HTTPoison Stateless API

http_posion_client.ex

@doc """
Struct to hold the configurable portions of HTTPoision
"""
defmodule HTTPoison.Client do
  defstruct process_url: &__MODULE__.process_url/1,
  process_request_body: &__MODULE__.process_request_body/1,
  process_request_headers: &__MODULE__.process_request_headers/1,
  process_request_options: &__MODULE__.process_request_options/1,
  process_response_body: &__MODULE__.process_response_body/1,
  process_headers: &__MODULE__.process_headers/1,
  process_status_code: &__MODULE__.process_status_code/1

  def process_url(url), do: url
  def process_request_body(body), do: body
  def process_request_headers(headers), do: headers
  def process_request_options(opts), do: opts
  def process_response_body(body), do: body
  def process_headers(headers), do: headers
  def process_status_code(status_code), do: status_code

end

@doc """
API that takes the client and applies the configuration
"""
defmodule HTTPoison.API do
  def post(client, url, body, headers \\ [], options \\ []) do
    request(client, :post, url, body, headers, options)
  end

  def get(client, url, headers \\ [], options \\ []) do
    request(client, :get, url, "", headers, options)
  end

  def delete(client, url, headers \\ [], options \\ []) do
    request(client, :delete, url, "", headers, options)
  end

  # Make the actual request
  # This uses an undocumented function in HTTPoision which is probably a bit sketchy
  defp request(client, method, url, body, headers, options) do
    url =
      if Keyword.has_key?(options, :params) do
        url <> "?" <> URI.encode_query(options[:params])
      else
        url
      end
      url = client.process_url.(to_string(url))
      body = client.process_request_body.(body)
      headers = client.process_request_headers.(headers)
      options = client.process_request_options.(options)
      HTTPoison.Base.request(__MODULE__, method, url, body, headers, options, client.process_status_code, client.process_headers, client.process_response_body)
  end
end

# How you would use it in your own code
@doc """
client = Example.Pulsar.client(:service)
HTTPosion.API.get(client, "/foo")
"""
defmodule Example.Pulsar do
  require Logger

  def client(key) do
    %HTTPoison.Client{process_url: process_url(key),
      process_request_body: &json_encode/1,
      process_response_body: &json_decode/1,
      process_request_headers: &ensure_json_header/1,
      process_request_options: &increase_timeouts/1,
    }
  end

  def process_url(key) do
    endpoint = endpoint(key)

    fn (url) ->
      endpoint <> url
    end
  end

  def json_encode(body) do
    Logger.debug("Connection body: #{inspect body}")
    body
    |> Poison.encode!()
  end

  def json_decode(body) do
    Logger.debug("Connection response: #{inspect body}")
    case Poison.decode(body) do
      {:ok, response} -> response
      error -> error
    end
  end

  def ensure_json_header(headers) do
    Enum.into(headers, [{"Content-Type", "application/json"}])
  end

  def increase_timeouts(opts) do
    opts
    |> PffLive.Pulsar.default_options(:timeout, 10000)
    |> PffLive.Pulsar.default_options(:recv_timeout, 30000)
  end

  defp config(key) do
    Application.fetch_env!(:example, key)
  end

  defp endpoint(key) do
    config(key)
    |> Keyword.get(:pulsar_endpoint)
  end

  def default_options(options, keyword, default) do
    val = Keyword.get(options, keyword)
    if val do
      options
    else
      [{keyword, default} | options]
    end
  end

end

There are multiple ways to achieve this with HTTPoison. You could redefine request/5 and receive the endpoint that you intend as an option. Then the "base url" will be defined at that stage.

I also experimented a bit with a Behaviour: https://github.com/edgurgel/httpehaviour

The behavior is interesting but doesn't seem to achieve what I want to do. Maybe a more generic idea:
How could someone create a generic JSON client that set the proper headers, transformed the body on the request and response, etc. but only change the endpoint so that they had an instance that they could use to call ServiceA and a different instance they could use to call ServiceB? That's the fundamental problem I was trying to solve. The only way I could see to do that was to turn the "client" into data instead of having it hardcoded in a module.

One other way is to pass an option which is always the last argument. Then build the url with the option passed?