Zero Allocation JSON Logger

The zerolog package provides a fast and simple logger dedicated to JSON output.

Zerolog's API is designed to provide both a great developer experience and stunning performance. Its unique chaining API allows zerolog to write JSON log events by avoiding allocations and reflection.

Uber's zap library pioneered this approach. Zerolog is taking this concept to the next level with a simpler to use API and even better performance.

To keep the code base and the API simple, zerolog focuses on JSON logging only. Pretty logging on the console is made possible using the provided (but inefficient) zerolog.ConsoleWriter.

Who uses zerolog

Find out who uses zerolog and add your company / project to the list.

Features

• Blazing fast
• Low to zero allocation
• Level logging
• Sampling
• Hooks
• Contextual fields
• context.Context integration
• net/http helpers
• Pretty logging for development

Installation

go get -u github.com/rs/zerolog/log

Getting Started

Simple Logging Example

For simple logging, import the global logger package github.com/rs/zerolog/log

package main

import (
	"github.com/rs/zerolog"
	"github.com/rs/zerolog/log"
)

func main() {
	// UNIX Time is faster and smaller than most timestamps
	// If you set zerolog.TimeFieldFormat to an empty string,
	// logs will write with UNIX time
	zerolog.TimeFieldFormat = ""

	log.Print("hello world")
}

// Output: {"time":1516134303,"level":"debug","message":"hello world"}

Note: The default log level for log.Print is debug

Leveled Logging

Simple Leveled Logging Example

package main

import (
	"github.com/rs/zerolog"
	"github.com/rs/zerolog/log"
)

func main() {
	zerolog.TimeFieldFormat = ""

	log.Info().Msg("hello world")
}

// Output: {"time":1516134303,"level":"info","message":"hello world"}

zerolog allows for logging at the following levels (from highest to lowest):

• panic (zerolog.PanicLevel, 5)
• fatal (zerolog.FatalLevel, 4)
• error (zerolog.ErrorLevel, 3)
• warn (zerolog.WarnLevel, 2)
• info (zerolog.InfoLevel, 1)
• debug (zerolog.DebugLevel, 0)

You can set the Global logging level to any of these options using the SetGlobalLevel function in the zerolog package, passing in one of the given constants above, e.g. zerolog.InfoLevel would be the "info" level. Whichever level is chosen, all logs with a level greater than or equal to that level will be written. To turn off logging entirely, pass the zerolog.Disabled constant.

Setting Global Log Level

This example uses command-line flags to demonstrate various outputs depending on the chosen log level.

package main

import (
	"flag"

	"github.com/rs/zerolog"
	"github.com/rs/zerolog/log"
)

func main() {
	zerolog.TimeFieldFormat = ""
	debug := flag.Bool("debug", false, "sets log level to debug")

	flag.Parse()

	// Default level for this example is info, unless debug flag is present
	zerolog.SetGlobalLevel(zerolog.InfoLevel)
	if *debug {
		zerolog.SetGlobalLevel(zerolog.DebugLevel)
	}

	log.Debug().Msg("This message appears only when log level set to Debug")
	log.Info().Msg("This message appears when log level set to Debug or Info")

	if e := log.Debug(); e.Enabled() {
		// Compute log output only if enabled.
		value := "bar"
		e.Str("foo", value).Msg("some debug message")
	}
}

Info Output (no flag)

$ ./logLevelExample
{"time":1516387492,"level":"info","message":"This message appears when log level set to Debug or Info"}

Debug Output (debug flag set)

$ ./logLevelExample -debug
{"time":1516387573,"level":"debug","message":"This message appears only when log level set to Debug"}
{"time":1516387573,"level":"info","message":"This message appears when log level set to Debug or Info"}
{"time":1516387573,"level":"debug","foo":"bar","message":"some debug message"}

Logging Fatal Messages

package main

import (
	"errors"

	"github.com/rs/zerolog"
	"github.com/rs/zerolog/log"
)

func main() {
	err := errors.New("A repo man spends his life getting into tense situations")
	service := "myservice"

	zerolog.TimeFieldFormat = ""

	log.Fatal().
		Err(err).
		Str("service", service).
		Msgf("Cannot start %s", service)
}

// Output: {"time":1516133263,"level":"fatal","error":"A repo man spends his life getting into tense situations","service":"myservice","message":"Cannot start myservice"}
//         exit status 1

NOTE: Using Msgf generates one allocation even when the logger is disabled.

