Log to console library, with configurable levels and tagging

elixir, filtering, logger, logger-backend



A Logger backend and frontend with enhanced filtering capabilities, flexible configuration and utility macros.


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

def deps do
    {:log, ">= 0.3"}

Configure the Logger to use Log.Backend instead of :console

config :logger,
  backends: [Log.Backend]

To use the backend it's sufficient to use Logger:

require Logger
Logger.info("This is a message", tags: [:a_message])

Log provides an advanced frontend to Logger which adds new levels, output filtering and other features that can be useful during development and in production. To access these features is necessary to use the module Log instead of Logger:

require Log
Log.info("This is a message", tags: [:a_message])

More advanced functionalities are explained in Advanced Usage section.

Advanced Usage

  • Tagging
  • do syntax
  • inspect
  • @log_tags attribute
  • Custom Logger with pre-defined tags
  • Available Log Levels
  • Log.Args


The core feature of Log is to be able to tag messages and later on filter messages not including some tags.

Log.info("message 1", tags: [:tag1, :tag2])
Log.info("message 2", tag: :tag1)

With the environment variable LOG_TAGS it's possible to filter some messages:

  • LOG_TAGS=tag2,tag1 displays both messages
  • LOG_TAGS=-tag2,tag1 displays only message 2
  • LOG_TAGS=-tag2 displays only message 2 (any message without tag2)
  • LOG_TAGS=-tag1 displays no messages
  • LOG_TAGS=tag3 displays no messages



Will allow output of any message with any (minimum 1) of the following tags:

  • tag1
  • tag2
  • tag3
  • tag4

The syntax for LOG_TAGS is the following:

  • , (comma) is used as a separator for tags
  • Empty strings are ignored, so LOG_TAGS=tag1,,,,tag2 is valid and it will filter any message without tag1 or tag2
Special Tags
  • _untagged includes any message without tags
  • _all includes all messages, tagged and untagged. Must be used alone

A tag can be prefixed with + and - modifiers. When a tag has - modifier, it must be absent from the message tags.

When a tag has + modifier, it must be present from the message tags.


A message is output only if it has:

  • tag1 and tag3
  • tag2 and tag3
  • tag1, tag2 and tag3

Do Syntax

In addition to passing a string or a function as log message, to lazy evaluate the content, it's also possible to use the do syntax to achieve the same lazy-evaluation:

require Log
Log.info(tags: [:a_tag, :another_tag]) do
  "This message #{interpolates} some variables"


IO.inspect is a widely used function. Log provides a custom logger named Log.Inspect which behaves like IO.inspect/2, while preserving the ability to filter log messages:

a = 1
b = Log.Inspect.info(a) + 2
# b is now 3

Like inspect, the return value of Log.Inspect is the data structure passed. Log output is:

[2000-01-01T01:01:01.001Z] INFO:

The same options available for IO.inspect are available, such as :label:

Log.Inspect.info(%{some: "data"}, label: "a message")

Which outputs:

[2000-01-01T01:01:01.001Z] INFO: a message
%{some: "data"}

Notice that differently from IO.inspect, Log.Inspect defaults pretty to true. The last feature of Log.Inspect is that it tags all the messages with the tag :inspect, so removing inspect messages can be easily performed by setting the environment variable LOG_TAGS to include -inspect.

Log Tags Attribute

When the attribute @log_tags is defined, any Log message will include the tags specified in the attribute

defmodule MyModule do
  require Log
  @log_tags [:tag1, :tag2]

  def hello do
    Log.info("message 1")
    Log.info("message 2")

  @log_tags [:tag3]

  def world do
    Log.info("message 3")

  def run do


Message 1 and 2 will be both tagged with :tag1 and :tag2, while message 3 will be tagged only with :tag3.

Custom Logger With Pre-Defined Tags

It's possible to create a Logger with some tags always added to every message where the module is used:

defmodule MyLog do
  use Log, tags: [:tag1, :tag2]
defmodule MyModule do
  require Log

  @log_tags [:tag3]

  def run do


"message" will have tags: :tag1, :tag2 and :tag3

Available Log Levels

If using Log frontend module instead of Logger directly, the following levels are available:

  • trace
  • debug
  • info
  • warn
  • error
  • fatal

Otherwise the log levels are limited to the Logger levels:

  • debug
  • info
  • warn
  • error


A common scenario is logging the entry point of a function, in such cases, displaying the arguments of the function is important. The module Log.Args helps by formatting variables in a readable way:

require Log.Args
Log.Args.info({"a message", %{some_id: 123, some_name: "Jon"}})

Will output:

[2000-01-01T01:01:01.001Z] INFO: a message (SomeId: 123, SomeName: Jon)

If String keys are used, no transformation to pascal case is performed. It's possible to use a keyword instead of a map.


