The Anatomy of a Log Request

In logger: A Lightweight, Modern and Flexible Logging Utility

knitr::opts_chunk$set(
    collapse = TRUE,
    comment = "#>"
)
library(logger)
## backup settings
oldconf <- list(
    threshold = log_threshold(),
    layout = log_layout(),
    formatter = log_formatter(),
    appender = log_appender())
## knitr not picking up stderr
log_appender(appender_stdout)

knitr::include_graphics("logger_structure.svg")

To make a successful log record, logger requires the below components:

• a log request, eg

  r log_error('Oops')

  • including the log level (importance) of the record, which will be later used to decide if the log record is to be delivered or not: ERROR in this case
  • R objects to be logged: a simple string in this case, although it could be a character vector or any R object(s) that can be converted into a character vector by the formatter function

• the environment and meta-information of the log request, eg actual timestamp, hostname of the computer, the name of the user running the R script, the pid of the R process, calling function and the actual call etc.

  r f <- function() get_logger_meta_variables(log_level = INFO) f()

• a logger definition to process the log request, including

  • log level threshold, eg INFO, which defines the minimum log level required for actual logging -- all log requests with lower log level will be thrown away

    r log_threshold() ERROR <= INFO log_error('Oops')

  • formatter function, which takes R objects and converts those into actual log message(s) to be then passed to the layout function for the log record rendering -- such as paste, sprintf, glue or eg the below custom example:

    r formatter <- function(...) paste(..., collapse = ' ', sep = ' ') formatter(1:3, c('foo', 'bar'))

  • layout function, which takes log message(s) and further information on the log request (such as timestamp, hostname, username, calling function etc) to render the actual log records eg human-readable text, JSON etc

    ```r library(jsonlite) layout <- function(level, msg) toJSON(level = level, timestamp = time, hostname = node, message = msg) layout(INFO, 'Happy Thursday!')

    > {'level': 'INFO', 'timestamp': '1970-01-01 00:00:00', 'hostname': 'foobar', 'message': 'Happy Thursday!'}

    ```

  • appender function, which takes fully-rendered log record(s) and delivers to somewhere, eg stdout, a file or a streaming service, eg

    r appender <- function(line) cat(line, '\n') appender('INFO [now] I am a log message')

Putting all these together (by explicitly setting the default config in the global namespace):

log_threshold(INFO)
log_formatter(formatter_glue)
log_layout(layout_simple)
log_appender(appender_console)
log_debug('I am a low level log message that will not be printed with a high log level threshold')
log_warn('I am a higher level log message that is very likely to be printed')

Note, that all logger definitions and requests are tied to a logging namespace, and one log request might trigger multiple logger definitions as well (stacking). Find more information on these in the Customizing the format and destination of log records vignette.

## restore settings
log_threshold(oldconf$threshold)
log_layout(oldconf$layout)
log_formatter(oldconf$formatter)
log_appender(oldconf$appender)

logger documentation built on Oct. 19, 2021, 9:07 a.m.