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Distributed programming made easy with Elixir

Getting Records from Mnesia

Unfortunately, Mnesia does not support SQL syntax, but it does support some basic filters like dirty_match_object (Listing 5).

Listing 5

Getting Records

iex> # Get all the records, use :_ for "all", add a sort at the end
iex> :mnesia.dirty_match_object({Pi3, :_ ,:_})  |> Enum.sort
[
  {Pi3, 0, '0'},
  {Pi3, 1, '0'},
  {Pi3, 2, '1'},
  ...
]
iex> # Get only Pi Values that are '1'
iex> :mnesia.dirty_match_object({Pi3, :_ ,'1'})  |> Enum.sort
[
  {Pi3, 2, '1'}
]

A dialog from a zenity --list command can display table output in columns (Figure 8). The last argument in the command is the data string, which fills in the defined columns. The --extra-button option returns the button text when the button is pressed.

Figure 8: A Zenity list in Bash.

The script in Listing 6 (Show_gpio.exs) reads the Mnesia results. As in the earlier example, Enum.map and Enum.join functions format the results as one long string for a Zenity list dialog (Figure 9).

Listing 6

Show_gpio.exs

01 #-------------------
02 # Show_Show_gpio.exs - show Mnesia table in a Zenity list dialog
03 #-------------------
04 defmodule Show_gpio do
05   def getdata() do
06     result = :mnesia.dirty_match_object({Pi3, :_ ,:_}) |> Enum.sort
07     # create a presentable string for output
08     output = Enum.map(result, fn x -> "#{elem(x,1)} #{elem(x,2)} " end) |> Enum.join
09     feedback = :os.cmd(:"zenity --list --title=Pi3_Table --text='Pin Values' --extra-button Refresh --column=Pin --column=Value #{output}")
10     if ("#{feedback}" =~ "Refresh") do
11       getdata()
12     end
13   end
14 end
15 # Start Mnesia
16 :mnesia.start()
17 # Wait for tables to update
18 :timer.sleep(1000)
19 # Show a Zenity list dialog.
20 # Refresh button to continue, other buttons to quit
21 Show_gpio.getdata()

Figure 9: Elixir script showing Mnesia data.

To test that things are working, use the first project (Zen2gpio.exs) to toggle GPIO pin values; then, the Show_gpio.exs dialog can be refreshed to check the new values.

When the final project is running, you have an example of Elixir concurrency. The Pi3 node is populating a Mnesia table, and it is handling remote GPIO writes and CPU temperature messages.

Final Comments

Elixir with the Erlang VM offers a lot of functionality out of the box. The next steps from here would be to look at messaging between nodes and creating projects with imported Elixir libraries.

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