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Go logging library. Canonical import path: You will need Go 1.11 or newer.

The main goal is to offer a friendly and ergonomic API. Getting the maximum possible amount of performance or zero-allocations are not goals, although simple benchmarks show it should be more than Fast Enough™ for most purposes (if not, there are a few max-performance libraries already).



zlog.Print("foo")                  // 15:55:17 INFO: foo
zlog.Printf("foo %d", 1)           // 15:55:17 INFO: foo 1
zlog.Error(errors.New("oh noes"))  // 15:55:17 ERROR: oh noes
                                   //          main.main
                                   //                  /home/martin/code/zlog/_example/basic.go:11
                                   //          runtime.main
                                   //                  /usr/lib/go/src/runtime/proc.go:203
                                   //          runtime.goexit
                                   //                  /usr/lib/go/src/runtime/asm_amd64.s:1357
zlog.Errorf("foo %d", 1)           // 15:55:17 ERROR: foo 1
                                   //          ..stack trace omitted..

This does what you expect: output a message to stdout or stderr. The Error() functions automatically print a stack trace. You'll often want to filter that trace to include just frames that belong to your app:

zlog.Config.StackFilter = errorutil.FilterPattern(
    errorutil.FilterTraceInclude, "zlog/_example")

zlog.Error(errors.New("oh noes"))  // 15:55:17 ERROR: oh noes
                                   //          main.main
                                   //                  /home/martin/code/zlog/_example/basic.go:16

You can add module information and fields for extra information:

log := zlog.Module("test")
log.Print("foo")                    // 15:56:12 test: INFO: foo

log = log.Fields(zlog.F{"foo": "bar"})
log.Print("foo")                    // 15:56:55 test: INFO: foo {foo="bar"}

Debug logs are printed only for modules marked as debug:

zlog.Module("bar").Debug("w00t")    // Prints nothing (didn't enable module "bar").
log := zlog.SetDebug("bar")         // Enable debug logs only for module "bar".
log.Module("bar").Debug("w00t")     // 15:56:55 bar: DEBUG: w00t

Trace logs are like debug logs, but are also printed when there is an error, even when debug is disabled for the module:

log := zlog.Module("foo")
log = log.Trace("useful info")
log.Error(errors.New("oh noes"))   // 19:44:26 foo: TRACE: useful info
                                   // 19:44:26 foo: ERROR: oh noes
                                   //          ..stack trace omitted..
log = log.ResetTrace()             // Remove all traces.

This is pretty useful for adding context to errors without clobbering your general log with mostly useless info.

You can also easily record timings; this is printed for modules marked as debug:

log := zlog.SetDebug("zzz").Module("zzz")

time.Sleep(1 * time.Second)
log = log.Since("one")             // zzz  1000ms  one

log.Since("two")                   // zzz    20ms  two

// Add timing as fields, always works regardless of Debug.
log.FieldsSince().Print("done")    // 19:48:15 zzz: INFO: done {one="1000ms" two="20ms"}

Many functions return a Log object. It's important to remember that Log objects are never modified in-place, so using log.Trace(..) without assigning it is does nothing. This also applies to SetDebug(), Module(), Since(), etc.

The Recover() helper function makes it easier to recover from panics in goroutines:

go func() {
    defer zlog.Recover()           // Recover panics and report with Error().
    panic("oh noes!")

See godoc for the full reference.


Configuration is done by setting the zlog.Config variable usually during initialisation of your app.

It's not possible to configure individual logger instances, as it's rarely needed (but I might change my mind if someone presents a good use-case).

See LogConfig godoc for docs.

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Package zlog is a logging library.



View Source
const (
	LevelInfo  = 0
	LevelErr   = 1
	LevelDbg   = 2
	LevelTrace = 3

Log levels.


This section is empty.


func Error

func Error(err error)

func Errorf

func Errorf(f string, v ...interface{})

func Print

func Print(v ...interface{})

func Printf

func Printf(f string, v ...interface{})

func ProfileCPU

func ProfileCPU(path string) func()

ProfileCPU writes a memory if the path is non-empty. This should be called on start and the returned function on end (e.g. defer):

func main() {
    defer zlog.ProfileCPU("")()


func ProfileHeap

func ProfileHeap(path string)

ProfileHeap writes a memory if the path is non-empty. This is usually called just before the program exits:

func main() {


func Recover

func Recover(cb ...func(Log) Log)

Recover from a panic.

Any panics will be recover()'d and reported with Error():

go func() {
    defer zlog.Recover()
    // ... do work...

