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Zero Allocation JSON Logger

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Zero Allocation JSON Logger

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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.

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

To keep the code base and the API simple, zerolog focuses on JSON logging only. As suggested on reddit, you may use tools like humanlog to pretty print JSON on the console during development.


  • Level logging
  • Sampling
  • Contextual fields
  • context.Context integration
  • net/http helpers


import ""

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

    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 one allocation even when the logger is disabled.

Fields can be added to log messages

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

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

Level logging


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

    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


// 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().WithContext(ctx)

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

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

Integration with net/http

The 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().
    Str("role", "my-service").
    Str("host", host).

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.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.
        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.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. // 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.


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

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