Appenders

In log4j etymology, Appenders are destinations where messages are written. Depending on the nature of the destination, the format of the messages may be controlled using a Layout.

The most basic appenders log messages to the console or to a file; these are described below.

For implementing your own appenders, see Details.

console_appender(layout = default_log_layout())

file_appender(file, append = TRUE, layout = default_log_layout())

Arguments

layout

A layout function taking a level parameter and additional arguments corresponding to the message. See layouts().

file

The file to write messages to.

append

When TRUE, the file is not truncated when opening for the first time.

Details

Appenders are implemented as functions with the interface function(level, ...). These functions are expected to write their arguments to a destination and return invisible(NULL).

See also

tcp_appender(), http_appender(), syslog_appender()

Examples

# The behaviour of an appender can be seen by using them directly; the
# following snippet will write the message to the console.
appender <- console_appender()
appender("INFO", "Input has length ", 0, ".")
#> INFO  [2025-10-14 20:30:36] Input has length 0.