The document outlines the creation and implementation of a module for downloading files in Elixir, specifically using the HTTPoison library for making HTTP requests. It includes code snippets demonstrating the setup of the downloader, handling of incoming data chunks, and writing these chunks to files. The process is tested to ensure functionality, and it also highlights the structure of the module and the interaction with external URLs.

3. ▶ mix new fun_with_processes --module FWP
* creating README.md
* creating .formatter.exs
* creating .gitignore
* creating mix.exs
* creating config
* creating config/config.exs
* creating lib
* creating lib/fun_with_processes.ex
* creating test
* creating test/test_helper.exs
* creating test/fun_with_processes_test.exs

4. defmodule FWP.MixProject do
# ...
defp deps do
[
{:httpoison, "~> 1.0"}
]
end
end

5. iex(1)> url = "https://elixir-lang.org/images/logo/logo.png"
iex(2)> dir = Application.get_env(:fun_with_processes, :downloads_dir)
iex(3)> file = Utils.open_file_for(url, dir)
iex(4)>

6. iex(1)> url = "https://elixir-lang.org/images/logo/logo.png"
iex(2)> dir = Application.get_env(:fun_with_processes, :downloads_dir)
iex(3)> file = Utils.open_file_for(url, dir)
iex(4)> {:ok, resp} = HTTPoison.get(url)
{:ok, %HTTPoison.Response{body: ...}}
iex(5)>

7. iex(1)> url = "https://elixir-lang.org/images/logo/logo.png"
iex(2)> dir = Application.get_env(:fun_with_processes, :downloads_dir)
iex(3)> file = Utils.open_file_for(url, dir)
iex(4)> {:ok, resp} = HTTPoison.get(url)
{:ok, %HTTPoison.Response{body: ...}}
iex(5)>

8. iex(1)> url = "https://elixir-lang.org/images/logo/logo.png"
iex(2)> dir = Application.get_env(:fun_with_processes, :downloads_dir)
iex(3)> file = Utils.open_file_for(url, dir)
iex(4)> {:ok, resp} = HTTPoison.get(url)
{:ok, %HTTPoison.Response{body: ...}}
iex(5)> IO.binwrite(file, resp.body)
iex(6)> File.close(file)

9. iex(1)> url = "https://elixir-lang.org/images/logo/logo.png"
iex(2)> dir = Application.get_env(:fun_with_processes, :downloads_dir)
iex(3)> file = Utils.open_file_for(url, dir)
iex(4)> {:ok, resp} = HTTPoison.get(url)
{:ok, %HTTPoison.Response{body: ...}}
iex(5)> IO.binwrite(file, resp.body)
iex(6)> File.close(file)

10. ▶ ls -l downloads
total 1
-rw-r--r-- 1 pawel.dawczak pawel.dawczak 8109 Jul 21 21:43 logo.png

11. ▶ ls -l downloads
total 1
-rw-r--r-- 1 pawel.dawczak pawel.dawczak 8109 Jul 21 21:43 logo.png

19. iex(1)> url = "https://elixir-lang.org/images/logo/logo.png"
iex(2)> HTTPoison.get(url, [], stream_to: self())
{:ok, %HTTPoison.AsyncResponse{id: #Reference<0.1.2.3>}}
iex(3)>

20. iex(1)> url = "https://elixir-lang.org/images/logo/logo.png"
iex(2)> HTTPoison.get(url, [], stream_to: self())
{:ok, %HTTPoison.AsyncResponse{id: #Reference<0.1.2.3>}}
iex(3)>

21. iex(1)> url = "https://elixir-lang.org/images/logo/logo.png"
iex(2)> HTTPoison.get(url, [], stream_to: self())
{:ok, %HTTPoison.AsyncResponse{id: #Reference<0.1.2.3>}}
iex(3)> flush
%HTTPoison.AsyncStatus{code: 200, id: #Reference<0.1.2.3>}
%HTTPoison.AsyncHeaders{...}
%HTTPoison.AsyncChunk{
chunk: <<137, 80, 78, 71, ...>>,
id: #Reference<0.1.2.3>
}
%HTTPoison.AsyncChunk{
chunk: <<29, 5, 22, 220, ...>>,
id: #Reference<0.1.2.3>
}
...
%HTTPoison.AsyncEnd{id: #Reference<0.1.2.3>}
:ok

