How can I create a custom log format for JSON files like say one for logstash?

[chintanp]

I say the custom format for logstash, like done by NodeJS logger Winston (https://github.com/winstonjs/logform#logstash).

This would be look something like so:

{"@message":"analysis status updated to processing ","@timestamp":"2019-11-23T21:39:41.853Z","@fields":{"level":"info","IP":"128.95.204.113"}}

This isnt, ofcourse, too critical as logstash can parse the JSON generated by lgr. Wanted to understand how I can totally transform the log format. I am able to add custom fields right now.

[s-fleck]

So to achieve this, you have to create your own Layout. This is pretty straight forward, but you need to know a bit about R6 classes and the inner workings of lgr [that are properly not documented sufficiently]. Basically you just have to inherit from LayoutJson and modify it's format_event() method to transform the log event before writing the JSON output.

LayoutLogstash <- R6::R6Class(
  "LayoutLogstash",
  inherit = lgr::LayoutJson,
  public = list(
    format_event = function(event) {
      dd <- list(
        "@message" = event$msg,
        "@timestamp" = event$timestamp,
        "@fields" = c(
          level = unname(event$level_name), 
          event$values[setdiff(names(event$values), c("timestamp", "msg", "level"))]
        )
      )
      do.call(
        jsonlite::toJSON,
        args = c(list(x = dd), get(".toJSON_args", private))
      )
    }  
  )
)

lg <- lgr::get_logger("test")
lg$add_appender(lgr::AppenderJson$new(layout = LayoutLogstash$new(), file = tempfile()), name = "logstash")
lg$info("test")
lg$appenders$logstash$show()

This is also a nicer place to label the levels with label_levels() than the filter solution from the other issue (because the filter modifies the event for all appenders connected to the logger)

[s-fleck]

edit: I forgott hat you can just use event$level_name to get the character representation of a log level, no need to call lgr::label_levels(event$level) manually