zerolog
Zero-allocation JSON logger
About zerolog
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# Zero Allocation JSON Logger
[](https://godoc.org/github.com/rs/zerolog) [](https://raw.githubusercontent.com/rs/zerolog/master/LICENSE) [](https://github.com/rs/zerolog/actions/workflows/test.yml) [](https://raw.githack.com/wiki/rs/zerolog/coverage.html)
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](#benchmarks). Its unique chaining API allows zerolog to write JSON (or CBOR) log events by avoiding allocations and reflection.
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.
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`](#pretty-logging).
## Who uses zerolog
Find out [who uses zerolog](https://github.com/rs/zerolog/wiki/Who-uses-zerolog) and add your company / project to the list.
## Features
- [Blazing fast](#benchmarks)
- [Low to zero allocation](#benchmarks)
- [Leveled logging](#leveled-logging)
- [Sampling](#log-sampling)
- [Hooks](#hooks)
- [Contextual fields](#contextual-logging)
- [`context.Context` integration](#contextcontext-integration)
- [Integration with `net/http`](#integration-with-nethttp)
- [JSON and CBOR encoding formats](#binary-encoding)
- [Pretty logging for development](#pretty-logging)
- [Error Logging (with optional Stacktrace)](#error-logging)
- [`log/slog` integration](#integration-with-logslog)
## Installation
```bash
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**
```go
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:
```go
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](#standard-types)
### Leveled Logging
#### Simple Leveled Logging Example
```go
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.
> **Default level.** If you never call `SetGlobalLevel`, the global logger writes to `os.Stderr` at the `trace` level (the lowest level), so **every** event - `trace`, `debug`, `info`, `warn`, `error`, `fatal`, `panic` - is written to stderr by default. In particular, a plain `log.Info(...)` call **will** appear on stderr out of the box. Calling `zerolog.SetGlobalLevel(zerolog.InfoLevel)` raises the minimum level so `trace` and `debug` messages are suppressed.
#### Setting Global Log Level
This example uses command-line flags to demonstrate various outputs depending on the chosen log level.
```go
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)
```bash
$ ./logLevelExample
{"time":1516387492,"level":"info","message":"This message appears when log level set to Debug or Info"}
```
Debug Output (debug flag set)
```bash
$ ./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.
```go
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
```go
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.
```go
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
```go
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
```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"}
```
### Pretty logging
To log a human-friendly, colorized output, use `zerolog.ConsoleWriter`:
```go
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:
```go
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:
```go
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
```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"}
```
### 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"}
```
### Add contextual fields to the global logger
```go
log.Logger = log.With().Str("foo", "bar").Logger()
```
### Add file and line number to log
Equivalent of `Llongfile`:
```go
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`:
```go
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:
```go
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
```go
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 logged.
// Other levels are not sampled.
sampled := log.Sample(zerolog.LevelSampler{
DebugSampler: &zerolog.BurstSampler{
Burst: 5,
Period: 1*time.Second,
NextSampler: &zerolog.BasicSampler{N: 100},
},
})
sampled.Debug().Msg("hello world")
// Output: {"time":1494567715,"level":"debug","message":"hello world"}
```
### Hooks
```go
type SeverityHook struct{}
func (h SeverityHook) Run(e *zerolog.Event, level zerolog.Level, msg string) {
if level != zerolog.NoLevel {
e.Str("severity", level.String())
}
}
hooked := log.Hook(SeverityHook{})
hooked.Warn().Msg("")
// Output: {"level":"warn","severity":"warn"}
```
### Pass a sub-logger by context
```go
ctx := log.With().Str("component", "module").Logger().WithContext(ctx)
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"}
```
### context.Context integration
Go contexts are commonly passed throughout Go code, and this can help you pass
your Logger into places it might otherwise be hard to inject. The `Logger`
instance may be attached to Go context (`context.Context`) using
`Logger.WithContext(ctx)` and extracted from it using `zerolog.Ctx(ctx)`.
For example:
```go
func f() {
logger := zerolog.New(os.Stdout)
ctx := context.Background()
// Attach the Logger to the context.Context
ctx = logger.WithContext(ctx)
someFunc(ctx)
}
func someFunc(ctx context.Context) {
// Get Logger from the go Context. if it's nil, then
// `zerolog.DefaultContextLogger` is returned, if
// `DefaultContextLogger` is nil, then a disabled logger is returned.
logger := zerolog.Ctx(ctx)
logger.Info().Msg("Hello")
}
```
A second form of `context.Context` integration allows you to pass the current
`context.Context` into the logged event, and retrieve it from hooks. This can be
useful to log trace and span IDs or other information stored in the go context,
and facilitates the unification of logging and tracing in some systems:
```go
type TracingHook struct{}
func (h TracingHook) Run(e *zerolog.Event, level zerolog.Level, msg string) {
ctx := e.GetCtx()
spanId := getSpanIdFromContext(ctx) // as per your tracing framework
e.Str("span-id", spanId)
}
func f() {
// Setup the logger
logger := zerolog.New(os.Stdout)
logger = logger.Hook(TracingHook{})
ctx := context.Background()
// Use the Ctx function to make the context available to the hook
logger.Info().Ctx(ctx).Msg("Hello")
}
```
### 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().
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 that handler, all our logs will come with some prepopulated fields.
c = c.Append(hlog.AccessHandler(func(r *http.Request, status, size int, duration time.Duration) {
hlog.FromRequest(r).Info().
Str("method", r.Method).
Stringer("url", r.URL).
Int("status", status).
Int("size", size).
Dur("duration", duration).
Msg("")
}))
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")
// 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")
}
```
## Multiple Log Output
`zerolog.MultiLevelWriter` may be used to send the log message to multiple outputs.
In this example, we send the log message to both `os.Stdout` and the in-built `ConsoleWriter`.
```go
func main() {
consoleWriter := zerolog.ConsoleWriter{Out: os.Stdout}
multi := zerolog.MultiLevelWriter(consoleWriter, os.Stdout)
logger := zerolog.New(multi).With().Timestamp().Logger()
logger.Info().Msg("Hello World!")
}
// Output (Line 1: Console; Line 2: Stdout)
// 12:36PM INF Hello World!
// {"level":"info","time":"2019-11-07T12:36:38+03:00","message":"Hello World!"}
```
## Global Settings
Some settings can be changed and will be applied to all loggers:
- `log.Logger`: You can set this value to customize the global logger (the one used by package level methods).Frequently Asked Questions
What is zerolog?
zerolog is a Logging library for the Go programming language. Zero-allocation JSON logger
How many GitHub stars does zerolog have?
zerolog has 12,519 GitHub stars in the directory's latest synchronization.
How do I install zerolog?
Install zerolog with the Go module system using `go get rs/zerolog`. Check the repository for the current installation instructions.
What category does zerolog belong to?
zerolog is listed under Logging, specifically Logging.