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.
**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"}
> 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)
> 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.
#### 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"}
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.
// 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
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:
*`zerolog.TimeFieldFormat`: Can be set to customize `Time` field value formatting. If set with `zerolog.TimeFormatUnix`, `zerolog.TimeFormatUnixMs` or `zerolog.TimeFormatUnixMicro`, times are formatted as UNIX timestamp.
*`zerolog.DurationFieldUnit`: Can be set to customize the unit for time.Duration type fields added by `Dur` (default: `time.Millisecond`).
*`zerolog.DurationFieldInteger`: If set to `true`, `Dur` fields are formatted as integers instead of floats (default: `false`).
*`zerolog.ErrorHandler`: Called whenever zerolog fails to write an event on its output. If not set, an error is printed on the stderr. This handler must be thread safe and non-blocking.
In addition to the default JSON encoding, `zerolog` can produce binary logs using [CBOR](https://cbor.io) encoding. The choice of encoding can be decided at compile time using the build tag `binary_log` as follows:
