Creating a supervisor with multiple children

I am trying to take an input from the user and then creating the number of genservers as input and Supervise them. My code is something like

defmodule GossSim do
  use Supervisor

def main(args) do
  # Since it is a mix generated project I had to put the main
  Supervisor.start_link(__MODULE__, args)

def start_link(args) do
  Supervisor.start_link(__MODULE__, args)

#The two methods down below create children dynamically
def get_child_list(child_list, 0), do: child_list

def get_child_list(child_list, num_of_nodes) do
  child = 
    id: :rand.uniform(num_of_nodes*100000),
    start: {Gossip_node, :start_link, [[0,true]]}
  if num_of_nodes > 0 do
    get_child_list( [child] ++ child_list, num_of_nodes-1)

def init(args) do
  children = get_child_list([], 10)
  # The outut of this inspect is pasted below
  IO.inspect children, label: "The children list is"
  // some function that does something parse_args_and_start(args)
  // num_of_nodes is 10
  children = get_child_list([], num_of_nodes)
  Supervisor.init(children, strategy: :simple_one_for_one)


I get the following error
bad child specification, invalid children: [%{id: 83303, start: {Gossip_node, :start_link, [[0, true]]}}, %{id: 186070, start: {Gossip_node, :start_link, [[0, true]]}}, %{id: 85483, start: {Gossip_node, :start_link, [[0, true]]}}, %{id: 40851, start: {Gossip_node, :start_link, [[0, true]]}}, %{id: 85216, start: {Gossip_node, :start_link, [[0, true]]}}, %{id: 390428, start: {Gossip_node, :start_link, [[0, true]]}}, %{id: 466922, start: {Gossip_node, :start_link, [[0, true]]}}, %{id: 187412, start: {Gossip_node, :start_link, [[0, true]]}}, %{id: 103750, start: {Gossip_node, :start_link, [[0, true]]}}, %{id: 176391, start: {Gossip_node, :start_link, [[0, true]]}}]

The Supervisor doc says that a supervisor is fine if it has a start and an id. Since children is a list I am adding multiple maps of children in it. Am i missing something. Gossip_node is a module in the same folder.

You should use Dynamic Supervisor instead.

Thanks for looking into it @kokolegorille
I started looking into that but then they say that

The Supervisor module was designed to handle mostly static children that are started in the given order when the supervisor starts. A DynamicSupervisor starts with no children. Instead, children are started on demand via start_child/2

I don’t need to start children on demand. When my supervisor starts up it knows what processes it needs to supervise. Anyways with that Dynamic Supervisor too I might have run into the same error when trying to supervise multiple children as I would have tried to add them into the list.

1 Like

This is going to be deprecated… in favor of DynamicSupervisor :slight_smile:


Have a look at this thread

In this case :one_for_one would be the best choice, although that will probably not fix this error.

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But that would not solve the dynamic number of genservers, specified by the user.

In one sense these are not really dynamic children. Yes, we are starting input specified number of children but from the supervisors POV it is starting the children all at start-up time so they are not dynamic. They could just as well be specified as a list of children to start.

Also there are no problems with dynamically starting children in a “normal” Supervisor. There is a Supervisor.start_child/2 which does this. This is the same as for DynamicSupervisor which also has a DynamicSupervisor.start_child/2. Both these functions need a child-spec. The main difference is how the children are terminated: in a Supervisor they are terminated in reverse order to which they were started; while in a DynamicSupervisor they are all terminated at the same time.

:simple_one_for_one is/was slightly different in that when you started the supervisor you gave one child spec which is then used as a template for all the children started/managed by it. It is/was a special case for running lots of the same thing.

Both Supervisor and DynamicSupervisor have a terminate_chlid/2 for killing children.