main
------ Worker_1
------ Worker_2
------ Something_else
-- Supervisor
------ Main
------ Worker_1
------ Worker_2
------ Something_else
...
So, our “unit” has 4 GenServers in the example above. What would be the best way to let them easily communicate with each other? I need to find out the pids of the related processes. Has someone done something like this before and can recommend a good way?
A simple usecase here is that Main tells the 3 other servers what to do and they report back to Main.
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dom
https://hexdocs.pm/elixir/1.6/Registry.html
david_ex
It sounds like you’re looking for the service/worker pattern: The basic Erlang service ⇒ worker pattern – The Intellectual Wilderness
Coincidentally, I’ve written a series of blog posts that use it in practice: http://davidsulc.com/blog/2018/07/09/pooltoy-a-toy-process-pool-manager-in-elixir-1-6/
Another possibility is to use a hybrid approach where Supervisor starts Main. Then Main will create a registry instance, and start its siblings with https://hexdocs.pm/elixir/1.6/Supervisor.html#start_child/2 where the provided child spec contains the registry name within the
:startvalue.Phillipp
I already had the idea of one Registry per “unit” in mind but were stuck at the point of discovering that registry process. Your idea sounds quite doable. I need to tinker around a bit and see how it turns out with actual code.
dom
You don’t need a registry per unit if you register the processes with tuples, like
{:unit_supervisor, unit_id},{:unit_worker, 1, unit_id}, etc.If your units don’t have a unique id already you can generate one (
make_ref) in the supervisor’s init callback and pass it down as an argument to the children, via their child spec.Phillipp
I actually would like to have one Registry per unit. If something bad happens with the Registry, it won’t affect the other units.
kokolegorille
That would kind of defeat the role of Registry, to have one place where to lookup processes by key…
But nothing forbids You to monitor others processes from Main, with a private ETS table, for example. This way Main would be used as a small registry too. You would catch EXIT, and adjust your ETS entries.
Phillipp
I already had cases where the GenServers that were monitored by my single Registry blew up the Registry due to fast restarting or whatever. Then everything went down. That’s why I like to have many Registries.
kokolegorille
You still can, but what You need is a Registry of Registries
michalmuskala
It should be perfectly enough to have one registry for the entire application. It’s performant enough.
Phillipp
@michalmuskala I am not saying it’s not performant enough. I just had problems with it in the past that the Registry crashed for some reasons.