How to Log something in Controller when Phoenix Server is running?

Okay, turns out it's pretty straight forward. You need to require the Logger elixir module in your controller and call one of it's methods to log your text.

defmodule PhoenixApp.TopicController do
    require Logger

    def index(conn, params) do
        Logger.info  "Logging this text!"
        Logger.debug "Var value: #{inspect(params)}"

        # ...
    end
end

Supported levels are:

• :debug - for debug-related messages
• :info - for information of any kind
• :warn - for warnings
• :error - for errors

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Source: Elixir - Logger Documentation

You can also just do IO.puts or IO.inspect and it'll show up, but IO.puts can be troublesome if what you're trying to print doesn't implement the String.Chars protocol