Logger Backends JSON

Yet another (but flexible) JSON backend for Logger. Pick whatever json encoder you want (poison, json, exjsx) or provide your own.

Roadmap

• Basic functionallity, dumb IO usage, configuration and using given parsing lib.
• Proper formatting of error messages taken from error_logger which come in as lists
• Improve documentation on hex docs.
• Buffered & async sending messages to IO :user process.
• Filtering of messages via configured function in addition to log levels.
• Adding examples of custom json encoders.
• Allowing custom message builders.
• Additional switchable IO backends, i.e. TCP, UDP, File

Installation

The package can be installed as:

1. Add logger_backends_json to your list of dependencies in mix.exs:

```elixir
def deps do
  [
    # your json library of choice, otherwise you will need to provide custom module.
    {:poison, "~> 3.0"},
    {:logger_backends_json, "~> 0.5.0"}
  ]
end
```

2. Update your config

```elixir
# A tuple with module and config name to use
config :logger, backends: [{Logger.Backends.JSON, :json}]

config :logger, :json,
  level: :info,
  metadata: %{foo: "bar"},
  encoder: Poison
```

Extra configuration

This library uses ConfigExt for loading dynamic configuration, like environment variables or functions on runtime.

defmodule Foo do
  def bar(baz), do: %{bar: inspect(baz), env: System.get_env("APP_ENV")}

  alias Logger.Backends.JSON.Event

  def create(%Event{} = event), do: %{text: event.message}
  def allow?(%Event{} = event), do: event.message != "ping"
end

System.put_env "LOG_LEVEL", "debug"

config :logger, :json,
  level: {:system, "LOG_LEVEL", :info},
  metadata: {:function, Foo, :bar, [:baz]},
  encoder: Poison,
  event: Foo

All possible options:

• encoder anything that implements encode(object) :: map|list => {:ok, json}, we test against poison, exjsx and json libs.
• level represents log level, same as in default Logger - debug, info, warn, error.
• metadata additional info to pass into json, should be Map in the end.
• event a module that implements build(%Event{}) :: map and allow?(%Event{}) :: boolean functions.

In any case you can also specify a function that will get evaluated on initialization, if you need to update it during runtime - just run Logger.configure_backend(...).

If you need to pass any extra info on each log, i.e. some stuff from ETS tables or whatever, you can do it by creating custom encoder and adding it there.

Frequent issues

1. This backend doesn't install any dependencies - it doesn't come with any default JSON encoder - so when you try to assign Poison as encoder, but you didn't installed it you will get a message like

```elixir
iex(1)>
=INFO REPORT==== 16-Nov-2016::09:57:32 ===
    application: logger
    exited: shutdown
    type: temporary
```

to solve it just install `:poison` (or any other json lib) by following their installation instructions, and everything should be back to good.

Sources & inspiration

• user process Erl shell workings and what is user process.
• elixir#4720 performance via calling user process directly.
• elixir#4728 performance via buffering IO and sending stuff async.
• various json loggers (i.e. json_logger and logger_logstash_backend)

License

This source code is released under the MIT License.