Zero Allocation JSON Logger

The zerolog package provides a fast and simple logger dedicated to JSON output. It is inspired by uber's zap but with a simpler API and a smaller code base.

Features

• Level logging
• Sampling
• Contextual fields

Performance

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

BenchmarkLogEmpty-8            50000000      19.8 ns/op     0 B/op      0 allocs/op
BenchmarkDisabled-8           100000000       4.73 ns/op    0 B/op      0 allocs/op
BenchmarkInfo-8                10000000      85.1 ns/op     0 B/op      0 allocs/op
BenchmarkContextFields-8       10000000      81.9 ns/op     0 B/op      0 allocs/op
BenchmarkLogFields-8            5000000     247 ns/op       0 B/op      0 allocs/op

Using Uber's zap comparison benchmark:

Log a message and 10 fields:

Library	Time	Bytes Allocated	Objects Allocated
zerolog	787 ns/op	80 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	80 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	76.2 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

Usage

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

A global logger can be use for simple logging

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

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

NOTE: To import the global logger, import the log subpackage github.com/rs/zerolog/log.

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

// Output: {"level":"fatal","time":1494567715,"message":"Cannot start myservice","error":"some error","service":"myservice"}
// Exit 1

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

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"}

Level logging

zerolog.SetGlobalLevel(zerolog.InfoLevel)

log.Debug().Msg("filtered out message")
log.Info().Msg("routed message")

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

// Output: {"level":"info","time":1494567715,"message":"routed message"}

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()

Log Sampling

sampled := log.Sample(10)
sampled.Info().Msg("will be logged every 10 messages")

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

Pass a sub-logger by context

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

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

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

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 mimimum 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.SampleFieldName: Can be set to customize the field name added when sampling is enabled.
• zerolog.TimeFieldFormat: Can be set to customize Time field value formatting. If set with an empty string, times are formated as UNIX timestamp.

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 formatted as a float. For int value, use DurInt.
• Dict: Adds a sub-key/value as a field of the event.
• Interface: Uses reflection to marshal the type.