22. iex(1)> url = "https://elixir-lang.org/images/logo/logo.png"
iex(2)> HTTPoison.get(url, [], stream_to: self())
{:ok, %HTTPoison.AsyncResponse{id: #Reference<0.1.2.3>}}
iex(3)> flush
%HTTPoison.AsyncStatus{code: 200, id: #Reference<0.1.2.3>}
%HTTPoison.AsyncHeaders{...}
%HTTPoison.AsyncChunk{
chunk: <<137, 80, 78, 71, ...>>,
id: #Reference<0.1.2.3>
}
%HTTPoison.AsyncChunk{
chunk: <<29, 5, 22, 220, ...>>,
id: #Reference<0.1.2.3>
}
...
%HTTPoison.AsyncEnd{id: #Reference<0.1.2.3>}
:ok

23. defmodule FWP.Downloader.StreamServer do
# ...
def init(opts) do
file_url = Keyword.fetch!(opts, :file_url)
dir = Keyword.fetch!(opts, :dir)
state = %{
file_url: file_url,
dir: dir,
file_path: nil,
file: nil
}
{:ok, state, {:continue, :init_download}}
end
end

24. defmodule FWP.Downloader.StreamServer do
# ...
def init(opts) do
file_url = Keyword.fetch!(opts, :file_url)
dir = Keyword.fetch!(opts, :dir)
state = %{
file_url: file_url,
dir: dir,
file_path: nil,
file: nil
}
{:ok, state, {:continue, :init_download}}
end
end

25. defmodule FWP.Downloader.StreamServer do
# ...
def init(opts) do
file_url = Keyword.fetch!(opts, :file_url)
dir = Keyword.fetch!(opts, :dir)
state = %{
file_url: file_url,
dir: dir,
file_path: nil,
file: nil
}
{:ok, state, {:continue, :init_download}}
end
end

26. defmodule FWP.Downloader.StreamServer do
# ...
def handle_continue(:init_download, state) do
end
end

27. defmodule FWP.Downloader.StreamServer do
# ...
def handle_continue(:init_download, state) do
{:ok, _} = HTTPoison.get(state.file_url, [], stream_to: self())
end
end

28. defmodule FWP.Downloader.StreamServer do
# ...
def handle_continue(:init_download, state) do
{:ok, _} = HTTPoison.get(state.file_url, [], stream_to: self())
file_path = Utils.file_path_for(state.file_url, state.dir)
file = Utils.open_file(file_path)
end
end

29. defmodule FWP.Downloader.StreamServer do
# ...
def handle_continue(:init_download, state) do
{:ok, _} = HTTPoison.get(state.file_url, [], stream_to: self())
file_path = Utils.file_path_for(state.file_url, state.dir)
file = Utils.open_file(file_path)
new_state = %{state | file_path: file_path, file: file}
{:noreply, new_state}
end
end

30. defmodule FWP.Downloader.StreamServer do
# ...
end

31. defmodule FWP.Downloader.StreamServer do
# ...
def handle_info(%HTTPoison.AsyncChunk{chunk: chunk}, state) do
Logger.debug("Receiving chunk...")
IO.binwrite(state.file, chunk)
{:noreply, state}
end
end

32. defmodule FWP.Downloader.StreamServer do
# ...
def handle_info(%HTTPoison.AsyncChunk{chunk: chunk}, state) do
Logger.debug("Receiving chunk...")
IO.binwrite(state.file, chunk)
{:noreply, state}
end
end

33. defmodule FWP.Downloader.StreamServer do
# ...
def handle_info(%HTTPoison.AsyncChunk{chunk: chunk}, state) do
Logger.debug("Receiving chunk...")
IO.binwrite(state.file, chunk)
{:noreply, state}
end
def handle_info(%HTTPoison.AsyncEnd{}, state) do
Logger.info("That's all folks!")
File.close(state.file)
{:stop, :normal, state}
end
end

