setup-elixir-logging

name	setup-elixir-logging
description	Configure rotated and compressed file logging for Elixir/Phoenix projects using :logger_std_h. Use when setting up production logging, configuring logger rotation, or when the user asks to add file logging to an Elixir project.

Setup Elixir Logging

Quick Start

When setting up file logging for an Elixir/Phoenix project, use the built-in :logger_std_h handler. It supports log rotation and compression out of the box without requiring external dependencies.

Add the following configuration to config/runtime.exs (typically inside the if config_env() == :prod do block):

  # File logging with rotation via :logger_std_h (Logger boot config).
  # See https://hexdocs.pm/logger/Logger.html#module-configuration
  # and https://www.erlang.org/doc/apps/kernel/logger_std_h.html
  #
  # Ensure the log directory exists before boot (e.g. mkdir in unit file or release overlay).
  # LOG_FILE — absolute or relative path (default: $RELEASE_ROOT/log/app.log, else log/app.log)
  # LOG_MAX_BYTES — rotate when file exceeds this size (default: 10_485_760)
  # LOG_MAX_FILES — rotated archives to keep (default: 5); .N.gz when compress_on_rotate is true
  log_file =
    cond do
      path = System.get_env("LOG_FILE") -> path
      root = System.get_env("RELEASE_ROOT") -> Path.join([root, "log", "app.log"])
      true -> "log/app.log"
    end

  max_bytes =
    case Integer.parse(System.get_env("LOG_MAX_BYTES", "10485760")) do
      {n, _} when n > 0 -> n
      _ -> 10_485_760
    end

  max_files =
    case Integer.parse(System.get_env("LOG_MAX_FILES", "5")) do
      {n, _} when n >= 0 -> n
      _ -> 5
    end

  config :logger, :default_handler,
    config: [
      file: String.to_charlist(log_file),
      filesync_repeat_interval: 5000,
      file_check: 5000,
      max_no_bytes: max_bytes,
      max_no_files: max_files,
      compress_on_rotate: true
    ]

Implementation Notes

1. No external dependencies: This uses Erlang's standard :logger_std_h which is included in OTP.
2. String.to_charlist: The file option must be a charlist, not a binary string.
3. Directory creation: The directory containing the log file must exist before the application starts. The logger will not create it automatically.
4. Environment Variables: The configuration uses environment variables to allow overriding the defaults in production without recompiling.