494 lines
15 KiB
Markdown
494 lines
15 KiB
Markdown
# Zero Allocation JSON Logger
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[](https://godoc.org/github.com/rs/zerolog) [](https://raw.githubusercontent.com/rs/zerolog/master/LICENSE) [](https://travis-ci.org/rs/zerolog) [](http://gocover.io/github.com/rs/zerolog)
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The zerolog package provides a fast and simple logger dedicated to JSON output.
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Zerolog's API is designed to provide both a great developer experience and stunning [performance](#benchmarks). Its unique chaining API allows zerolog to write JSON log events by avoiding allocations and reflection.
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Uber's [zap](https://godoc.org/go.uber.org/zap) library pioneered this approach. Zerolog is taking this concept to the next level with a simpler to use API and even better performance.
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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`.
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## Features
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* Blazing fast
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* Low to zero allocation
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* Level logging
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* Sampling
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* Hooks
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* Contextual fields
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* `context.Context` integration
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* `net/http` helpers
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* Pretty logging for development
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## Installation
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```go
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go get -u github.com/rs/zerolog/log
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```
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## Getting Started
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### Simple Logging Example
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For simple logging, import the global logger package **github.com/rs/zerolog/log**
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```go
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package main
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import (
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"github.com/rs/zerolog"
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"github.com/rs/zerolog/log"
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)
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func main() {
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// UNIX Time is faster and smaller than most timestamps
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// If you set zerolog.TimeFieldFormat to an empty string,
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// logs will write with UNIX time
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zerolog.TimeFieldFormat = ""
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log.Print("hello world")
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}
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// Output: {"time":1516134303,"level":"debug","message":"hello world"}
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```
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> Note: The default log level for `log.Print` is *debug*
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### Leveled Logging
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#### Simple Leveled Logging Example
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```go
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package main
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import (
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"github.com/rs/zerolog"
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"github.com/rs/zerolog/log"
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)
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func main() {
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zerolog.TimeFieldFormat = ""
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log.Info().Msg("hello world")
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}
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// Output: {"time":1516134303,"level":"info","message":"hello world"}
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```
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**zerolog** allows for logging at the following levels (from highest to lowest):
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- panic (`zerolog.PanicLevel`, 5)
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- fatal (`zerolog.FatalLevel`, 4)
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- error (`zerolog.ErrorLevel`, 3)
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- warn (`zerolog.WarnLevel`, 2)
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- info (`zerolog.InfoLevel`, 1)
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- debug (`zerolog.DebugLevel`, 0)
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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.
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#### Setting Global Log Level
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This example uses command-line flags to demonstrate various outputs depending on the chosen log level.
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```go
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package main
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import (
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"flag"
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"github.com/rs/zerolog"
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"github.com/rs/zerolog/log"
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)
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func main() {
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zerolog.TimeFieldFormat = ""
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debug := flag.Bool("debug", false, "sets log level to debug")
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flag.Parse()
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// Default level for this example is info, unless debug flag is present
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zerolog.SetGlobalLevel(zerolog.InfoLevel)
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if *debug {
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zerolog.SetGlobalLevel(zerolog.DebugLevel)
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}
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log.Debug().Msg("This message appears only when log level set to Debug")
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log.Info().Msg("This message appears when log level set to Debug or Info")
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if e := log.Debug(); e.Enabled() {
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// Compute log output only if enabled.
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value := "bar"
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e.Str("foo", value).Msg("some debug message")
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}
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}
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```
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Info Output (no flag)
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```bash
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$ ./logLevelExample
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{"time":1516387492,"level":"info","message":"This message appears when log level set to Debug or Info"}
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```
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Debug Output (debug flag set)
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```bash
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$ ./logLevelExample -debug
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{"time":1516387573,"level":"debug","message":"This message appears only when log level set to Debug"}
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{"time":1516387573,"level":"info","message":"This message appears when log level set to Debug or Info"}
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{"time":1516387573,"level":"debug","foo":"bar","message":"some debug message"}
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```
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#### Logging Fatal Messages
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```go
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package main
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import (
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"errors"
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"github.com/rs/zerolog"
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"github.com/rs/zerolog/log"
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)
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func main() {
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err := errors.New("A repo man spends his life getting into tense situations")
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service := "myservice"
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zerolog.TimeFieldFormat = ""
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log.Fatal().
