package log provides a minimal interface for structured logging in services. It may be wrapped to encode conventions, enforce type-safety, provide leveled logging, and so on. It can be used for both typical application log events, and log-structured data streams.

Structured logging

Structured logging is, basically, conceding to the reality that logs are data, and warrant some level of schematic rigor. Using a stricter, key/value-oriented message format for our logs, containing contextual and semantic information, makes it much easier to get insight into the operational activity of the systems we build. Consequently, package log is of the strong belief that "the benefits of structured logging outweigh the minimal effort involved".

Migrating from unstructured to structured logging is probably a lot easier than you'd expect.

// Unstructured
log.Printf("HTTP server listening on %s", addr)

// Structured
logger.Log("transport", "HTTP", "addr", addr, "msg", "listening")

Usage

Typical application logging

w := log.NewSyncWriter(os.Stderr)
logger := log.NewLogfmtLogger(w)
logger.Log("question", "what is the meaning of life?", "answer", 42)

// Output:
// question="what is the meaning of life?" answer=42

Contextual Loggers

func main() {
	var logger log.Logger
	logger = log.NewLogfmtLogger(log.NewSyncWriter(os.Stderr))
	logger = log.With(logger, "instance_id", 123)

	logger.Log("msg", "starting")
	NewWorker(log.With(logger, "component", "worker")).Run()
	NewSlacker(log.With(logger, "component", "slacker")).Run()
}

// Output:
// instance_id=123 msg=starting
// instance_id=123 component=worker msg=running
// instance_id=123 component=slacker msg=running

Interact with stdlib logger

Redirect stdlib logger to Go kit logger.

import (
	"os"
	stdlog "log"
	kitlog "github.com/go-kit/kit/log"
)

func main() {
	logger := kitlog.NewJSONLogger(kitlog.NewSyncWriter(os.Stdout))
	stdlog.SetOutput(kitlog.NewStdlibAdapter(logger))
	stdlog.Print("I sure like pie")
}

// Output:
// {"msg":"I sure like pie","ts":"2016/01/01 12:34:56"}

Or, if, for legacy reasons, you need to pipe all of your logging through the stdlib log package, you can redirect Go kit logger to the stdlib logger.

logger := kitlog.NewLogfmtLogger(kitlog.StdlibWriter{})
logger.Log("legacy", true, "msg", "at least it's something")

// Output:
// 2016/01/01 12:34:56 legacy=true msg="at least it's something"

Timestamps and callers

var logger log.Logger
logger = log.NewLogfmtLogger(log.NewSyncWriter(os.Stderr))
logger = log.With(logger, "ts", log.DefaultTimestampUTC, "caller", log.DefaultCaller)

logger.Log("msg", "hello")

// Output:
// ts=2016-01-01T12:34:56Z caller=main.go:15 msg=hello

Supported output formats

Logfmt (see also)
JSON

Enhancements

package log is centered on the one-method Logger interface.

type Logger interface {
	Log(keyvals ...interface{}) error
}

This interface, and its supporting code like is the product of much iteration and evaluation. For more details on the evolution of the Logger interface, see The Hunt for a Logger Interface, a talk by Chris Hines. Also, please see #63, #76, #131, #157, #164, and #252 to review historical conversations about package log and the Logger interface.

Value-add packages and suggestions, like improvements to the leveled logger, are of course welcome. Good proposals should

Be composable with contextual loggers,
Not break the behavior of log.Caller in any wrapped contextual loggers, and
Be friendly to packages that accept only an unadorned log.Logger.

Benchmarks & comparisons

There are a few Go logging benchmarks and comparisons that include Go kit's package log.

imkira/go-loggers-bench includes kit/log
uber-common/zap, a zero-alloc logging library, includes a comparison with kit/log