Agent used to keep the simple state of the devices connection to nerves-hub.org.

The state is a tuple where the first element is an atom of :connected or :disconnected and the second element is the value of System.monotonic_time/1 at the time of setting the new state.

In practice, this state is set anytime the device connection to nerves-hub.org channel changes. Likewise, it is set after a HTTP request fails or succeeds. This makes it useful when you want to consider the connection to nerves-hub.org as part of the overall health of the device and perform explicit actions based on the result, such as using the Erlang :heart module to force a reboot if the callback check fails.

# Set a callback for heart to check every 5 seconds. If the function returns anything other than
# `:ok`, it will cause reboot.
:heart.set_callback(NervesHubLink.Connection, :check!)

Or, you can use the check as part of a separate function with other health checks as well

defmodule MyApp.Checker do
  def health_check do
    with :ok <- NervesHubLink.Connection.check,
         :ok <- MyApp.another_check,
         :ok <- MyApp.yet_another_check,
    do
      :ok
    else
      err -> err
    end
  end
end

# Somewhere else in MyApp
:heart.set_callback(MyApp.Checker, :health_check)