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Err(err).
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Str("service", service).
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Msgf("Cannot start %s", service)
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}
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// Output: {"time":1516133263,"level":"fatal","error":"A repo man spends his life getting into tense situations","service":"myservice","message":"Cannot start myservice"}
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// exit status 1
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```
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> NOTE: Using `Msgf` generates one allocation even when the logger is disabled.
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### Contextual Logging
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#### Fields can be added to log messages
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```go
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log.Info().
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Str("foo", "bar").
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Int("n", 123).
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Msg("hello world")
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// Output: {"level":"info","time":1494567715,"foo":"bar","n":123,"message":"hello world"}
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```
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### Create logger instance to manage different outputs
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```go
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logger := zerolog.New(os.Stderr).With().Timestamp().Logger()
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logger.Info().Str("foo", "bar").Msg("hello world")
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// Output: {"level":"info","time":1494567715,"message":"hello world","foo":"bar"}
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```
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### Sub-loggers let you chain loggers with additional context
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```go
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sublogger := log.With().
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Str("component": "foo").
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Logger()
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sublogger.Info().Msg("hello world")
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// Output: {"level":"info","time":1494567715,"message":"hello world","component":"foo"}
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```
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### Pretty logging
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```go
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if isConsole {
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log.Logger = log.Output(zerolog.ConsoleWriter{Out: os.Stderr})
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}
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log.Info().Str("foo", "bar").Msg("Hello world")
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// Output: 1494567715 |INFO| Hello world foo=bar
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```
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### Sub dictionary
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```go
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log.Info().
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Str("foo", "bar").
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Dict("dict", zerolog.Dict().
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Str("bar", "baz").
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Int("n", 1)
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).Msg("hello world")
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// Output: {"level":"info","time":1494567715,"foo":"bar","dict":{"bar":"baz","n":1},"message":"hello world"}
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```
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### Customize automatic field names
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```go
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zerolog.TimestampFieldName = "t"
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zerolog.LevelFieldName = "l"
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zerolog.MessageFieldName = "m"
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log.Info().Msg("hello world")
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// Output: {"l":"info","t":1494567715,"m":"hello world"}
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```
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### Log with no level nor message
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```go
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log.Log().Str("foo","bar").Msg("")
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// Output: {"time":1494567715,"foo":"bar"}
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```
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### Add contextual fields to the global logger
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```go
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log.Logger = log.With().Str("foo", "bar").Logger()
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```
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### Thread-safe, lock-free, non-blocking writer
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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:
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```go
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d := diodes.NewManyToOne(1000, diodes.AlertFunc(func(missed int) {
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fmt.Printf("Dropped %d messages\n", missed)
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}))
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w := diode.NewWriter(os.Stdout, d, 10*time.Millisecond)
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log := zerolog.New(w)
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log.Print("test")
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```
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You will need to install `code.cloudfoundry.org/go-diodes` to use this feature.
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### Log Sampling
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```go
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sampled := log.Sample(&zerolog.BasicSampler{N: 10})
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sampled.Info().Msg("will be logged every 10 messages")
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// Output: {"time":1494567715,"level":"info","message":"will be logged every 10 messages"}
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```
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More advanced sampling:
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```go
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// Will let 5 debug messages per period of 1 second.
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// Over 5 debug message, 1 every 100 debug messages are logged.
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// Other levels are not sampled.
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sampled := log.Sample(zerolog.LevelSampler{
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DebugSampler: &zerolog.BurstSampler{
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Burst: 5,
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Period: 1*time.Second,
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NextSampler: &zerolog.BasicSampler{N: 100},
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},
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})
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sampled.Debug().Msg("hello world")
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// Output: {"time":1494567715,"level":"debug","message":"hello world"}
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```
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### Hooks
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```go
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type SeverityHook struct{}
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func (h SeverityHook) Run(e *zerolog.Event, level zerolog.Level, msg string) {
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if level != zerolog.NoLevel {
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e.Str("severity", level.String())
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}
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}
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hooked := log.Hook(SeverityHook{})
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hooked.Warn().Msg("")
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// Output: {"level":"warn","severity":"warn"}
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```
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### Pass a sub-logger by context
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```go
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ctx := log.With("component", "module").Logger().WithContext(ctx)
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log.Ctx(ctx).Info().Msg("hello world")
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// Output: {"component":"module","level":"info","message":"hello world"}
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```
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### Set as standard logger output
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```go
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log := zerolog.New(os.Stdout).With().
