# Starting only what a FLAME runner needs

When FLAME boots a remote runner, it starts your application there, so your application has to decide which supervision children belong on that runner. Here is how ElixirDrops.Application tags each child and filters the list at boot:

```elixir
defmodule Demo do
  def children(child_specs, is_parent?, is_flame?) do
    Enum.flat_map(child_specs, fn
      {:always, spec} -> [spec]
      {:parent, spec} -> if is_parent?, do: [spec], else: []
      {:flame, spec} -> if is_flame?, do: [spec], else: []
    end)
  end
end

specs = [always: :pubsub, parent: :endpoint, flame: :screenshot_pool]

# is_parent? = is_nil(FLAME.Parent.get())
# is_flame?  = !is_parent? || FLAME.Backend.impl() == FLAME.LocalBackend
IO.inspect(Demo.children(specs, true, false), label: "parent, FLAME.FlyBackend")
IO.inspect(Demo.children(specs, false, true), label: "remote runner, FLAME.FlyBackend")
IO.inspect(Demo.children(specs, true, true), label: "single node, FLAME.LocalBackend")
```

```
parent, FLAME.FlyBackend: [:pubsub, :endpoint]
remote runner, FLAME.FlyBackend: [:pubsub, :screenshot_pool]
single node, FLAME.LocalBackend: [:pubsub, :endpoint, :screenshot_pool]
```

`FLAME.Parent.get/0` reads the `FLAME_PARENT` environment variable that a backend sets on the machine it boots, and returns `nil` when it is missing. So `is_parent?` is true on the node you deployed and false on a runner. `FLAME.Backend.impl/0` is just `Application.get_env(:flame, :backend, FLAME.LocalBackend)`, the same value on both sides.

Read `is_flame?` carefully. It is true on every runner, and also true on your own node whenever the configured backend is `FLAME.LocalBackend`. That second half matters because `FLAME.LocalBackend` boots no machine at all. It builds a parent struct in memory and runs your function on the node you already have, so `FLAME_PARENT` is never set and `is_parent?` stays true. There is no second node for the tags to split.

The practical consequence: with a real backend, `:flame` children start on the runner and not on your node. Under `FLAME.LocalBackend` every tag matches at once, so a child you meant for the runner starts next to your Endpoint in dev. The tag names are an ElixirDrops convention, not something FLAME defines or reads.

https://hexdocs.pm/flame/FLAME.Backend.html


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Created by: almirsarajcic
Date: September 14, 2026
URL: https://elixirdrops.net/d/fjeqAM8C