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Programming language: Elixir
License: Apache License 2.0
Tags: Logging    

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Bunyan: An Elixir Logger

  • easily extended with additional data sources and writers

  • can be networked (with one or more nodes also collecting messages from other nodes)

  • should be performant, as it

    • does not compile calls to the logger below the compile time log level;
    • bypasses the implicit serialization of the event-based loggersโ€”it is asynchronous throughout;
    • uses broadcasts where appropriate; and
    • uses iolists to construct messages where it can.
  • humane formatting of multi-line messages (including error_logger and SASL)

  • supports per-source and per-writer configuration, and the ability to log to multiple files and devices

  • works with logrotate (send it a SIGHUP and it will close and reopen the log file).


This is the omnibus version of Bunyan, which includes the API, Erlang error logger, and remote logging plugins, along with the writers for consoles, files, and remote nodes. If you want to configure a more minimal installation, have a look at Bunyan Core.

Show Me!

Here's some code that writes log messages. Obviously, in real code the values wouldn't be hardcoded...

require Bunyan
import Bunyan

info("Starting database compaction", %{
  database: "prod_orders",
  mode:     :concurrent,

# ...

info("1,396 orphaned carts found")

# ...

warn("These carts have referrers and cannot be deleted:",
     [ 48792, 49352, 49720, 50155, 57782 ])

# ...

error("This cart is locked and cannot be deleted", cart)

# ...

info("1,390 ophaned carts removed")

And here's the result if you've configured Bunyan to log to a console with ANSI color support.

[Example of Bunyan console output](./assets/images/log_example_1.png)

Try It

There's a simple demo of networked, hierarchical logging at https://github.com/bunyan-logger/demo-remote-logging.


Summary of the Summary

{ :bunyan, ">= 0.0.0" }

Basic logging:

require Bunyan

Bunyan.info "message or function"
Bunyan.info "message or function", ยซextraยป

Message can have embedded newlines, which will be honored. ยซextraยป can be any Elixir term: maps are encouraged, as they are formatted nicely.

API Overview

Logging Functions

You must require Bunyan before using any of these four functions.

  • Bunyan.debug(msg_or_fun, extra \\ nil)
  • Bunyan.info(msg_or_fun, extra \\ nil)
  • Bunyan.warn(msg_or_fun, extra \\ nil)
  • Bunyan.error(msg_or_fun, extra \\ nil)

If the first parameter is a function, it will only be invoked if the source log level is at or below the level of the message being generated.

The second parameter is optional Elixir term. It will be displayed beneath the main text of the message. If it is a map, it will be shown in a tabular structure.

Runtime Configuration
  • tba


     SOURCES                    CORE                     WRITERS
     โ€พโ€พโ€พโ€พโ€พโ€พโ€พ                    โ€พโ€พโ€พโ€พ                     โ€พโ€พโ€พโ€พโ€พโ€พโ€พ
โ•ญโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ•ฎ                                    โ•ญโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ•ฎ
โ”‚ Programmatic โ”‚ \                             ---->โ”‚ Write to stdoutโ”‚
โ”‚     API      โ”‚  \                           /     โ”‚ files, etc     โ”‚
โ•ฐโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ•ฏ   \                         /      โ•ฐโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ•ฏ
                    \      โ•ญโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ•ฎ /
โ•ญโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ•ฎ     ----> โ”‚   Collect    โ”‚/        โ•ญโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ•ฎ
โ”‚ Erlang error โ”‚ โ•Œโ•Œโ•Œโ•Œโ•Œโ•Œโ•Œโ•Œ> โ”‚ย      &       โ”‚โ•Œโ•Œโ•Œโ•Œโ•Œโ•Œโ•Œ> โ”‚   Write to     โ”‚
โ”‚   logger     โ”‚     ----> โ”‚ Distribute   โ”‚\        โ”‚  remote node   โ”‚
โ•ฐโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ•ฏ    /      โ•ฐโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ•ฏ \       โ•ฐโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ•ฏ
                   /                         \
โ•ญโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ•ฎ  /                           \     โ•ญโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ•ฎ
โ”‚    Remote    โ”‚ /                             \    โ”‚     etc        โ”‚
โ”‚    reader    โ”‚                                --->โ”‚                |
โ•ฐโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ•ฏ                                    โ•ฐโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ•ฏ

Bunyan takes logging input from a variety of sources. It is distributed with three:

  • an API which provides the functions you can call in your application,
  • an Erlang error handler which intercepts, reformats, and injects Erlang and OTP errors, and
  • a source that can accept log messages from other instances of this logger (which we call remote logging)

These sources are all plugins: you have to add them as dependencies (or use the batteries-included hex package) if you want to use them. Y

The sources send log messages to the collector. This in turn forwards the messages to the various log writers. Two writers come as standard:

  • one writes to standard error (or a file on disk);
  • the other writes to a remote instance of Bunyan.