Contextual Logging

Fields can be added to log messages

log.Info().
    Str("foo", "bar").
    Int("n", 123).
    Msg("hello world")

// Output: {"level":"info","time":1494567715,"foo":"bar","n":123,"message":"hello world"}

Create logger instance to manage different outputs

logger := zerolog.New(os.Stderr).With().Timestamp().Logger()

logger.Info().Str("foo", "bar").Msg("hello world")

// Output: {"level":"info","time":1494567715,"message":"hello world","foo":"bar"}

Sub-loggers let you chain loggers with additional context

sublogger := log.With().
                 Str("component": "foo").
                 Logger()
sublogger.Info().Msg("hello world")

// Output: {"level":"info","time":1494567715,"message":"hello world","component":"foo"}

Pretty logging

if isConsole {
    log.Logger = log.Output(zerolog.ConsoleWriter{Out: os.Stderr})
}

log.Info().Str("foo", "bar").Msg("Hello world")

// Output: 1494567715 |INFO| Hello world foo=bar

Sub dictionary

log.Info().
    Str("foo", "bar").
    Dict("dict", zerolog.Dict().
        Str("bar", "baz").
        Int("n", 1)
    ).Msg("hello world")

// Output: {"level":"info","time":1494567715,"foo":"bar","dict":{"bar":"baz","n":1},"message":"hello world"}

Customize automatic field names

zerolog.TimestampFieldName = "t"
zerolog.LevelFieldName = "l"
zerolog.MessageFieldName = "m"

log.Info().Msg("hello world")

// Output: {"l":"info","t":1494567715,"m":"hello world"}

Log with no level nor message

log.Log().Str("foo","bar").Msg("")

// Output: {"time":1494567715,"foo":"bar"}

Add contextual fields to the global logger

log.Logger = log.With().Str("foo", "bar").Logger()

Thread-safe, lock-free, non-blocking writer

If your writer might be slow or not thread-safe and you need your log producers to never get slowed down by a slow writer, you can use a diode.Writer as follow:

d := diodes.NewManyToOne(1000, diodes.AlertFunc(func(missed int) {
    fmt.Printf("Dropped %d messages\n", missed)
}))
w := diode.NewWriter(os.Stdout, d, 10*time.Millisecond)
log := zerolog.New(w)
log.Print("test")

You will need to install code.cloudfoundry.org/go-diodes to use this feature.

Log Sampling

sampled := log.Sample(&zerolog.BasicSampler{N: 10})
sampled.Info().Msg("will be logged every 10 messages")

// Output: {"time":1494567715,"level":"info","message":"will be logged every 10 messages"}

More advanced sampling:

// Will let 5 debug messages per period of 1 second.
// Over 5 debug message, 1 every 100 debug messages are logged.
// Other levels are not sampled.
sampled := log.Sample(zerolog.LevelSampler{
    DebugSampler: &zerolog.BurstSampler{
        Burst: 5,
        Period: 1*time.Second,
        NextSampler: &zerolog.BasicSampler{N: 100},
    },
})
sampled.Debug().Msg("hello world")

// Output: {"time":1494567715,"level":"debug","message":"hello world"}

Hooks

type SeverityHook struct{}

func (h SeverityHook) Run(e *zerolog.Event, level zerolog.Level, msg string) {
    if level != zerolog.NoLevel {
        e.Str("severity", level.String())
    }
}

hooked := log.Hook(SeverityHook{})
hooked.Warn().Msg("")

// Output: {"level":"warn","severity":"warn"}

Pass a sub-logger by context

ctx := log.With("component", "module").Logger().WithContext(ctx)

log.Ctx(ctx).Info().Msg("hello world")

// Output: {"component":"module","level":"info","message":"hello world"}

Set as standard logger output

log := zerolog.New(os.Stdout).With().
    Str("foo", "bar").
    Logger()

stdlog.SetFlags(0)
stdlog.SetOutput(log)

stdlog.Print("hello world")

// Output: {"foo":"bar","message":"hello world"}

Integration with net/http

The github.com/rs/zerolog/hlog package provides some helpers to integrate zerolog with http.Handler.

In this example we use alice to install logger for better readability.

log := zerolog.New(os.Stdout).With().
    Timestamp().
    Str("role", "my-service").
    Str("host", host).
    Logger()

c := alice.New()

// Install the logger handler with default output on the console
c = c.Append(hlog.NewHandler(log))

// Install some provided extra handler to set some request's context fields.
// Thanks to those handler, all our logs will come with some pre-populated fields.
c = c.Append(hlog.AccessHandler(func(r *http.Request, status, size int, duration time.Duration) {
    hlog.FromRequest(r).Info().
        Str("method", r.Method).
        Str("url", r.URL.String()).
        Int("status", status).
        Int("size", size).
        Dur("duration", duration).
        Msg("")
}))
c = c.Append(hlog.RemoteAddrHandler("ip"))
c = c.Append(hlog.UserAgentHandler("user_agent"))
c = c.Append(hlog.RefererHandler("referer"))
c = c.Append(hlog.RequestIDHandler("req_id", "Request-Id"))

// Here is your final handler
h := c.Then(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    // Get the logger from the request's context. You can safely assume it
    // will be always there: if the handler is removed, hlog.FromRequest
    // will return a no-op logger.
    hlog.FromRequest(r).Info().
        Str("user", "current user").
        Str("status", "ok").
        Msg("Something happened")

    // Output: {"level":"info","time":"2001-02-03T04:05:06Z","role":"my-service","host":"local-hostname","req_id":"b4g0l5t6tfid6dtrapu0","user":"current user","status":"ok","message":"Something happened"}
}))
http.Handle("/", h)

if err := http.ListenAndServe(":8080", nil); err != nil {
    log.Fatal().Err(err).Msg("Startup failed")
}

