LogfmtEx

A log formatter for the logfmt format popularized by Heroku.

Installation

The package can be installed by adding logfmt_ex to your list of dependencies in mix.exs:

def deps do
  [
    {:logfmt_ex, "~> 0.2"}
  ]
end

In your app's logger configuration, specify the LogfmtEx module and its format function to be used to format your logs:

config :logger, :console,
  format: {LogfmtEx, :format}

Additionally, add LogfmtEx to your application.ex with any Configuration values as additional options.

defmodule MyApp do
  use Application

  def start(_type, _args) do
    children = [
      # first entry, so formatter is available to all following modules.
      {LogfmtEx, [message_key: "msg", timestamp_format: :iso8601],
      MyApp.Web,
      MyApp.CoolThing,
      ...

Configuration

The following options are available to be passed to Logfmt.start_link/1:

• :delimiter - defaults to =.
• :format - A list of atoms that defines the order in which key/value pairs will written to the log line. Defaults to [:timestamp, :level, :message, :metadata]. Valid parameters are
  • :timestamp - the timestamp of the log message
  • :level - the log level
  • :message - the log message itself
  • :metadata - metadata as key=value paris
  • :node - the node name
• timestamp_key - changes the key used for the timestamp field. Defaults to timestamp.
• timestamp_format - How the timestamp is formatted. Defaults to :elixir. The options are
  • :elixir - Uses the same formatting functions found in the standard elixir log formatter. Example: "12:38:38.055 1973-03-12"
  • :iso8601 - Formats the timestamp according to ISO8601-2019. Example: 2000-02-29T23:00:07
• level_key - the key used for the log level. Defaults to level.
• message_key - the key used for the message field. Defaults to message, but msg is a popular alternative.

If you want to use application configuration for the logger, you can easily pull in the formatting options:

config.exs:

config :logger, :console,
  format: {LogfmtEx, :format},
  metadata: [:user_id, :pid, :file]

config LogfmtEx, :opts,
  format: [:level, :message, :node, :timestamp, :metadata],
  timestamp_key: "ts",
  message_key: "msg"

application.ex:

defmodule MyApp do

  use Application

  def start(_type, _args) do
    children = [
      {LogfmtEx, Application.get_env(LogfmtEx, :opts)},
      MyApp.Web
    ]

    Supervisor.start_link(children, strategy: :one_for_one)
  end
end

for which Logger.info("I am a message", user_id: 123) would ouput something along the lines of

level=info msg="I am a message" ts="12:38:38.055 1973-03-12" user_id=123 pid=#PID<0.223.0> file=test/logfmt_ex_test.exs\n

Encoding

Structs can be encoded via the LogfmtEx.ValueEncoder protocol.

  defimpl LogfmtEx.ValueEncoder, for: Regex do
    def encode(regex), do: inspect(regex)
  end

Testing and Development

This library uses asdf to manage runtime versions of Elixir and Erlang.

Alternatives

LogfmtEx is a simple logfmt formatter specifically for the Elixir console backend. If you're looking for a library to encode and decode logfmt, take a look at logfmt-elixir instead.