Reference Manual

Version 6.3

Table of Contents




Module Summary

Default formatter for Logger.


Module logger_formatter was introduced in OTP 21.0.


Each Logger 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. See sections Handlers and Formatters in the Kernel User's Guide for more information.

logger_formatter is the default formatter used by Logger.

Data Types

config() =
    #{chars_limit => integer() >= 1 | unlimited,
      depth => integer() >= 1 | unlimited,
      legacy_header => boolean(),
      max_size => integer() >= 1 | unlimited,
      report_cb => logger:report_cb(),
      single_line => boolean(),
      template => template(),
      time_designator => byte(),
      time_offset => integer() | [byte()]}

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

chars_limit = integer() > 0 | 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.

Defaults to unlimited.

depth = integer() > 0 | 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.

Defaults to unlimited.

legacy_header = boolean()

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 list [logger_formatter,header] to the template. See the description of the template() type for more information.

Defaults to false.

max_size = integer() > 0 | 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.

Defaults to unlimited.

report_cb = logger:report_cb()

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 exists 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.

single_line = boolean()

If set to true, each log event is printed as a single line. To achieve this, logger_formatter sets the field width to 0 for all ~p and ~P control sequences in the format a string (see io:format/2), and replaces all newlines in the message with ", ". White spaces following directly after newlines are removed. Notice that newlines added by the template parameter are not replaced.

Defaults to true.

template = template()

The template describes how the formatted string is composed by combining different data values from the log event. See the description of the template() type for more information about this.

time_designator = byte()

Timestamps are formatted according to RFC3339, and the time designator is the character used as date and time separator.

Defaults to $T.

The value of this parameter is used as the time_designator option to calendar:system_time_to_rcf3339/2.

time_offset = integer() | [byte()]

The time offset, either a string or an integer, to be used when formatting the timestamp.

An empty string is interpreted as local time. The values "Z", "z" or 0 are interpreted as Universal Coordinated Time (UTC).

Strings, other than "Z", "z", or "", must be on the form ±[hh]:[mm], for example "-02:00" or "+00:00".

Integers must be in microseconds, meaning that the offset 7200000000 is equivalent to "+02:00".

Defaults to an empty string, meaning that timestamps are displayed in local time. However, for backwards compatibility, if the SASL configuration parameter utc_log=true, the default is changed to "Z", meaning that timestamps are displayed in UTC.

The value of this parameter is used as the offset option to calendar:system_time_to_rcf3339/2.

metakey() = atom() | [atom()]

template() =
    [metakey() | {metakey(), template(), template()} | string()]

The template is a list of atoms, atom lists, tuples and strings. The atoms level or msg, are treated as placeholders for the severity level and the log message, respectively. Other atoms or atom lists are interpreted as placeholders for metadata, where atoms are expected to match top level keys, and atom lists represent paths to sub keys when the metadata is a nested map. For example the list [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,

Tuples in the template express if-exist tests for metadata keys. For example, the following tuple says that if key1 exists in the metadata map, print "key1=Value", where Value is the value that key1 is associated with in the metadata map. If key1 does not exist, print nothing.

{key1, ["key1=",key1], []}

Strings in the template are printed literally.

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

The log event used in the examples is:

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

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

Example log entry:

=ERROR REPORT==== 17-May-2018::18:30:19.453447 ===
name: my_name
exit_reason: "It crashed"

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

legacy_header = true, single_line = true

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

Notice that the template is here the same as for single_line=false, but the resulting log entry differs in that there is only one line after the heading:

=ERROR REPORT==== 17-May-2018::18:31:06.952665 ===
name: my_name, exit_reason: "It crashed"
legacy_header = false, single_line = true

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

Example log entry:

2018-05-17T18:31:31.152864+02:00 error: name: my_name, exit_reason: "It crashed"
legacy_header = false, single_line = false

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

Example log entry:

2018-05-17T18:32:20.105422+02:00 error:
name: my_name
exit_reason: "It crashed"


check_config(Config) -> ok | {error, term()}
OTP 21.0


Config = config()

The function is called by Logger when the formatter configuration for a handler is set or modified. It returns ok if the configuration is valid, and {error,term()} if it is faulty.

The following Logger API functions can trigger this callback:

format(LogEvent, Config) -> unicode:chardata()
OTP 21.0


Config = config()

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. See section Log Message in the Kernel User's Guide for more information about report callbacks and valid forms of log messages.
  • The message size is limited according to the values of configuration parameters chars_limit and depth.
  • 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.

See Also