Again, these are both optional.

You can configure multiple instances of each type of writer. This would allow you to (for example) log everything to standard error, errors only to a log file, and warnings and errors to two separate remote loggers.

You can easily add new sources and new writers to Bunyan.


Bunyan is organized as a core module, bunyan_core, and a set of plugins. If you just use this project (bunyan) as a dependency, you'll automatically get them all.

Alternatively, you can use bunyan_core as a dependency, and then add just the plugins that you want to use. You only have to add a plugin as a dependency if you declare it in the configuration (see below).

The various components are:

Name Function
bunyan_core The Collector component, which distributes incoming messages to writers
buyan_source_erlang_error_logger Injects Erlang, OTP, and SASL errors
bunyan_source_remote_reader Injects log messages sent from a remote node
bunyan_writer_device Write messages to a console, file, or other device.
bunyan_writer_remote Write messages to a remote node

There are two additional components, bunyan_shared and bunyan_formatter, that are used by the plugins. You do not need to declare these as dependencies.

Log Levels

The log levels are debug, info, warn, and error, with debug being the lowest and error the highest.

You can set log levels indpendently for each of the sources and each of the writers.

The level set on a source determines which messages are sent on to the writters. The level set on a writer determines which messages get written.

In addition, the API source has an additional option to set the compile time log level. Calls to logging functions will not be compiled into your code if they are below this level.


Configuration can be specified in the regular config.xxx files. Much of it can also be set at runtime using the Bunyan.config function.

The top level configuration looks like this:

  read_from: [
    ยซlist of sourcesยป
  write_to: [
    ยซlist of writersยป
  • read_from:

A list of sources (see below).

  • write_to:

A list of writers (also below).

Source Configuration

Each source configuration entry can be either a module name, or a tuple containing a module name and a keyword list of options:

Bunyan.Source.API or { Bunyan.Source.API, compile_time_log_level: :info }

Each source module has its own set of configuration options.

Source: Bunyan.Source.API

Provides a programmatic API that lets applications create log messages and configure the logger.


  • runtime_log_level: :debug, :info, :warn, or :error

Only calls to the corresponding API functions at or above this level will be passed to the collector.

Defaults to :debug in development, :info otherwise.

  • compile_time_log_level: :debug, :info, :warn, or :error

Calls to the corresponding API functions below this this level will be ignored: no code will be generated for them. Because of this, you don't want to give function calls with side effects as parameters to logging functions.

Defaults to :debug in development, :info otherwise.

Source: Bunyan.Source.ErlangErrorLogger

Handles messages and reports generated by the Erlang error_logger. It also handles SASL and OTP messages that the error logger forwards.

  • runtime_log_level: :debug, :info, :warn, or :error

Only calls to the corresponding API functions at or above this level will be passed to the collector.

Defaults to :debug in development, :info otherwise.

Writer Configuration

Bunyan comes with two writers (but you can add your ownโ€”see below).

Writer: Bunyan.Writer.Device

Writes log messages to standard error after formatting them for human consumption.

  • name:

The OTP name associated with this device. Set this if you want to run multiple device writers, as each must have a distinct name. You'll also need to specify this if you want to set device-specific options in your code (as the name is used to identify which device to update).

  • device:

This writer will send messages to the specified device. This is either the name of an IO handler (such as :user or :standard_error) or a string containing a file name. It would be produce to make this filename an absolue path.

Defaults to :user

  • pid_file_name:

If the log device is a file on disk, and if this option is set to the name of a pid file, the operating system pid (not the Erland pid) of the writer will be stored in the pid file. This allows utilities such as logrotate to send a USR1 signal to the writer, which will cause the writer to close and reopen the log file.

  • runtime_log_level: :debug, :info, :warn, or :error

Only calls to the corresponding API functions at or above this level will be written to standard error.

Defaults to :debug in development, :info otherwise.

  • main_format_string:

The format used for the first line of a log message (see Formats below)

Defaults to "$time [$level] $message_first_line"

  • additional_format_string:

The format used on the remaining lines of the log message.

Defaults to "$message_rest\n$extra".

  • level_colors: %{ ... }

Specifies the colors to be used when displaying the $level field. This is a map of one or more entries where the keys are the log level and the value is a string used to prefix that level. You can use IO.ANSI to generate these strings.

The defaults are:

   level_colors:   %{
     @debug => faint(),
     @info  => green(),
     @warn  => yellow(),
     @error => light_red() <> bright()
  • message_colors:

The colors used for the message text at various log levels.

  message_colors: %{
    @debug => faint(),
    @info  => reset(),
    @warn  => yellow(),
    @error => light_red()
  • timestamp_color:

The attributes used to display the $time, $date, and $datetime fields. Defaults to faint().

  • extra_color:

The attributes used to display the $extra field. Defaults to italic()<>faint().

