Life is beautiful when your application log looks nice!
Author tk3369
7 Stars
Updated Last
3 Years Ago
Started In
May 2020

Build Status codecov.io Project Status


This package provides an easy way to build transformer loggers as defined in LoggingExtras.jl.


A standard log record consists of the following components:

  • level: the logging level like Error, Warn, Info, and Debug
  • messasge: a string
  • kwargs: key-value pairs (where key is a Symbol)

In practice, the log output may need to be enhanced with additional information. Further, it is conceivable that the data in these fields may need to be moved around within the record. For examples:

  1. Prepend current timestamp to message or add it to kwargs
  2. Format the log as a single line by moving all kwargs into message
  3. Format the complete log record as a JSON string in message field
  4. etc.

Common Transformer Loggers

A few commonly used transformer loggers are provided as part of this package. They can be accessed as follows.


Migrate all kwargs fields by appending them to the message string.

julia> with_logger(OneLineTransformerLogger(current_logger())) do
           name = "Pluto"
           planet = false
           @info "hello world" name planet
[ Info: hello world name=Pluto planet=false


Add a timestamp at the beginnging or the end of the message string.

julia> with_logger(TimestampTransformerLogger(current_logger(), BeginningMessageLocation();
                                              format = "yyyy-mm-dd HH:MM:SSz")) do
           @info "hello"

[ Info: 2020-07-04 21:00:58-07:00 hello


The JSONTransformerLogger is a little special in that it is expected to be used with the MessageOnlyLogger sink (so that we can e.g. skip adding the log level as a prefix).

The level and message data are automatically included within the JSON string, for which their labels are customizable. See doc string for details.

julia> with_logger(JSONTransformerLogger(MessageOnlyLogger(); indent = 2)) do
           name = "Pluto"
           planet = false
           @info "hello world" name planet
  "message": "hello world",
  "level": "Info",
  "name": "Pluto",
  "planet": false


Color the message string based upon the log level. Try it! :-)

julia> preferred_colors = Dict(
           Logging.Debug => ColorSpec(:grey, false),
           Logging.Info  => ColorSpec(:cyan, true),
           Logging.Warn  => ColorSpec(:yellow, true),
           Logging.Error => ColorSpec(:red, true),

julia> clogger = ColorMessageTransformerLogger(current_logger(), preferred_colors);

ColorMessageTransformerLogger usage


Fix the width of the message string by padding spaces at the end. If the message string is longer than the specified width, then it is truncated.

julia> logger = compose(
           logger -> FixedMessageWidthTransformerLogger(logger, 40),

julia> with_logger(logger) do
           for i in 1:4
               x = round(rand(), digits = 10)
               y = round(rand(), digits = 10)
               @info "Iteration #$i" x y
[ Info: Iteration #1                             x=0.9901128795 y=0.7439288248
[ Info: Iteration #2                             x=0.7326098234 y=0.9881316083
[ Info: Iteration #3                             x=0.1050601631 y=0.2100138712
[ Info: Iteration #4                             x=0.2812764292 y=0.0242226631

Creating your own transformer logger...

This is the whole reason why this package exists! :-)

There are 4 main operations in a transformer:

  1. Inject - add data to the log record at either message or kwargs location.
  2. Migrate - move data between level, message, or kwargs.
  3. Mutate - mutate data in level, message, or kwargs
  4. Remove - empty out message or remove all elements in kwargs.


# Inject a timestamp to the beginning of the message
logger = TransformerLogger(current_logger()) do log
             inject(log, BeginningMessageLocation(), () -> now())

# Inject a timestamp to the kwargs location
logger = TransformerLogger(current_logger()) do log
             inject(log, KwargsLocation(), () -> (:timestamp => now(),))

# Migrate all kwargs to the message string
logger = TransformerLogger(current_logger()) do log
             migrate(log, KwargsProperty(), MessageProperty())

# Multiple operations (using Pipe.jl)
logger = TransformerLogger(current_logger()) do log
             @pipe log |>
                   migrate(_log_, KwargsProperty(), MessageProperty())
                   inject(_, BeginningMessageLocation(), () -> now())


This package was originally conceived as part of this coding live stream. Special credit to Chris de Graff for joining the live stream and helping out.

It has been redesigned significantly since v0.2.0.

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