The first callback will be called before the Error() call, and can be used to modify the Log instance, for example to add fields:

defer zlog.Recover(func(l Log) Log {
    return l.Fields(zlog.F{"id": id})

Any other callbacks will be called after the Error() call. Modifying the Log instance has no real use.


type F

type F map[string]interface{}

F are log fields.

type Log

type Log struct {
	Ctx          context.Context
	Msg          string   // Log message; set with Print(), Debug(), etc.
	Err          error    // Original error, set with Error().
	Level        int      // 0: print, 1: err, 2: debug, 3: trace
	Modules      []string // Modules added to the logger.
	Data         F        // Fields added to the logger.
	DebugModules []string // List of modules to debug.
	Traces       []string // Traces added to the logger.
	// contains filtered or unexported fields

Log module.

func Field

func Field(k string, v interface{}) Log

func Fields

func Fields(f F) Log

func FieldsLocation

func FieldsLocation() Log

FieldsLocation records the caller location.

func FieldsRequest

func FieldsRequest(r *http.Request) Log

FieldsRequest adds information from a HTTP request as fields.

func Module

func Module(m string) Log

Module adds a module to this Log entry.

You can add multiple Modules.

func SetDebug

func SetDebug(m ...string) Log

func (Log) Context

func (l Log) Context(ctx context.Context)

Context adds a context to the Log entry.

This isn't used by zlog, and mostly so that outputs can use it if needed.

func (Log) Debug

func (l Log) Debug(v ...interface{})

Debug records debugging information. This won't do anything if the current module isn't beind debugged.

func (Log) Debugf

func (l Log) Debugf(f string, v ...interface{})

Debugf records debugging information. This won't do anything if the current module isn't beind debugged.

func (Log) Error

func (l Log) Error(err error)

Error prints an error.

func (Log) Errorf

func (l Log) Errorf(f string, v ...interface{})

Errorf prints an error.

func (Log) Field

func (l Log) Field(k string, v interface{}) Log

Field sets one data field.

func (Log) Fields

func (l Log) Fields(f F) Log

Fields append data to the Log object.

func (Log) FieldsLocation

func (l Log) FieldsLocation() Log

FieldsLocation records the caller location.

func (Log) FieldsRequest

func (l Log) FieldsRequest(r *http.Request) Log

FieldsRequest adds information from a HTTP request as fields.

func (Log) FieldsSince

func (l Log) FieldsSince() Log

FieldsSince adds timing information recorded with Since as fields.

func (Log) Module

func (l Log) Module(m string) Log

func (Log) Print

func (l Log) Print(v ...interface{})

Print an informational error.

func (Log) Printf

func (l Log) Printf(f string, v ...interface{})

Printf an informational error.

func (Log) ResetTrace

func (l Log) ResetTrace() Log

ResetTrace removes all trace logs added with Trace() and Tracef().

func (Log) SetDebug

func (l Log) SetDebug(m ...string) Log

func (Log) Since

func (l Log) Since(msg string) Log

Since records the duration since the last Since() or Module() call with the given message.

The result will be printed to stderr if this module is in the debug list. It can also be added to a Log with FieldsSince().

func (Log) Trace

func (l Log) Trace(v ...interface{}) Log

func (Log) Tracef

func (l Log) Tracef(f string, v ...interface{}) Log

type LogConfig

type LogConfig struct {
	// Outputs for a Log entry.
	// The default is to print to stderr for errors, and stdout for everything
	// else. Generally you want to keep this as a backup and add additional
	// outputs, instead of replacing this. For example:
	//    zlog.Config.Outputs = append(zlog.Config.Outputs, func(l Log) {
	//        if l.Level != LevelErr { // Only process errors.
	//            return
	//        }
	//        // .. send to external error notification service ..
	//    })
	//    zlog.Config.Outputs = append(zlog.Config.Outputs, func(l Log) {
	//        if l.Level == LevelErr { // Only process non-errors.
	//            return
	//        }
	//        // .. send to external logging service ..
	//    })
	Outputs []OutputFunc

	// Filter stack traces, only used if the error is a
	// with a stack trace. Useful to filter out HTTP middleware and other
	// useless stuff.
	// Example:
	//   zlog.Config.StackFilter = errorutil.FilterPattern(
	// 	     errorutil.FilterTraceInclude, "")
	StackFilter *errorutil.Patterns

	// Always print debug information for these modules. Debug will be enabled
	// for all modules with the special word "all".
	Debug []string

	// Format function used by the default stdout/stderr output. This takes a
	// Log entry and formats it for output.
	// TODO: the boundary between "outputs" and "zlog internals" are kinda leaky
	// here; it's used for Trace() logs now. Should think about refactoring
	// re-doing this in another way.
	// Maybe add type OutputConfig{ .. } for this (and FmtTime)?
	Format func(Log) string

	// Time/date format as accepted by time.Format(); used in the default
	// Format() function.
	// The default is to just print the time, which works well in development.
	// For production you probably want to use time.RFC3339 or some such.
	// This is used in the standard format() function, not not elsewhere.
	FmtTime string

LogConfig is the configuration struct.

var Config LogConfig

Config for this package.

func (LogConfig) RunOutputs

func (c LogConfig) RunOutputs(l Log)

func (*LogConfig) SetDebug

func (c *LogConfig) SetDebug(d string)

SetDebug sets the Debug field from a comma-separated list of module names.

type OutputFunc

type OutputFunc func(Log)

OutputFunc is an output function, used in Config.Outputs.