Registry with DynamicSupervisor

TwistingTwists · January 12, 2024, 9:00am

Taking code from livebook here: It tries to do a Registry.lookup for see if the task has been registered in the registry.

github.com

livebook-dev/livebook/blob/8ba6baec3a78e01a07d2e64ed06c6ca4f42fde86/lib/livebook/utils/unique_task.ex#L56-L65


      
          case Registry.lookup(@registry, key) do
            [{pid, _}] ->
              pid
          
            [] ->
              case DynamicSupervisor.start_child(@supervisor, {@task, {key, fun}}) do
                {:ok, pid} -> pid
                {:error, {:already_started, pid}} -> pid
              end
          end

So, the {:error, {:already_started, pid}} branch should never be called. Here is the reason:

Whenever, task starts, it will register itself under Registry with {:via , Registry , {k, fun})
If there is a Task => it is in Registry else, simply start the child via DynamicSupervisor.start_child/2

I think I m missing something.

LostKobrakai · January 12, 2024, 9:20am

You’re missing the case, where another task starts up in between the calls to Registry.lookup and DynamicSupervisor.start_child. It won’t be there when you ask the registry if it exists, but it will be there when you try to start another instance with the same name.