I have to say, I was initially a bit perplexed by Registry.keys/2. I think I misunderstood its functionality based on some incorrect, preconceived notions.

I discovered this thread when trying to figure out my use case, so I figured I’d add a post in case it’s useful to someone in future.

I just wanted to get a list of all registered keys in a registry and assumed (without reading the docs to be fair) that Registry.keys/2 would do that, in much the same way that Registry.count/1 returns a count of all items in a registry.

I read through the docs a bit and realised that Registry.select/2 does this; i.e:

# get a list of keys
Registry.select(MyRegistry, [{{:"$1", :_, :_}, [], [:"$1"]}])

# get a list of two-element key/pid tuples
Registry.select(MyRegistry, [{{:"$1", :"$2", :_}, [], [{{:"$1", :"$2"}}]}])

Looks like you have it figured out, but I have a sample project here - GitHub - amokan/registry_sample: Example application using the new Registry module in Elixir 1.4.0 · GitHub

In particular, you can look at how I used the Supervisor.which_children to get the registry keys here - registry_sample/lib/registry_sample/account_supervisor.ex at master · amokan/registry_sample · GitHub

Second argument passed to Registry.keys/2 is a pid of a process which put an entry into registry (i.e. the one which called Registry.register/3 which is done behind the scenes when using :via option). I think that in your case it is a pid of MySupervisor because this is the process which called start_link/1 of your workers.

Try it with supervisor’s pid and let us know if that worked!