LoggerLagerBackend

A lager (https://github.com/erlang-lager/lager) backend for Elixir’s Logger
(https://hexdocs.pm/logger/Logger.html).

That is, it routes messages generated with Logger.<level>() to lager. This
is useful if you have a mixed Erlang/Elixir project and have decided to
standardize on lager as a logging framework.

Known limitations:

• You’re on your own to configure and start lager.
• Only lager‘s default sink (:lager_event) is used.
• Performance is probably not ideal - all messages received are sent to lager
  regardless of the configured level and lager‘s compile-time optimization
  does not happen. However, Logger will still do its own optimization
  upstream of this backend.
• Metadata is passed straight from Logger to lager, hoping that the keys
  match. It seems to be the case for the basics (module, function…) but
  may need looking into.

Installation

Add logger_lager_backend to your list of dependencies in mix.exs:

def deps do
  [{:logger_lager_backend, "~> 0.1.0"}]
end

Configuration

Instruct Logger to use logger_lager_backend:

config :logger,
  backends: [LoggerLagerBackend],
  handle_otp_reports: false,
  level: :debug

This sends all messages of level debug or higher to lager. They will then
be subject to filtering and routing according to whichever lager config you
have in place.

We also use handle_otp_reports: false to avoid having messages from the
Erlang’s built-in error_logger module appear twice in the output.

Troubleshooting

FORMAT ERRORs in log

If you get FORMAT ERROR messages like this one:

FORMAT ERROR: "~s" [[<<"GenServer :redis_sub_0_8 terminating">>,<<"\n** (stop) ">>|<<":redis_down">>]

You’re probably hitting erlang-lager/lager#326. Upgrade
lager to 3.2.0 or more recent.

Elixir.Logger.Supervisor error on startup

If you get the following message on startup:

[error] Supervisor 'Elixir.Logger.Supervisor' had child 'Elixir.Logger.ErrorHandler' started with
  'Elixir.Logger.Watcher':watcher(error_logger, 'Elixir.Logger.ErrorHandler',
  {true,false,500}, link) at <0.422.0> exit with reason normal in context child_terminated

Make sure the :lager application is started before :logger, by putting
:lager first in your applications list in mix.exs:

def application do
  [applications: [:lager, :logger, …],
   mod: {MyApp, []}]
end

Related projects

• lager_logger does the
  opposite of this backend: it sends lager messages to Logger.
• exlager offers an Elixir frontend to
  lager, which can be an alternative to this backend if you control all the
  code that does logging. But if you are using e.g. Ecto, which writes to
  Logger, exlager will not help you.