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 (or CBOR) 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 efficient structured 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
• Leveled logging
• Sampling
• Hooks
• Contextual fields
• context.Context integration
• Integration with net/http
• JSON and CBOR encoding formats
• Pretty logging for development
• Error Logging (with optional Stacktrace)
• log/slog integration

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
    zerolog.TimeFieldFormat = zerolog.TimeFormatUnix

    log.Print("hello world")
}

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

Note: By default log writes to os.Stderr Note: The default log level for log.Print is trace

Contextual Logging

zerolog allows data to be added to log messages in the form of key:value pairs. The data added to the message adds "context" about the log event that can be critical for debugging as well as myriad other purposes. An example of this is below:

package main

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

func main() {
    zerolog.TimeFieldFormat = zerolog.TimeFormatUnix

    log.Debug().
        Str("Scale", "833 cents").
        Float64("Interval", 833.09).
        Msg("Fibonacci is everywhere")

    log.Debug().
        Str("Name", "Tom").
        Send()
}

// Output: {"level":"debug","Scale":"833 cents","Interval":833.09,"time":1562212768,"message":"Fibonacci is everywhere"}
// Output: {"level":"debug","Name":"Tom","time":1562212768}

You'll note in the above example that when adding contextual fields, the fields are strongly typed. You can find the full list of supported fields here

Leveled Logging

Simple Leveled Logging Example

package main

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

func main() {
    zerolog.TimeFieldFormat = zerolog.TimeFormatUnix

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

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

It is very important to note that when using the zerolog chaining API, as shown above (log.Info().Msg("hello world"), the chain must have either the Msg or Msgf method call. If you forget to add either of these, the log will not occur and there is no compile time error to alert you of this.

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)
• trace (zerolog.TraceLevel, -1)

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 = zerolog.TimeFormatUnix
    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 without Level or Message

You may choose to log without a specific level by using the Log method. You may also write without a message by setting an empty string in the msg string parameter of the Msg method. Both are demonstrated in the example below.

package main

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

func main() {
    zerolog.TimeFieldFormat = zerolog.TimeFormatUnix

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

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

Error Logging

You can log errors using the Err method

package main

import (
    "errors"

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

func main() {
    zerolog.TimeFieldFormat = zerolog.TimeFormatUnix

    err := errors.New("seems we have an error here")
    log.Error().Err(err).Msg("")
}

// Output: {"level":"error","error":"seems we have an error here","time":1609085256}

The default field name for errors is error, you can change this by setting zerolog.ErrorFieldName to meet your needs.

Error Logging with Stacktrace

Using github.com/pkg/errors, you can add a formatted stacktrace to your errors.

package main

import (
    "github.com/pkg/errors"
    "github.com/rs/zerolog/pkgerrors"

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

func main() {
    zerolog.TimeFieldFormat = zerolog.TimeFormatUnix
    zerolog.ErrorStackMarshaler = pkgerrors.MarshalStack

    err := outer()
    log.Error().Stack().Err(err).Msg("")
}

func inner() error {
    return errors.New("seems we have an error here")
}

func middle() error {
    err := inner()
    if err != nil {
        return err
    }
    return nil
}

func outer() error {
    err := middle()
    if err != nil {
        return err
    }
    return nil
}

// Output: {"level":"error","stack":[{"func":"inner","line":"20","source":"errors.go"},{"func":"middle","line":"24","source":"errors.go"},{"func":"outer","line":"32","source":"errors.go"},{"func":"main","line":"15","source":"errors.go"},{"func":"main","line":"204","source":"proc.go"},{"func":"goexit","line":"1374","source":"asm_amd64.s"}],"error":"seems we have an error here","time":1609086683}

zerolog.ErrorStackMarshaler must be set in order for the stack to output anything.

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 = zerolog.TimeFormatUnix

    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.

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

To log a human-friendly, colorized output, use zerolog.ConsoleWriter:

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

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

// Output: 3:04PM INF Hello World foo=bar

To customize the configuration and formatting:

output := zerolog.ConsoleWriter{Out: os.Stdout, TimeFormat: time.RFC3339}
output.FormatLevel = func(i interface{}) string {
    return strings.ToUpper(fmt.Sprintf("| %-6s|", i))
}
output.FormatMessage = func(i interface{}) string {
    return fmt.Sprintf("***%s****", i)
}
output.FormatFieldName = func(i interface{}) string {
    return fmt.Sprintf("%s:", i)
}
output.FormatFieldValue = func(i interface{}) string {
    return strings.ToUpper(fmt.Sprintf("%s", i))
}

log := zerolog.New(output).With().Timestamp().Logger()

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

// Output: 2006-01-02T15:04:05Z07:00 | INFO  | ***Hello World**** foo:BAR

To use custom advanced formatting:

output := zerolog.ConsoleWriter{Out: os.Stdout, NoColor: true,
    PartsOrder:    []string{"level", "one", "two", "three", "message"},
    FieldsExclude: []string{"one", "two", "three"}}
output.FormatLevel = func(i interface{}) string { return strings.ToUpper(fmt.Sprintf("%-6s", i)) }
output.FormatFieldName = func(i interface{}) string { return fmt.Sprintf("%s:", i) }
output.FormatPartValueByName = func(i interface{}, s string) string {
    var ret string
    switch s {
    case "one":
        ret = strings.ToUpper(fmt.Sprintf("%s", i))
    case "two":
        ret = strings.ToLower(fmt.Sprintf("%s", i))
    case "three":
        ret = strings.ToLower(fmt.Sprintf("(%s)", i))
    }
    return ret
}
log := zerolog.New(output)

log.Info().Str("foo", "bar").
    Str("two", "TEST_TWO").
    Str("one", "test_one").
    Str("three", "test_three").
    Msg("Hello World")

// Output: INFO   TEST_ONE test_two (test_three) 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"}

Add contextual fields to the global logger

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

Add file and line number to log

Equivalent of Llongfile:

log.Logger = log.With().Caller().Logger()
log.Info().Msg("hello world")

// Output: {"level": "info", "message": "hello world", "caller": "/go/src/your_project/some_file:21"}

Equivalent of Lshortfile:

zerolog.CallerMarshalFunc = func(pc uintptr, file string, line int) string {
    return filepath.Base(file) + ":" + strconv.Itoa(line)
}
log.Logger = log.With().Caller().Logger()
log.Info().Msg("hello world")

// Output: {"level": "info", "message": "hello world", "caller": "some_file:21"}

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 follows:

wr := diode.NewWriter(os.Stdout, 1000, 10*time.Millisecond, func(missed int) {
        fmt.Printf("Logger Dropped %d messages", missed)
    })
log := zerolog.New(wr)
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:

```go // Will let 5 debug messages per period of 1 second. // Over 5 debug message, 1 every 100 debug messages are l