34. defmodule FWP.Downloader.StreamServer do
# ...
def handle_info(%HTTPoison.AsyncChunk{chunk: chunk}, state) do
Logger.debug("Receiving chunk...")
IO.binwrite(state.file, chunk)
{:noreply, state}
end
def handle_info(%HTTPoison.AsyncEnd{}, state) do
Logger.info("That's all folks!")
File.close(state.file)
{:stop, :normal, state}
end
end

35. defmodule FWP.Downloader.StreamServer do
# ...
def handle_info(%HTTPoison.AsyncChunk{chunk: chunk}, state) do
Logger.debug("Receiving chunk...")
IO.binwrite(state.file, chunk)
{:noreply, state}
end
def handle_info(%HTTPoison.AsyncEnd{}, state) do
Logger.info("That's all folks!")
File.close(state.file)
{:stop, :normal, state}
end
end

36. defmodule FWP.Downloader.StreamServer do
# ...
def handle_info(%HTTPoison.AsyncChunk{chunk: chunk}, state) do
Logger.debug("Receiving chunk...")
IO.binwrite(state.file, chunk)
{:noreply, state}
end
def handle_info(%HTTPoison.AsyncEnd{}, state) do
Logger.info("That's all folks!")
File.close(state.file)
{:stop, :normal, state}
end
def handle_info(msg, state) do
Logger.debug("Received: #{inspect(msg)}")
{:noreply, state}
end
end

37. iex(3)> FWP.Downloader.StreamServer.start_link(file_url: url, dir: dir)
{:ok, #PID<0.198.0>}
iex(4)>

38. iex(3)> FWP.Downloader.StreamServer.start_link(file_url: url, dir: dir)
{:ok, #PID<0.198.0>}
iex(4)>
21:26:54.267 [debug] Received: %HTTPoison.AsyncStatus{..., id: #Reference<1>}
21:26:54.270 [debug] Received: %HTTPoison.AsyncHeaders{..., id: #Reference<1>}
21:26:54.275 [debug] Receiving chunk…
...
21:26:54.638 [info] That's all folks!

39. ▶ ls -l downloads
total 2
-rw-r--r-- 1 pawel.dawczak pawel.dawczak 8109 Jul 21 21:43 logo.png
-rw-r--r-- 1 pawel.dawczak pawel.dawczak 115K Jul 21 21:56 phoenix.png

40. ▶ ls -l downloads
total 2
-rw-r--r-- 1 pawel.dawczak pawel.dawczak 8109 Jul 21 21:43 logo.png
-rw-r--r-- 1 pawel.dawczak pawel.dawczak 115K Jul 21 21:56 phoenix.png

44. defmodule FWP.Downloader.StreamServerTest do
# ...
@tag :capture_log
test "accepts chunks and writes them to the file" do
url = "http://foo.com/test-success.txt"
file_path = Utils.file_path_for("test-success.txt", @test_dir)
File.rm_rf!(file_path)
Process.flag(:trap_exit, true)
{:ok, server} =
StreamServer.start_link(file_url: url, dir: @test_dir, engine: HTTPMock)
send(server, %HTTPoison.AsyncChunk{chunk: [?H, ?e]})
send(server, %HTTPoison.AsyncChunk{chunk: [?l]})
send(server, %HTTPoison.AsyncChunk{chunk: [?l, ?o]})
send(server, %HTTPoison.AsyncEnd{})
assert_receive {:EXIT, ^server, :normal}
assert File.exists?(file_path)
assert File.read!(file_path) == "Hello"
end
end

45. defmodule FWP.Downloader.StreamServerTest do
# ...
@tag :capture_log
test "accepts chunks and writes them to the file" do
url = "http://foo.com/test-success.txt"
file_path = Utils.file_path_for("test-success.txt", @test_dir)
File.rm_rf!(file_path)
Process.flag(:trap_exit, true)
{:ok, server} =
StreamServer.start_link(file_url: url, dir: @test_dir, engine: HTTPMock)
send(server, %HTTPoison.AsyncChunk{chunk: [?H, ?e]})
send(server, %HTTPoison.AsyncChunk{chunk: [?l]})
send(server, %HTTPoison.AsyncChunk{chunk: [?l, ?o]})
send(server, %HTTPoison.AsyncEnd{})
assert_receive {:EXIT, ^server, :normal}
assert File.exists?(file_path)
assert File.read!(file_path) == "Hello"
end
end

