LoggerFileBackend

A simple Logger backend which writes logs to a file. It does not handle log rotation for you, but it does tolerate log file renames, so it can be used in conjunction with external log rotation.

Note The following of file renames does not work on Windows, because File.Stat.inode is used to determine whether the log file has been (re)moved and, on non-Unix, File.Stat.inode is always 0.

Configuration

LoggerFileBackend 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 {LoggerFileBackend, id}, where id is any elixir term (usually an atom).

For example, let's say we want to log error messages to "/var/log/my_app/error.log". To do that, we will need to configure a backend. Let's call it {LoggerFileBackend, :error_log}.

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

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

With this configuration, the :logger application will start one LoggerFileBackend named {LoggerFileBackend, :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: [{LoggerFileBackend, :error_log}]

# configuration for the {LoggerFileBackend, :error_log} backend
config :logger, :error_log,
  path: "/var/log/my_app/error.log",
  level: :error

Check out the examples below for runtime configuration and configuration for multiple log files.

LoggerFileBackend supports the following configuration values:

• path - the path to the log file
• 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

Examples

Runtime configuration

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

Application config for multiple log files

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

config :logger, :info,
  path: "/path/to/info.log",
  level: :info

config :logger, :error,
  path: "/path/to/error.log",
  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: [{LoggerFileBackend, :ui}]

config :logger, :ui,
  path: "/path/to/ui.log",
  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: [{LoggerFileBackend, :device_1}]

config :logger, :device_1
  path: "/path/to/device_1.log",
  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