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.
Infos
- Elixir: https://elixir-lang.org/
- Erlang: https://www.erlang.org/
- Elixir installation: https://elixir-lang.org/install.html
- GPIO utility: http://wiringpi.com/the-gpio-utility/
- Zenity: https://help.gnome.org/users/zenity/2.32/
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