Problem reading file system from Elixir app

Questions / Help
1

I’m having trouble getting my Elixir app to start on fly.io. I’m using the Dockerfile that flyctl created, although it looks straightforward enough to me. My app has a GenServer application that reads some static files when it starts up. If I docker build the image locally and shell in to it, the files are present (eg. /lib/myapp-0.1.0/priv/vocabs/condition.ttl).

But the logs on fly.io show:

2022-10-27T11:26:12Z app[601b7009] lhr [info]
{"Kernel pid terminated",application_controller,"{application_start_failure,myapp,
{{shutdown,{failed_to_start_child,'Elixir.KnowledgeGraph.Graph',
{#{'__exception__' => true,'__struct__' => 'Elixir.File.Error',
action => <<\"open\">>,
path => <<\"priv/vocabs/condition.ttl\">>, reason => enoent},
[{'Elixir.File','open!',3,[{file,\"lib/file.ex\"},{line,1511}]},
{'Elixir.RDF.Serialization.Reader','do_read_file!',4,
[{file,\"lib/rdf/serialization/reader.ex\"},{line,81}]},

To get enoent, either the file priv/vocabs/condition.ttl didn’t get copied to the image (but I can see that it did, assuming it’s in the right place), or there’s something else I need to do to allow the Elixir process to access the file system in a container.

This is my first time building a Docker image for Elixir, although I’ve done it several times for Ruby and Python apps. But I thought that reading from the file system was usually straightforward. So I’m feeling a bit stuck!

2

Can you share the code that reads from the priv dir? Thanks!

3

Sure. My own code is pretty simple:

  defvocab Condition,
    base_iri: "http://example.org/def/core/habitat/",
    file: "condition.ttl"

The docs for defvocab instruct that the path to the vocab file should be relative to priv/vocabs, and indeed this works just fine when he app is running in dev.

The defvocab macro is part of the RDF.ex library. Tracking through the (many) layers, I think we ultimately end up at:

  @spec read_file!(module, Path.t(), keyword) :: Graph.t() | Dataset.t()
  def read_file!(decoder, file, opts \\ []) do
    decoder
    |> Serialization.use_file_streaming!(opts)
    |> do_read_file!(decoder, file, opts)
  end

  defp do_read_file!(false, decoder, file, opts) do
    file
    |> File.open!(file_mode(decoder, opts), &IO.read(&1, @io_read_mode))
    |> case do
      {:error, error} when is_tuple(error) -> error |> inspect() |> raise()
      {:error, error} -> raise(error)
      :eof -> decoder.decode!("", opts)
      content -> decoder.decode!(content, opts)
    end
  end

source

But it does suggest that an obvious debug step for me to do is to try to read that file directly, before defvocab has a go, and log the output.

4

Sorry, I mean the file argument specifically that you pass to that function. You likely need to be referencing :code.priv_dir when you build the file path.

5

So my code only has condition.ttl. That gets expanded to priv/vocabs/condition.ttl by the RDF.ex library here:

  defp filename!(opts) do
    if filename = Keyword.get(opts, :file) do
      cond do
        File.exists?(filename) ->
          filename

        File.exists?(expanded_filename = Path.expand(filename, @vocabs_dir)) ->
          expanded_filename

        true ->
          raise File.Error, path: filename, action: "find", reason: :enoent
      end
    end
  end

In my code, I’m passing in %{file: "condition.ttl"} as the opts to that function.

6

And @vocabs_dir is defined:

  # Note: We're not using :code.priv_dir/1 here on purpose, since vocabulary files should be
  # searched in the vocabs dir of the project in which the vocabulary namespace is defined.
  @vocabs_dir "priv/vocabs"
7

Tbh, I don’t understand the comment above that definition.

8

it needs to be referencing @vocabs_dir Path.join(:code.priv_dir(:your_otp_app), "vocabs")

The priv dir of the release is not priv/, it will be something like _build/prod/lib/your_app/priv/, so using :code.priv_dir returns the proper full path to it.

9

Thanks @chrismccord. I’ll report an issue to the upstream RDF library (that @vocabs_dir attribute declaration is not in my code).