to get region an instance is running at

myaws_metadata.ex

defmodule MyAws.Metadata
  # AWS InstanceMetaData URL
  @meta_path_region "http://169.254.169.254/latest/meta-data/placement/availability-zone"

  @region_regexp ~r/(?<continent>eu|us)-(?<zone>[^-]+)-(?<number>[\d]+)/i

  @doc """
  Get aws region. 
  First, it tries with :some_app, :aws_region variable stablished in config (hopefuly using env vars), and if no
  it assumes the code is running in an ec2 instance, so it tries to get the region from instance metadata.
  """
  @spec get_region() :: binary()
  def get_region, do: Application.get_env(:some_app,:aws_region, get_region_from_instance_metadata())
  
  @spec get_region_from_instance_metadata() :: binary()
  defp get_region_from_instance_metadata do
    %{"continent" => continent, "zone" => zone, "number" => number} =
      Regex.named_captures(
        @region_regexp,
        ExAws.InstanceMeta.request(ExAws.Config.new(:s3), @meta_path_region)
      )
    continent <> "-" <> zone <> "-" <> number
  end
  
end