Logger.Backends.Logfmt (LoggerLogfmt v1.0.1)

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A Logfmt formatter for Elixir's Logger.

This module provides functions to format log messages in the logfmt format, a structured logging format that is easy to parse and human-readable.

Features

  • Flexible format configuration with customizable fields
  • Support for metadata filtering (whitelist/blacklist modes)
  • Multiple timestamp formats (Elixir, ISO8601, Unix epoch)
  • Automatic quoting and escaping of values
  • Nested map support with dot notation

Configuration

Configure the formatter in your config/config.exs:

config :logger, :logfmt,
  format: [:timestamp, :level, :message, :metadata],
  metadata: [:application, :request_id],
  mode: :whitelist,
  timestamp_format: :iso8601

Format Options

The :format option accepts a list of atoms that determine which fields to include in the output:

  • :timestamp - Log event timestamp
  • :level - Log level (debug, info, warn, error)
  • :message - Log message
  • :domain - Logger domain
  • :node - Node name
  • :pid - Process identifier
  • :metadata - Additional metadata
  • :file - Source file
  • :line - Line number

Examples

iex> result = Logger.Backends.Logfmt.format(:info, "User logged in", {{2024, 1, 15}, {10, 30, 45, 123}}, [user_id: 42])
iex> output = IO.iodata_to_binary(result)
iex> output =~ ~r/timestamp=2024-01-15T10:30:45.123/
true
iex> output =~ ~r/level=info/
true
iex> output =~ ~r/message="User logged in"/
true

Summary

Functions

format(level, message, timestamp, metadata, opts \\ [])

Formats a log message in logfmt format.

Functions

format(level, message, timestamp, metadata, opts \\ [])

Formats a log message in logfmt format.

Parameters

  • level - The log level (:debug, :info, :warn, :error)
  • message - The log message
  • timestamp - The timestamp tuple {{year, month, day}, {hour, minute, second, millisecond}}
  • metadata - A keyword list of metadata
  • opts - Optional keyword list of formatting options

Returns

An iolist containing the formatted log message.

Examples

iex> result = Logger.Backends.Logfmt.format(:info, "Hello", {{2024, 1, 1}, {12, 0, 0, 0}}, [])
iex> output = IO.iodata_to_binary(result)
iex> output =~ ~r/level=info/
true
iex> output =~ ~r/message=Hello/
true