  • use_ansi_color?:

If falsy, the various color attributes will be ignored, and the log messages will not be colored.

Defaults to true if writing to a console, false otherwise.

Writer: Bunyan.Writer.Remote
:send_to,         # name of the remote logger process
:send_to_node,    # the node or nodes when the remote reader lives. If nil, send to all
:send_to_nodes,   # alias for `send_to_node`
:min_log_level,   # only send >= this,

Used to forward log messages to another instance of Bunyan.

  • runtime_log_level: :debug, :info, :warn, or :error

Only log messages at or above this level will be forwarded to the remote logger.

Defaults to :warn.

  • send_to:

The name of the logger to send the log messages to. This name must have been given as an accept_remote_as option to that logger.

  • send_to_node: or send_to_nodes:

The name of a node, or a list of nodes. If specified, log messages will be forwarded to the loggers on these nodes (connecting to the nodes first if required). If not specified, the log message will be broadcast to all connected nodes, and any log reader using the given name will receive it.

  • max_pending_size: and max_pending_wait:

The remote logger tries to cut down on network traffic by batching messages before forwarding them. Once it receives the first message in a batch, it will then start a timer for max_pending_wait: milliseconds (default 200). When this timer expires, or when max_pending_size (default 100) messages are pending, all the messages will be sent, and the batching process reinitiated.

Log Message Format Specifications

The Device writer tries to create nicely formatted output. For example, it will try to indent multi-line messages so the start of the text of the message lines up, and it recognizes things such as maps when laying out nontext data.

What it writes is under your control. You specify this using format strings. Each message is potentially formatted using two formats. The first of these, the main_format_string is used to write the first line of the message. Typically this string will include some kind of time stamp and a log level, as well as the first line of the actual log message.

The additional_format_string is used to format the rest of the log message (if any). The output generated under the control of this format will automatically be indented to line up with the start of the message in the first line.

Newlines in the message or in the format string will automatically cause the message to be split and indented.

A format string consists of regular text and field names. The regular text is simply copied into the resulting message. The contents of the corresponding fields are substituted for the field names.

  • $date

The date the log message was sent (yy-mm-dd).

  • $time

The time the log message was sent (hh:mm:ss.mmm). because we'll all big girls and boys, this time will be in UTC.

  • $datetime

"$date $time"

  • $message

The whole log message. If the message contains newlines, it will be split, with the second and subsequent lines appearing beneath the first and left-aligned with it.

  • $msg_first_line

Just the first line of the message

  • $msg_rest

Lines 2... of the message

  • $level

The log level as a single character (D, I, W, or E)

  • $node

The node that generated the message

  • $pid

The pid that generated the message

  • $remote_info

A combination of the pid and node, with a trailing newline. If a log message originates from the same node that the logger is running on, nothing will be generated.

  • $extra

For messages generated via the API, this will be the contents of the second parameter, formatted nicely.

For reports coming from the Erlang error logger, this will be the raw content of the report.

A Sample Configuration

This is probably way more than you'd ever need to specify, but I wanted to show all the options:

  read_from:              [
  write_to:               [
      Bunyan.Writer.Device, [
        name:  :stdout_logging,

        main_format_string:        "$time [$level] $remote_info$message_first_line",
        additional_format_string:  "$message_rest\n$extra",

        level_colors:   %{
          @debug => faint(),
          @info  => green(),
          @warn  => yellow(),
          @error => light_red() <> bright(),
        message_colors: %{
          @debug => faint(),
          @info  => reset(),
          @warn  => yellow(),
          @error => light_red(),
        timestamp_color: faint(),
        extra_color:     italic() <> faint(),

        use_ansi_color?: true
      Bunyan.Writer.Device, [
        name:  :critical_errors,
        device:            "/var/log/myapp_errors.log",
        pid_file_name:     "/var/run/myapp.pid",
        rumtime_log_level: :error,
        use_ansi_color?:   false,
       Bunyan.Writer.Remote, [

          # assumes there's a Bunyan.Source.GlobalReader with
          # `global_name: MyApp.GlobalLogger` running on
          # the given two nodes

          send_to: MyApp.GlobalLogger,
          send_to_nodes:  [
            :"[email protected]",
            :"[email protected]",
          runtime_log_level: :warn,

To Do

  • [ ] Runtime configuration (hooks are in place)
  • [ ] Guides for creating your own sources and writers
  • [ ] Finish off reformatting of Erlang error logger and sasl messages (framework in place)
  • [ ] Add support for OTP 21 :logger

Why Another Logger?

I needed a distributed logger as part of the Toyland project, and couldn't find what I needed. I also wanted to experiment with something more decoupled than the available options.

A Big, Big Thank You!

To Benjamin Coppock, who let me take over the project name Bunyan on Hex. His original project is still available as

{ bunyan: "0.1.0" }