46. defmodule FWP.Downloader.StreamServerTest do
# ...
@tag :capture_log
test "accepts chunks and writes them to the file" do
url = "http://foo.com/test-success.txt"
file_path = Utils.file_path_for("test-success.txt", @test_dir)
File.rm_rf!(file_path)
Process.flag(:trap_exit, true)
{:ok, server} =
StreamServer.start_link(file_url: url, dir: @test_dir, engine: HTTPMock)
send(server, %HTTPoison.AsyncChunk{chunk: [?H, ?e]})
send(server, %HTTPoison.AsyncChunk{chunk: [?l]})
send(server, %HTTPoison.AsyncChunk{chunk: [?l, ?o]})
send(server, %HTTPoison.AsyncEnd{})
assert_receive {:EXIT, ^server, :normal}
assert File.exists?(file_path)
assert File.read!(file_path) == "Hello"
end
end

47. defmodule FWP.Downloader.StreamServerTest do
# ...
@tag :capture_log
test "accepts chunks and writes them to the file" do
url = "http://foo.com/test-success.txt"
file_path = Utils.file_path_for("test-success.txt", @test_dir)
File.rm_rf!(file_path)
Process.flag(:trap_exit, true)
{:ok, server} =
StreamServer.start_link(file_url: url, dir: @test_dir, engine: HTTPMock)
send(server, %HTTPoison.AsyncChunk{chunk: [?H, ?e]})
send(server, %HTTPoison.AsyncChunk{chunk: [?l]})
send(server, %HTTPoison.AsyncChunk{chunk: [?l, ?o]})
send(server, %HTTPoison.AsyncEnd{})
assert_receive {:EXIT, ^server, :normal}
assert File.exists?(file_path)
assert File.read!(file_path) == "Hello"
end
end

48. defmodule FWP.Downloader.StreamServer do
# ...
def init(opts) do
file_url = Keyword.fetch!(opts, :file_url)
dir = Keyword.fetch!(opts, :dir)
state = %{
file_url: file_url,
dir: dir,
file_path: nil,
file: nil
}
{:ok, state, {:continue, :init_download}}
end
end

49. defmodule FWP.Downloader.StreamServer do
# ...
def init(opts) do
file_url = Keyword.fetch!(opts, :file_url)
dir = Keyword.fetch!(opts, :dir)
engine = Keyword.get(opts, :engine, HTTPoison)
state = %{
file_url: file_url,
dir: dir,
engine: engine,
file_path: nil,
file: nil
}
{:ok, state, {:continue, :init_download}}
end
end

50. defmodule FWP.Downloader.StreamServer do
# ...
def handle_continue(:init_download, state) do
{:ok, _} = HTTPoison.get(state.file_url, [], stream_to: self())
file_path = Utils.file_path_for(state.file_url, state.dir)
file = Utils.open_file(file_path)
new_state = %{state | file_path: file_path, file: file}
{:noreply, new_state}
end
end

51. defmodule FWP.Downloader.StreamServer do
# ...
def handle_continue(:init_download, state) do
{:ok, _} = HTTPoison.get(state.file_url, [], stream_to: self())
file_path = Utils.file_path_for(state.file_url, state.dir)
file = Utils.open_file(file_path)
new_state = %{state | file_path: file_path, file: file}
{:noreply, new_state}
end
end

52. defmodule FWP.Downloader.StreamServer do
# ...
def handle_continue(:init_download, state) do
{:ok, _} = state.engine.get(state.file_url, [], stream_to: self())
file_path = Utils.file_path_for(state.file_url, state.dir)
file = Utils.open_file(file_path)
new_state = %{state | file_path: file_path, file: file}
{:noreply, new_state}
end
end

53. ▶ mix test

54. ▶ mix test
.
Finished in 0.07 seconds
1 test, 0 failures
Randomized with seed 941275

55. ▶ mix test
.
Finished in 0.07 seconds
1 test, 0 failures
Randomized with seed 941275

56. @pawel_dawczak @pdawczak
https://github.com/pdawczak/fun_with_processes