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Loki Handler for slog

This handler allows logging with slog to Loki. By default, logs are sent asynchronously in batches and are formatted using slog.TextHandler (logfmt).

Installation

go get github.com/magnetde/slog-loki

Example Usage in Go

package main

import (
	"log/slog"
	loki "github.com/magnetde/slog-loki"
)

func main() {
	// NewHandler(url, ...options)
	handler := loki.NewHandler("http://localhost:3200", loki.WithName("my-go-binary"))
	defer handler.Close()

	slog.SetDefault(slog.New(handler))

	// ...
}

The package slog-multi is recommended for simultaneous logging to the console and Loki.

Example Usage in Grafana

{name="my-go-binary"} | logfmt | level =~ "WARN|ERROR"

This query displays all log entries of the application with a severity of at least "warning". The format depends on the handler set with WithHandler.

Options

• WithName(string):
  Adds the additional label "name" to all log entries sent to Loki. If the default handler is used, it is also added as an attribute to the formatted log message.
• WithLabel(string, any):
  Similar to WithName, this adds an additional label to all log entries sent to Loki.
• WithLabelsEnabled(...Label):
  Sends additional labels from log entry attributes. By default, only the name attribute is sent. Available labels include:
  • Labels added with WithName or WithLabel are always added to log entries.
  • LabelTime: Adds the time as a label.
  • LabelLevel: Adds the log level as a label.
  • LabelMessage: Adds the message as a label.
  • LabelSource: Adds the source with format "[file]:[line]:[function]".
  • LabelAttrs: Adds all extra attributes as labels.
  • LabelAll: Adds all fields and attributes as labels.
• WithHandler(func(w io.Writer) slog.Handler):
  Allows setting a handler to change the format of log entries. The handler should write the log entries to the writer w. For example, to create log entries in JSON format:

  WithHandler(func(w io.Writer) slog.Handler {
      return slog.NewJSONHandler(w, &slog.HandlerOptions{
          // ...
      })
  })

• WithSynchronous(bool):
  By default (async mode), log entries are processed in a separate Go routine. If synchronous sending is used, batch interval and size are ignored.
• WithBatchInterval(time.Duration):
  Sets the batch interval. Every batch interval, all collected log entries will be dispatched (default: 15 seconds).
• WithBatchSize(int):
  Maximum batch size. If the number of collected log entries exceeds this size, all collected entries will be sent to Loki (default: 1024).
• WithErrorHandler(bool):
  Sets the error handler for send errors in asynchronous mode. This is ignored in synchronous mode.