Example polling a log file

polling_log_reader.ex

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Log Reader - polls log file and sends new lines to channel
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
defmodule LogReader do
  use GenServer

  @log_file "priv/some_log.log"
  @poll_interval 5 * 1000 # 5 seconds

  def run_test() do
    {:ok, _} = LogWriter.start_link()
    {:ok, _} = LogReader.start_link()
  end

  def start_link(_ \\ []) do
    GenServer.start_link(__MODULE__, :ok, name: LogReader)
  end

  def init(:ok) do
    # open the log file and set the pointer to the end so that we only grab
    # new log messages
    {:ok, fp} = File.open(@log_file, [:read])
    :file.position(fp, :eof)
    poll()
    {:ok, fp}
  end

  def handle_info(:read_log_lines, fp) do
    # consume any new log lines and pass them off the channel
    fp |> read_til_eof |> send_to_channel
    poll()
    {:noreply, fp}
  end

  def read_til_eof(fp),
    do: read_til_eof(IO.binread(fp, :line), fp, [])
  def read_til_eof(:eof, _fp, buffer), do: buffer
  def read_til_eof(line, fp, buffer),
    do: read_til_eof(IO.binread(fp, :line), fp, buffer ++ [line])

  # this could be handing off the new lines to some service or sending directly
  # to the channel or whatever
  def send_to_channel([]), do: :ok
  def send_to_channel(lines),
    do: for line <- lines, do: IO.puts line

  defp poll(),
    do: Process.send_after(self(), :read_log_lines, @poll_interval)

end

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Log Writer - simulates log writes
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
defmodule LogWriter do
  use GenServer

  @log_file "priv/some_log.log"

  def start_link(_ \\ []) do
    GenServer.start_link(__MODULE__, :ok, name: LogWriter)
  end

  def init(:ok) do
    {:ok, fp} = File.open(@log_file, [:append])
    poll()
    {:ok, fp}
  end

  def handle_info(:write_log, fp) do
    for _ <- 0..:rand.uniform(5), do: IO.puts fp, make_log_message()
    poll()
    {:noreply, fp}
  end

  def make_log_message() do
    time = :os.system_time(:milli_seconds) |> to_string()
    body = :crypto.strong_rand_bytes(15) |> Base.url_encode64
    "[#{time}] #{body}"
  end

  defp poll() do
    interval = :rand.uniform(10) * 1000
    Process.send_after(self(), :write_log, interval)
  end
end