Registry.keys

You're seeing just the function keys, go back to Registry module for more information.
Link to this function

keys(registry, pid)

View Source (since 1.4.0)

Specs

keys(registry(), pid()) :: [key()]

Returns the known keys for the given pid in registry in no particular order.

If the registry is unique, the keys are unique. Otherwise they may contain duplicates if the process was registered under the same key multiple times. The list will be empty if the process is dead or it has no keys in this registry.

Examples

Registering under a unique registry does not allow multiple entries:

iex> Registry.start_link(keys: :unique, name: Registry.UniqueKeysTest)
iex> Registry.keys(Registry.UniqueKeysTest, self())
[]
iex> {:ok, _} = Registry.register(Registry.UniqueKeysTest, "hello", :world)
iex> Registry.register(Registry.UniqueKeysTest, "hello", :later) # registry is :unique
{:error, {:already_registered, self()}}
iex> Registry.keys(Registry.UniqueKeysTest, self())
["hello"]

Such is possible for duplicate registries though:

iex> Registry.start_link(keys: :duplicate, name: Registry.DuplicateKeysTest)
iex> Registry.keys(Registry.DuplicateKeysTest, self())
[]
iex> {:ok, _} = Registry.register(Registry.DuplicateKeysTest, "hello", :world)
iex> {:ok, _} = Registry.register(Registry.DuplicateKeysTest, "hello", :world)
iex> Registry.keys(Registry.DuplicateKeysTest, self())
["hello", "hello"]