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Str("foo", "bar").
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Logger()
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stdlog.SetFlags(0)
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stdlog.SetOutput(log)
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stdlog.Print("hello world")
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// Output: {"foo":"bar","message":"hello world"}
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```
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### Integration with `net/http`
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The `github.com/rs/zerolog/hlog` package provides some helpers to integrate zerolog with `http.Handler`.
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In this example we use [alice](https://github.com/justinas/alice) to install logger for better readability.
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```go
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log := zerolog.New(os.Stdout).With().
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Timestamp().
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Str("role", "my-service").
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Str("host", host).
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Logger()
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c := alice.New()
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// Install the logger handler with default output on the console
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c = c.Append(hlog.NewHandler(log))
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// Install some provided extra handler to set some request's context fields.
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// Thanks to those handler, all our logs will come with some pre-populated fields.
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c = c.Append(hlog.AccessHandler(func(r *http.Request, status, size int, duration time.Duration) {
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hlog.FromRequest(r).Info().
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Str("method", r.Method).
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Str("url", r.URL.String()).
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Int("status", status).
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Int("size", size).
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Dur("duration", duration).
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Msg("")
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}))
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c = c.Append(hlog.RemoteAddrHandler("ip"))
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c = c.Append(hlog.UserAgentHandler("user_agent"))
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c = c.Append(hlog.RefererHandler("referer"))
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c = c.Append(hlog.RequestIDHandler("req_id", "Request-Id"))
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// Here is your final handler
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h := c.Then(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
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// Get the logger from the request's context. You can safely assume it
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// will be always there: if the handler is removed, hlog.FromRequest
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// will return a no-op logger.
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hlog.FromRequest(r).Info().
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Str("user", "current user").
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Str("status", "ok").
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Msg("Something happened")
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// 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"}
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}))
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http.Handle("/", h)
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if err := http.ListenAndServe(":8080", nil); err != nil {
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log.Fatal().Err(err).Msg("Startup failed")
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}
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```
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## Global Settings
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Some settings can be changed and will by applied to all loggers:
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* `log.Logger`: You can set this value to customize the global logger (the one used by package level methods).
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* `zerolog.SetGlobalLevel`: Can raise the minimum level of all loggers. Set this to `zerolog.Disable` to disable logging altogether (quiet mode).
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* `zerolog.DisableSampling`: If argument is `true`, all sampled loggers will stop sampling and issue 100% of their log events.
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* `zerolog.TimestampFieldName`: Can be set to customize `Timestamp` field name.
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* `zerolog.LevelFieldName`: Can be set to customize level field name.
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* `zerolog.MessageFieldName`: Can be set to customize message field name.
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* `zerolog.ErrorFieldName`: Can be set to customize `Err` field name.
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* `zerolog.TimeFieldFormat`: Can be set to customize `Time` field value formatting. If set with an empty string, times are formated as UNIX timestamp.
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// DurationFieldUnit defines the unit for time.Duration type fields added
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// using the Dur method.
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* `DurationFieldUnit`: Sets the unit of the fields added by `Dur` (default: `time.Millisecond`).
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* `DurationFieldInteger`: If set to true, `Dur` fields are formatted as integers instead of floats.
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## Field Types
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### Standard Types
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* `Str`
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* `Bool`
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* `Int`, `Int8`, `Int16`, `Int32`, `Int64`
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* `Uint`, `Uint8`, `Uint16`, `Uint32`, `Uint64`
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* `Float32`, `Float64`
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### Advanced Fields
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* `Err`: Takes an `error` and render it as a string using the `zerolog.ErrorFieldName` field name.
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* `Timestamp`: Insert a timestamp field with `zerolog.TimestampFieldName` field name and formatted using `zerolog.TimeFieldFormat`.
