Stout

Stout is a better version of the lager_default_formatter formatter. It follows the same pattern, a list of either IO data or tags. In Stout, each tag can be accompanied by a set of format options.

Features

• Color support (explicit for all tags and automatic for severity)
• Formatting tags will only print something if the tag exist in the Lager metadata. For example, wrapping a tag in [ and ] using {format, "[~s]"} would only show the brackets if the tag exists.
• Automatic formatting of all metadata tags.

Example

The following code:

lager:debug([{binary, <<"hello">>}], "Debug", []),
lager:info([{number, 2.3}], "Info", []),
lager:notice([{pid, self()}], "Notice", []),
lager:warning("Warning"),
lager:error("Error"),
lager:critical("Critical"),
lager:alert("Alert"),
lager:emergency("Emergency").

With the following configuration:

[
    {date, blackb}, " ",
    {time, yellow}, " ",
    {severity, [upper, {format, "~.7s"}, color, {format, "~s "}]},
    {'$metadata', [], [node, application, line]}, " ",
    message, "\n"
]

Which would yield:

Items

IOData :: iolist() | binary() | list() | char()

Any IO data (iolist(), binary(), list(), or char()) is printed as is in each log message.

Tag :: atom()

A tag (atom()) is picked from the message metadata. Predefined tags in Stout messages are:

• date The date from Lager's datetime record.
• time The time from Lager's datetime record.
• severity The severity of the message.
• message The actual log message.

TagOpt :: {Tag, option()} | {Tag, [option()]}

A tag with options is formatted according to the options given (see Options below).

{'$metadata', Formats :: [TagOpt], Skipped :: [Tag]}

The metadata item formats all tags in the message in a key value style (key=value). Special overrides can be given in the Formats list which is a list of tags with options ([TagOpt]). Any tag in Skipped will not be printed.

Options

Options are applied one after the other in the sequence they appear in. The order can be significant depending on which options are used. For example, adding padding first and then a color would color the padding ([{format, "~s "}, blue]). Coloring first and padding afterwards, would only color the text and not the padding ([blue, {format, "~s "}]) because the padding is applied around the alread blue text.

upper

Transform to upper case.

lower

Transform to lower case.

color

Colorize severity levels (only supported for the severity tag).

{format, Fmt}

Format string according to io:format/3.

Color :: color()

A color supported by the color library.

Handler Configuration

To enable this in Lager for both console and file, try the following handler configuration for Lager:

{handlers, [
    {lager_console_backend, [
        debug,
        {stout, [
            {date, blackb}, " ",
            {time, yellow}, " ",
            {severity, [upper, {format, "~.7s"}, color, {format, "~s "}]},
            {'$metadata', [], [node, application, line]}, " ",
            message, "\n"
        ]}
    ]},
    {lager_file_backend, [
        {level, debug},
        {file, "log/debug.log"},
        {formatter, stout},
        {formatter_config, [
            {date, blackb}, " ",
            {time, yellow}, " ",
            {severity, [upper, {format, "~.7s"}, color, {format, "~s "}]},
            {'$metadata', [], [node, application, line]}, " ",
            message, "\n"
        ]}
    ]}
]}