LoggerFileBackendWin View Source

A simple Elixir Logger backend for Windows, which writes logs to a file. It can rotate files at the start of each new day (local time) and when a maximum size is reached, if you wish it to be so.

Note If you are running this with the Phoenix framework, please review the Phoenix specific instructions later on in this file.

Configuration

LoggerFileBackendWin is a custom backend for the elixir :logger application. As such, it relies on the :logger application to start the relevant processes. However, unlike the default :console backend, we may want to configure multiple log files, each with different log levels, formats, etc. Also, we want :logger to be responsible for starting and stopping each of our logging processes for us. Because of these considerations, there must be one :logger backend configured for each log file we need. Each backend has a name like {LoggerFileBackendWin, name}, where name is any elixir term (usually an atom).

For example, let's say we want to log error messages to "<APPDIR>/logs/error_log_<DATE>.<FILE#>.log". To do that, we will need to configure a backend.

Our config.exs would have an entry similar to this:

# tell logger to load a LoggerFileBackend processes
config :logger,
  backends: [{LoggerFileBackendWin, :error_log}]

With this configuration, the :logger application will start one LoggerFileBackendWin named {LoggerFileBackendWin, :error_log}. We still need to set the correct file path and log levels for the backend, though. To do that, we add another config stanza. Together with the stanza above, we'll have something like this:

# tell logger to load a LoggerFileBackend processes
config :logger,
  backends: [{LoggerFileBackendWin, :error_log}]

# configuration for the {LoggerFileBackend, :error_log} backend
config :logger, :error_log,
  dir: "logs",
  level: :error

This will use the name defined in the backend configuration for the filename. You can set a custom name using the filename option. The complete filename looks like: "#{dir}/#{filename}_#{Date.to_string(date).#{file_number}.log". The file number starts at 0 and newer ones are incremented by one. We do not rename them, the highest number is always the most recent.

LoggerFileBackendWin supports the following configuration values:

  • dir - the directory where log files are saved
  • filename - the file name to write to
  • level - the logging level for the backend
  • format - the logging format for the backend
  • metadata - the metadata to include
  • metadata_filter - metadata terms which must be present in order to log
  • rotate - file rotation configuration

File Rotation

config :logger, :error_log,
  dir: "logs",
  level: :error,
  rotate: %{
    daily: true,
    days: 30,
    keep: 3,
    max_bytes: 1_000_000
  }

The above configuration will:

  • rotate files daily
  • keep 30 days worth of logs
  • create a new log file once the current one exceeds max_bytes
  • keeping up to 3 x 1,000,000 byte files for each day

So you'll end up with something like this:

error_log_2021-06-18.1.log
error_log_2021-06-18.0.log

error_log_2021-06-17.0.log

error_log_2021-06-16.2.log
error_log_2021-06-16.1.log
error_log_2021-06-16.0.log

Examples

Runtime configuration

Logger.add_backend {LoggerFileBackend, :debug}
Logger.configure_backend {LoggerFileBackend, :debug},
  dir: "/path/to",
  filename: "debug"
  format: ...,
  metadata: ...,
  metadata_filter: ...

Application config for multiple log files

config :logger,
  backends: [{LoggerFileBackendWin, :info},
             {LoggerFileBackendWin, :error}]

config :logger, :info,
  level: :info

config :logger, :error,
  level: :error

Filtering specific metadata terms

This example only logs :info statements originating from the :ui OTP app; the :application metadata key is auto-populated by Logger.

config :logger,
  backends: [{LoggerFileBackendWin, :ui}]

config :logger, :ui,
  level: :info,
  metadata_filter: [application: :ui]

This example only writes log statements with a custom metadata key to the file.

# in a config file:
config :logger,
  backends: [{LoggerFileBackendWin, :device_1}]

config :logger, :device_1,
  level: :debug,
  metadata_filter: [device: 1]

# Usage:
# anywhere in the code:
Logger.info("statement", device: 1)

# or, for a single process, e.g., a GenServer:
# in init/1:
Logger.metadata(device: 1)
# ^ sets device: 1 for all subsequent log statements from this process.

# Later, in other code (handle_cast/2, etc.)
Logger.info("statement") # <= already tagged with the device_1 metadata

Additional Phoenix Configurations

Phoenix makes use of its own mix.exs file to track dependencies and additional applications. Add the following to your mix.exs:

def application do
    [applications: [
      ...,
      :logger_file_backend,
      ...
      ]
    ]
end
  
defp deps do
  [ 
    ...
    {:logger_file_backend_win, "~> 0.0.1"},
    ...
  ]
end

Attribution

This project is little more than logger_file_backend modified to rotate files on Windows.