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* `Time`: Adds a field with the time formated with the `zerolog.TimeFieldFormat`.
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* `Dur`: Adds a field with a `time.Duration`.
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* `Dict`: Adds a sub-key/value as a field of the event.
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* `Interface`: Uses reflection to marshal the type.
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## Benchmarks
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All operations are allocation free (those numbers *include* JSON encoding):
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```
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BenchmarkLogEmpty-8 100000000 19.1 ns/op 0 B/op 0 allocs/op
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BenchmarkDisabled-8 500000000 4.07 ns/op 0 B/op 0 allocs/op
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BenchmarkInfo-8 30000000 42.5 ns/op 0 B/op 0 allocs/op
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BenchmarkContextFields-8 30000000 44.9 ns/op 0 B/op 0 allocs/op
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BenchmarkLogFields-8 10000000 184 ns/op 0 B/op 0 allocs/op
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```
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There are a few Go logging benchmarks and comparisons that include zerolog.
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- [imkira/go-loggers-bench](https://github.com/imkira/go-loggers-bench)
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- [uber-common/zap](https://github.com/uber-go/zap#performance)
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Using Uber's zap comparison benchmark:
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Log a message and 10 fields:
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| Library | Time | Bytes Allocated | Objects Allocated |
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| :--- | :---: | :---: | :---: |
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| zerolog | 767 ns/op | 552 B/op | 6 allocs/op |
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| :zap: zap | 848 ns/op | 704 B/op | 2 allocs/op |
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| :zap: zap (sugared) | 1363 ns/op | 1610 B/op | 20 allocs/op |
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| go-kit | 3614 ns/op | 2895 B/op | 66 allocs/op |
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| lion | 5392 ns/op | 5807 B/op | 63 allocs/op |
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| logrus | 5661 ns/op | 6092 B/op | 78 allocs/op |
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| apex/log | 15332 ns/op | 3832 B/op | 65 allocs/op |
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| log15 | 20657 ns/op | 5632 B/op | 93 allocs/op |
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Log a message with a logger that already has 10 fields of context:
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| Library | Time | Bytes Allocated | Objects Allocated |
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| :--- | :---: | :---: | :---: |
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| zerolog | 52 ns/op | 0 B/op | 0 allocs/op |
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| :zap: zap | 283 ns/op | 0 B/op | 0 allocs/op |
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| :zap: zap (sugared) | 337 ns/op | 80 B/op | 2 allocs/op |
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| lion | 2702 ns/op | 4074 B/op | 38 allocs/op |
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| go-kit | 3378 ns/op | 3046 B/op | 52 allocs/op |
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| logrus | 4309 ns/op | 4564 B/op | 63 allocs/op |
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| apex/log | 13456 ns/op | 2898 B/op | 51 allocs/op |
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| log15 | 14179 ns/op | 2642 B/op | 44 allocs/op |
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Log a static string, without any context or `printf`-style templating:
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| Library | Time | Bytes Allocated | Objects Allocated |
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| :--- | :---: | :---: | :---: |
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| zerolog | 50 ns/op | 0 B/op | 0 allocs/op |
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| :zap: zap | 236 ns/op | 0 B/op | 0 allocs/op |
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| standard library | 453 ns/op | 80 B/op | 2 allocs/op |
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| :zap: zap (sugared) | 337 ns/op | 80 B/op | 2 allocs/op |
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| go-kit | 508 ns/op | 656 B/op | 13 allocs/op |
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| lion | 771 ns/op | 1224 B/op | 10 allocs/op |
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| logrus | 1244 ns/op | 1505 B/op | 27 allocs/op |
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| apex/log | 2751 ns/op | 584 B/op | 11 allocs/op |
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| log15 | 5181 ns/op | 1592 B/op | 26 allocs/op |
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## Caveats
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There is no fields deduplication out-of-the-box.
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Using the same key multiple times creates new key in final JSON each time.
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```go
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logger := zerolog.New(os.Stderr).With().Timestamp().Logger()
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logger.Info().
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Timestamp().
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Msg("dup")
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// Output: {"level":"info","time":1494567715,"time":1494567715,"message":"dup"}
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```
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However, it’s not a big deal though as JSON accepts dup keys, the last one prevails.
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