zlog/README.md

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# Zero Allocation JSON Logger
[![godoc](http://img.shields.io/badge/godoc-reference-blue.svg?style=flat)](https://godoc.org/github.com/rs/zerolog) [![license](http://img.shields.io/badge/license-MIT-red.svg?style=flat)](https://raw.githubusercontent.com/rs/zerolog/master/LICENSE) [![Build Status](https://travis-ci.org/rs/zerolog.svg?branch=master)](https://travis-ci.org/rs/zerolog) [![Coverage](http://gocover.io/_badge/github.com/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.
Zerolog's API is designed to provide both a great developer experience and stunning [performance](#performance). Its unique chaining API allows zerolog to write JSON log events by avoiding allocations and reflection.
The uber's [zap](https://godoc.org/go.uber.org/zap) library pioneered this approach. Zerolog is taking this concept to the next level with 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. As [suggested on reddit](https://www.reddit.com/r/golang/comments/6c9k7n/zerolog_is_now_faster_than_zap/), you may use tools like [humanlog](https://github.com/aybabtme/humanlog) to pretty print JSON on the console during development.
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## Features
* Level logging
* Sampling
* Contextual fields
* `context.Context` integration
* `net/http` helpers
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## Usage
```go
import "github.com/rs/zerolog/log"
```
### A global logger can be use for simple logging
```go
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`.
```go
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
```
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NOTE: Using `Msgf` generates one allocation even when the logger is disabled.
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### Fields can be added to log messages
```go
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
```go
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
```go
sublogger := log.With().
Str("component": "foo").
Logger()
sublogger.Info().Msg("hello world")
// Output: {"level":"info","time":1494567715,"message":"hello world","component":"foo"}
```
### Level logging
```go
zerolog.SetGlobalLevel(zerolog.InfoLevel)
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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")
}
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// Output: {"level":"info","time":1494567715,"message":"routed message"}
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```
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### Sub dictionary
```go
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"}
```
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### Customize automatic field names
```go
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
```go
log.Log().Str("foo","bar").Msg("")
// Output: {"time":1494567715,"foo":"bar"}
```
### Add contextual fields to the global logger
```go
log.Logger = log.With().Str("foo", "bar").Logger()
```
### Log Sampling
```go
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"}
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```
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### Pass a sub-logger by context
```go
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ctx := log.With("component", "module").Logger().WithContext(ctx)
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log.Ctx(ctx).Info().Msg("hello world")
// Output: {"component":"module","level":"info","message":"hello world"}
```
### Set as standard logger output
```go
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](https://github.com/justinas/alice) to install logger for better readability.
```go
log := zerolog.New(os.Stdout).With().
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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.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")
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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"}
}))
http.Handle("/", h)
if err := http.ListenAndServe(":8080", nil); err != nil {
log.Fatal().Err(err).Msg("Startup failed")
}
```
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## 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.
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* `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.
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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
// 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.
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## 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.
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* `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`.
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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.
* `Interface`: Uses reflection to marshal the type.
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## Performance
All operations are allocation free (those numbers *include* JSON encoding):
```
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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 46.3 ns/op 0 B/op 0 allocs/op
BenchmarkContextFields-8 30000000 47.1 ns/op 0 B/op 0 allocs/op
BenchmarkLogFields-8 10000000 186 ns/op 0 B/op 0 allocs/op
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```
Using Uber's zap [comparison benchmark](https://github.com/uber-go/zap#performance):
Log a message and 10 fields:
| Library | Time | Bytes Allocated | Objects Allocated |
| :--- | :---: | :---: | :---: |
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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 |
| :zap: 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 |
| :--- | :---: | :---: | :---: |
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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 |
| :zap: 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 |
| :--- | :---: | :---: | :---: |
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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 |
| standard library | 453 ns/op | 80 B/op | 2 allocs/op |
| :zap: 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 |