Fixed-size buffer hook for Logrus  

This logrus hook keeps the logs in fixed-size buffer and flushes the buffer when certain condition is met, e.g. when error level is logged.

Usage

Here is an example of setup where Debug and Info logs are stored in the buffer and as soon as one of Warning, Error, Fatal, Panic levels is logged then the buffer is flushed. Notice that as the buffer is of fixed size, as long as you log only Debug and Info the older logs eventually get overwritten by newer logs.

package main

import (
	"os"

	"github.com/sirupsen/logrus"
	bufferhook "github.com/molecule-man/logrus-buffer-hook.git"
)

func main() {
	logger := logrus.New()
	logger.SetLevel(logrus.DebugLevel)
	// discard main output writer, otherwise logs will be written twice
	logger.SetOutput(ioutil.Discard)

	// the buffer will be of fixed size of 10 kb
	hook := bufferhook.New(os.Stderr, 10000)
	// here we  define that all levels starting from `Warning` (so also `Error`,
	// `Fatal` and `Panic`) should cause buffer (containing `Info` and `Debug`
	// logs) to be flushed
	hook.FlushCondition = bufferhook.FlushOnLevel(logrus.WarnLevel)

	logger.AddHook(hook)
}

Prevent unnecessary serialization from happening

In previous example it was shown that logger's default writer has to be discarded with logger.SetOutput(ioutil.Discard). However then logrus will serialize log entry twice. Once by logger (before sending it to io.Discard) and the second time by the hook itself. To prevent this unnecessary serialization it's recommended to use NullFormatter that does nothing:

// ...
hook.Formatter = logger.Formatter
// ...
logger.Formatter = &bufferhook.NullFormatter{}