View Source Overview

test workflow Contributor Covenant

logger_formatter_json

This is a formatter for the Erlang logger application that outputs JSON.

It implements the formatter API for the high-performance logger application introduced in OTP 21.

It formats log messages and logger metadata as JSON, supporting naming conventions from services such as Datadog and Google Cloud

It is written in Erlang with no dependencies except for the Erlang JSON library thoas. It can be used by pure Erlang projects as well as other BEAM languages such as Elixir.

installation

Installation

Erlang:

Add logger_formatter_json to the list of dependencies in rebar.config:

{deps, [logger_formatter_json]}.

Elixir:

Add logger_formatter_json to the list of dependencies in mix.exs:

def deps do
  [
    {:logger_formatter_json, "~> 0.7"},
  ]
end

usage

Usage

JSON output is mostly useful for production, as it makes it easier for tools to parse log output, but it can be excessively verbose for development. If you use a lot of metatadata, a library like flatlog produces output that is easier for humans to read.

In order to make all log output in consistent JSON format, including system messages, configure the formatter as the default for all applications running on the VM.

Erlang:

Configure the kernel default handler in the sys.config file for the release:

[
 {kernel, [
    {logger, [
        {handler, default, logger_std_h,
         #{formatter => {logger_formatter_json, #{}}}
        }
    ]},
    {logger_level, info}
 ]}
].

Elixir:

The Elixir logging system starts after the kernel logger, so it is tricky to configure.

Instead of configuring the logger in Elixir, we can override the default formatter in the Elixir application starts.

In config/prod.exs or config/runtime.exs, define the formatter config:

config :foo, :logger_formatter_config, {:logger_formatter_json, %{}}

or, with options (see below):

config :foo, :logger_formatter_config, {:logger_formatter_json,
 %{
   template: [
     :msg,
     :time,
     :level,
     :file,
     :line,
     # :mfa,
     :pid,
     :request_id,
     :trace_id,
     :span_id
   ]
 }}

You can set more metadata options for the Elixir logging system in config/prod.exs:

config :logger,
  level: :info,
  utc_log: true

config :logger, :console,
  metadata: [:time, :level, :file, :line, :mfa, :pid, :request_id, :trace_id, :span_id]

Next, in in your application startup file, e.g. lib/foo/application.ex, add a call to reconfigure the logger:

def start(_type, _args) do
  logger_formatter_config = Application.get_env(:foo, :logger_formatter_config)

  if logger_formatter_config do
    :logger.update_handler_config(:default, :formatter, logger_formatter_config)
  end

If you want all the messages from the initial startup in JSON as well, you have to configure the logger as a VM arg for the release.

In rel/vm.args.eex, set up the logger:

-kernel logger '[{handler, default, logger_std_h, #{formatter => {logger_formatter_json, #{}}}}]'

or, with options:

-kernel logger '[{handler, default, logger_std_h, #{formatter => {logger_formatter_json, #{template => [msg, time, level, file, line, mfa, pid, trace_id, span_id]}}}}]'

There used to be a way of doing this in Elixir, but it seems to have stopped working. In config/prod.exs or config/runtime.exs, define the formatter config:

if System.get_env("RELEASE_MODE") do
  config :kernel, :logger, [
    {:handler, :default, :logger_std_h,
     %{
       formatter: {:logger_formatter_json, %{}}
     }}
  ]
end

The check for the RELEASE_MODE environment variable makes the code only run when building a release.

configuration

Configuration

The formatter accepts a map of options, e.g.:

config :foo, :logger, [
  {:handler, :default, :logger_std_h,
   %{
     formatter:
       {:logger_formatter_json, %{
            names: %{
                time: "date",
                level: "status",
                msg: "message"
            }
        }}
   }}
]

names is a map of keys in the metadata map to string keys in the JSON output.

The module has predefined sets of keys for datadog and gcp.

config :foo, :logger, [
  {:handler, :default, :logger_std_h,
   %{
     formatter:
       {:logger_formatter_json, %{
            names: :datadog
        }}
   }}
]

You can also specify a list to add your own tags to the predefined ones, e.g. of options, e.g. names: [datadog, %{foo: "bar"}].

types is a map which identifies keys with a special format that the module understands (level, system_time, mfa).

template is a list of metadata to format. This lets you put keys in specific order to make them easier to read in the output.

For example:

template: [
  :msg,
  :time,
  :level,
  :file,
  :line,
  :mfa,
  :pid,
  :trace_id,
  :span_id
]

List elements are metadata key names, with a few special keys:

  • msg represents the text message, if any.

If you call logger:info("the message"), then it would be rendered in the JSON as {"msg": "the message", ...}. You can map the key msg to e.g. message via the names config option.

  • all represents all the metadata keys.
  • rest represents all the metadata keys which have not been handled explicitly.

You can specify a group of keys as a tuple like {group, <name>, [<list of metadata keys>]}, and they will be collected into a map in the output.

For example:

{group, source_location, [file, line, mfa]},
{group, tags, [rest]}

This would result in a log message like:

{
    ...
    "source_location": {"file:" "mymodule.ex", "line": 17, "mfa": "mymodule:thefunction/1"},
    "tags": {"foo": "bar", "biz": "baz"}
}

The default template is [msg, all].

You can also use a tuple to specify a standard set of keys to be used:

{keys, basic}: [time, level, msg]

{keys, trace}: [trace_id, span_id]

{keys, gcp}:

[
    msg,
    time,
    level,
    trace_id,
    span_id,
    {group, source_location, [file, line, mfa]},
    {group, tags, [rest]}
]

You can specify multiple templates, so you can add your own metadata keys to one of the standard templates, e.g. [{keys, basic}, request_id, trace_id, span_id].

Much thanks to Fred Hebert, as always.