log

There is more to it, than picking the next logger and call it a day, to get logging right. First, you have to be clear about your intention. What do you really need, logging, monitoring or tracing?

about logging

Logging is about tracking events and their related data. Think about administrators as your customer. In the old days, log files where managed entirely by the application itself, e.g. by appending to existing files and rotating when required. Today, most operating systems provide advanced logging facilities attached to stdout. On Linux, this is likely the systemd journal. Thus, there is usually no need anymore to handle this in the applications' layer.

However, the most important finding about logging is, that this is NOT the correct place to (solely) inform about alerts, warnings or panic situations: consider, that nobody will read your logs. A common way to collect, inspect and filter log data is using elastic search and kibana.

about tracing

Tracing is about following the programs' flow, typically from a user perspective. This may be accomplished by heavy and very verbose logging instruments, but indeed tools like Jaeger are more suited for this.

about monitoring

Monitoring is about instrumenting, collecting, aggregating and analyzing metrics to understand the behavior of your application and system, especially under load. Well known tools are Prometheus and Grafana.

how to log right?

If you are still here and are sure, that you need to log information, keep the following rules of thumb in mind:

avoid any vendor lock-in into existing logging frameworks by linking directly to them.
realize, that log levels do not make sense. Real errors will cause your application to halt just like a panic or an os.Exit. Anything else is an information, and the evaluation is in the eye of the beholder. It is worth to read the article from Dave Cheney.
logging hurts performance, and in high performance code, the argument propagation will still cause allocations and even worse - escaping to the heap. To enable verbose logs, guard all according calls with compile time flags, so that the actual calls can get eliminated entirely.
use a standard scheme for your fields, like the ECS field reference.

This is our recommended logging interface:

// Field is an interface to an explicit key/value tuple, to be clear about structured information.
type Field interface {
    // Key returns the unique key of this structured Field.
    Key() string
    // Value returns an arbitrary message, which itself may be structured.
    Value() interface{}
}

type Logger interface {
    // Println processes and prints the arguments as fields. The interpretation and formatting depends on the
    // concrete implementation and may range from fmt.Println over log.Println to a full structured logger.
    // Implementations are encouraged to type-switch on each field.
    Println(fields ...interface{})
}

This is our recommended guard:

// +build !debug

package log

// Debug is a build tag determined at build time, so that the compiler can remove dead code.
const Debug = false

Available implementations

logrus (★ 15k)
zap (★ 10k)
zerolog (★ 3k)
apex (★ 1k)

usage

If you want zero dependencies, just copy the recommended logging interface and provide injection capabilities through factory methods. Alternatively, this package provides a standard factory and three simple but ready-to use logger implementations. To get the best of both worlds, we recommend to just start with the dependency, and optimize later by setting a factory to any of the implementations above, when required. Note, that the default logger is simple.PrintColored, if started from within your IDE and otherwise simple.PrintStructured. However, you can change it using SetDefault to whatever you like.

There are also the following default special treatments: *

package main

import (
  "github.com/golangee/log"
  "github.com/golangee/log/ecs"
  "context"
)

func main(){
    // prints from IDE: 2020-12-14T10:46:37+01:00 my.logger hello world
    // prints in prod: {"@timestamp":"2020-12-14T10:46:37+01:00","log.logger":"my.logger","message":"hello world"}
    log.Println("hello world")

    // prints from IDE: 2020-12-14T10:46:37+01:00 my.logger INFO auto message https://automatic.url automatic error *errors.errorString
    // prints in prod: {"@timestamp":"2020-12-14T10:46:37+01:00","error.message":"automatic error","error.type":"*errors.errorString","log.level":"info","log.logger":"my.logger","message":"auto message","url.path":"https://automatic.url"}
    log.Println("info", "auto message", "https://automatic.url", fmt.Errorf("automatic error"))
	
    // how to create custom logger setup
    myLogger := log.NewLogger(ecs.Log("my.logger"))

    // prints from IDE: 2020-11-20T15:26:07+01:00 my.logger hello
    // prints in prod: {"@timestamp":"2020-11-20T15:26:07+01:00","log.level":"trace","log.logger":"my.logger","message":"hello"}
    myLogger.Println(ecs.Msg("hello")) 
    myLogger.Println(ecs.Msg("world"))

    // guard verbose and/or expensive logs
    if log.Debug{
    	myLogger.Println(ecs.Msg("info point 1"), ecs.ErrStack()) 
    }

    // in your http middleware you should use the context
    reqCtx := log.WithLogger(context.Background(), log.WithFields(myLogger, ecs.Log("my.request.logger")))
    log.FromContext(reqCtx).Println(ecs.Msg("from a request"))
}

golangee / log

log