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logger_formatter Default formatter for Logger.

Each log handler has a configured formatter specified as a module and a configuration term. The purpose of the formatter is to translate the log events to a final printable string (unicode:chardata()) which can be written to the output device of the handler.

logger_formatter is the default formatter used by Logger.
Configuration

The configuration term for logger_formatter is a map, and the following keys can be set as configuration parameters:

chars_limit = pos_integer() | unlimited

A positive integer representing the value of the option with the same name to be used when calling io_lib:format/3. This value limits the total number of characters printed for each log event. Notice that this is a soft limit. For a hard truncation limit, see option max_size.

Default is unlimited.

chars_limit has no effect on log messages on string form. These are expected to be short, but can still be truncated by the max_size parameter.

 depth = pos_integer() | unlimited

A positive integer representing the maximum depth to which terms shall be printed by this formatter. Format strings passed to this formatter are rewritten. The format controls ~p and ~w are replaced with ~P and ~W, respectively, and the value is used as the depth parameter. For details, see io:format/2,3 in STDLIB.

Default is unlimited.

depth has no effect on log messages on string form. These are expected to be short, but can still be truncated by the max_size parameter.

 max_size = pos_integer() | unlimited

A positive integer representing the absolute maximum size a string returned from this formatter can have. If the formatted string is longer, after possibly being limited by chars_limit or depth, it is truncated.

Default is unlimited.

 single_line = boolean()

If set to true, all newlines in the message are replaced with ", ", and whitespaces following directly after newlines are removed. Note that newlines added by the template parameter are not replaced.

Default is true.

 legacy_header = boolen()

If set to true a header field is added to logger_formatter's part of Metadata. The value of this field is a string similar to the header created by the old error_logger event handlers. It can be included in the log event by adding the tuple {logger_formatter,header} to the template. See section Default Templates for more information.

Default is false.

 report_cb = fun((logger:report()) -> {io:format(),[term()]})

A report callback is used by the formatter to transform log messages on report form to a format string and arguments. The report callback can be specified in the metadata for the log event. If no report callback exist in metadata, logger_formatter will use logger:format_report/1 as default callback.

If this configuration parameter is set, it replaces both the default report callback, and any report callback found in metadata. That is, all reports are converted by this configured function.

The value must be a function with arity 1, returning {Format,Args}, and it will be called with a report as only argument.

 template = template()

The template is a list of atoms, tuples and strings. The atoms level or msg, are treated as placeholders for the severity level and the log message, repectively. Other atoms or tuples are interpreted as placeholders for metadata, where atoms are expected to match top level keys, and tuples represent paths to sub keys when the metadata is a nested map. For example the tuple {key1,key2} is replaced by the value of the key2 field in the nested map below. The atom key1 on its own is replaced by the complete value of the key1 field. The values are converted to strings.

#{key1=>#{key2=>my_value, ...} ...}

Strings in the template are printed literally.

The default template differs depending on the values of legacy_header and single_line. See Default Templates for more information

 utc = boolean()

If set to true, all dates are displayed in Universal Coordinated Time.

Default is false.
Default templates

The default value for the template configuration parameter depends on the value of single_line and legacy_header as follows.

The log event used in the examples is:

?LOG_ERROR("name: ~p~nexit_reason: ~p",[my_reg_name,"It crashed"]) legacy_header=true

Default template: [{logger_formatter,header},"\n",msg,"\n"]

Example log entry:

=ERROR REPORT==== 29-Dec-2017::13:30:51.245123 === name: my_reg_name exit_reason: "It crashed"

Notice that all eight levels might occur in the heading, not only ERROR, WARNING or INFO as the old error_logger produced. And microseconds are added at the end of the timestamp.

 single_line=true

Default template: [time," ",level,": ",msg,"\n"]

Example log entry:

2017-12-29 13:31:49.640317 error: name: my_reg_name, exit_reason: "It crashed" legacy_header=false, single_line=false

Default template: [time," ",level,":\n",msg,"\n"]

Example log entry:

2017-12-29 13:32:25.191925 error: name: my_reg_name exit_reason: "It crashed"
Formats the given message.

This the formatter callback function to be called from handlers. The log event is processed as follows:

If the message is on report form, it is converted to {Format,Args} by calling the report callback. The size is limited according to the values of configuration parameters chars_limit and depth. Notice that this does not apply to messages on string form. The full log entry is composed according to the template. If the final string is too long, it is truncated according to the value of configuration parameter max_size.