Replace Logger :console backend with Log.Backend in your configuration.

Output Filtering Configuration

The environment variables are an interface to filter log output dinamically. The available environment variables are the following:

  • CONSOLE_DEVICE = stdout | stderr defaults to stderr
  • LOG_TAGS = _all | _untagged | tag_name | -tag_name | +tag_name
    • - requires the tag to be missing
    • + requires the tag to be present
    • No sign means "One or more of no sign tags must be present"
  • LOG_TAGS_LEVEL = _min | _max | trace | debug | info | warn | error | fatal Any message at this level or above will not be filtered out by tags. Useful to always display warns or errors
    • _min is an alias for trace
    • _max is an alias for fatal
    • User defined levels are also supported in LOG_TAGS_LEVEL
  • LOG_LEVEL = _none | _min | _max | trace | debug | info | warn | error | fatal Any message below this level won't be displayed
    • _none means no message will be logged
    • _min is an alias for trace
    • _max is an alias for fatal
    • User defined levels are also supported in LOG_LEVEL
  • LOG_DEBUG = on | off when set to on prints debug messages and errors, as well as tags information
  • LOG_FORMATTERS = on | off when set to on, colorizes output
  • LOG_FORMAT_TAGS = on | off when set to on, displays the tags of the message
  • LOG_MODULE = on | off when set to on, displays the module where log line is being invoked

Timestamp Configuration

It is recommended, but not required, to set logging timestamp to UTC, to avoid confusing issues with DST changes. This can be done in config.exs using:

import Config

config :logger, utc_log: true

Or with the native Logger function:

Logger.configure(utc_log: true)

The timestamp is always formatted according to ISO-8601 format, however no timezone modifier is displayed except for UTC. In case timestam is configured to use UTC, Z is appended to the timestamp.

Additional configuration

Other configuration options are provided that can be set directly on the Logger backend:

  • Alias a module namespace
  • Exclude some namespaces from writing output
  • Change output color on a per-level basis

These options can be set either through config.exs

import Config

config :logger, Log.Backend,
  module_alias: %{
    LogTest.Deeply.Nested.Module.WithLog => ""
  exclude_namespaces: [],
  colors: %{}

Or with the native Logger function:

    module_alias: %{},
    exclude_namespaces: [],
    colors: %{}

Alias a Module Namespace

Given the modules:

  • A.Module.Namespace.For.Something
  • Other.Module.Namespace.For.SomethingElse

and the configuration:

    module_alias: %{
      A.Module.Namespace => "",
      Other.Module.Namespace => "OMN"

When the log message "a message" is written from module A.Module.Namespace.For.Something it will be displayed as:

[2000-01-01T01:01:01.001Z] For.Something INFO: a message

When the log message "a message" is written from module Other.Module.Namespace.For.SomethingElse it will be displayed as:

[2000-01-01T01:01:01.001Z] OMN.For.SomethingElse INFO: a message

Exclude Namespaces

Given the modules:

  • A.Module.Namespace.For.Something
  • A.Module.Namespace.For.SomethingElse

and the configuration:

    exclude_namespaces: [

When the log message "a message" is written from module A.Module.Namespace.For.Something or from A.Module.Namespace.For.SomethingElse, no message is written.

Change Output Color

Given the following configuration:

    colors: %{
      debug: IO.ANSI.green(),
      error: [IO.ANSI.red(), IO.ANSI.bright()]
  • error color will is bold, red
  • debug color is green

The colors map accepts level (as atoms) as keys, and IO.ANSI.ansidata as values.

Customized Logger

It's possible to create a customized logger, which accepts a data structure of your choice, as well as return a value of your choice. It's sufficient to override the bare_log by following Log.Args footprint:

defmodule UpLog do
  use Log, tags: [:upcase]

  @impl true
  def bare_log(data, meta)

  def bare_log(data, meta) when is_function(data) do
      fn ->
        data.() |> String.upcase()

  def bare_log(data, meta) do
    Log.API.bare_log(&String.upcase/1, meta)

UpLog can be used like Log:

require UpLog
UpLog.info("a message")

And will output the following message:

[2000-01-01T01:01:01.001Z] INFO: A MESSAGE


  • Guidelines for logging
  • Restructure filters
  • Testing
  • Performance


This project is deeply inspired by Eventide and its logger.