Global Settings

Some settings can be changed and will by applied to all loggers:

• log.Logger: You can set this value to customize the global logger (the one used by package level methods).
• zerolog.SetGlobalLevel: Can raise the minimum level of all loggers. Set this to zerolog.Disable to disable logging altogether (quiet mode).
• zerolog.DisableSampling: If argument is true, all sampled loggers will stop sampling and issue 100% of their log events.
• zerolog.TimestampFieldName: Can be set to customize Timestamp field name.
• zerolog.LevelFieldName: Can be set to customize level field name.
• zerolog.MessageFieldName: Can be set to customize message field name.
• zerolog.ErrorFieldName: Can be set to customize Err field name.
• zerolog.TimeFieldFormat: Can be set to customize Time field value formatting. If set with an empty string, times are formated as UNIX timestamp. // DurationFieldUnit defines the unit for time.Duration type fields added // using the Dur method.
• DurationFieldUnit: Sets the unit of the fields added by Dur (default: time.Millisecond).
• DurationFieldInteger: If set to true, Dur fields are formatted as integers instead of floats.

Field Types

Standard Types

• Str
• Bool
• Int, Int8, Int16, Int32, Int64
• Uint, Uint8, Uint16, Uint32, Uint64
• Float32, Float64

Advanced Fields

• Err: Takes an error and render it as a string using the zerolog.ErrorFieldName field name.
• Timestamp: Insert a timestamp field with zerolog.TimestampFieldName field name and formatted using zerolog.TimeFieldFormat.
• Time: Adds a field with the time formated with the zerolog.TimeFieldFormat.
• Dur: Adds a field with a time.Duration.
• Dict: Adds a sub-key/value as a field of the event.
• Interface: Uses reflection to marshal the type.

Benchmarks

All operations are allocation free (those numbers include JSON encoding):

BenchmarkLogEmpty-8        100000000    19.1 ns/op	   0 B/op       0 allocs/op
BenchmarkDisabled-8        500000000     4.07 ns/op	   0 B/op       0 allocs/op
BenchmarkInfo-8            30000000	    42.5 ns/op	   0 B/op       0 allocs/op
BenchmarkContextFields-8   30000000	    44.9 ns/op	   0 B/op       0 allocs/op
BenchmarkLogFields-8       10000000	   184 ns/op	   0 B/op       0 allocs/op

There are a few Go logging benchmarks and comparisons that include zerolog.

• imkira/go-loggers-bench
• uber-common/zap

Using Uber's zap comparison benchmark:

Log a message and 10 fields:

Library	Time	Bytes Allocated	Objects Allocated
zerolog	767 ns/op	552 B/op	6 allocs/op
⚡ zap	848 ns/op	704 B/op	2 allocs/op
⚡ zap (sugared)	1363 ns/op	1610 B/op	20 allocs/op
go-kit	3614 ns/op	2895 B/op	66 allocs/op
lion	5392 ns/op	5807 B/op	63 allocs/op
logrus	5661 ns/op	6092 B/op	78 allocs/op
apex/log	15332 ns/op	3832 B/op	65 allocs/op
log15	20657 ns/op	5632 B/op	93 allocs/op

Log a message with a logger that already has 10 fields of context:

Library	Time	Bytes Allocated	Objects Allocated
zerolog	52 ns/op	0 B/op	0 allocs/op
⚡ zap	283 ns/op	0 B/op	0 allocs/op
⚡ zap (sugared)	337 ns/op	80 B/op	2 allocs/op
lion	2702 ns/op	4074 B/op	38 allocs/op
go-kit	3378 ns/op	3046 B/op	52 allocs/op
logrus	4309 ns/op	4564 B/op	63 allocs/op
apex/log	13456 ns/op	2898 B/op	51 allocs/op
log15	14179 ns/op	2642 B/op	44 allocs/op

Log a static string, without any context or printf-style templating:

Library	Time	Bytes Allocated	Objects Allocated
zerolog	50 ns/op	0 B/op	0 allocs/op
⚡ zap	236 ns/op	0 B/op	0 allocs/op
standard library	453 ns/op	80 B/op	2 allocs/op
⚡ zap (sugared)	337 ns/op	80 B/op	2 allocs/op
go-kit	508 ns/op	656 B/op	13 allocs/op
lion	771 ns/op	1224 B/op	10 allocs/op
logrus	1244 ns/op	1505 B/op	27 allocs/op
apex/log	2751 ns/op	584 B/op	11 allocs/op
log15	5181 ns/op	1592 B/op	26 allocs/op

Caveats

There is no fields deduplication out-of-the-box. Using the same key multiple times creates new key in final JSON each time.

logger := zerolog.New(os.Stderr).With().Timestamp().Logger()
logger.Info().
       Timestamp().
       Msg("dup")
// Output: {"level":"info","time":1494567715,"time":1494567715,"message":"dup"}

However, it’s not a big deal though as JSON accepts dup keys, the last one prevails.