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A simple golang logger with log-level capabilities
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.gitignore Initial implementation of structured log support Jan 12, 2020


A simple golang logger with log-level and structured logging capabilities. Wlog also supports split-output to both stdout and optionally a file.

Version 1.0.2 introduces logging hooks. See below for an example on how to install and use hooks.

Available log-levels:

  • Debug
  • Info
  • Warning
  • Error
  • Fatal


go get


package main

import ""

func main() {

  // Set log level to Debug (most verbose). Default is Info

  // Log some messages with different log-levels
  wlog.Debug("This is a debug message")
  wlog.Debugf("This is a %s", "formatted debug message")
  wlog.Info("This is an info message")
  wlog.Warning("This is a warning message")
  wlog.Error("This is an error message")
  wlog.Fatal("This is a fatal message that will call os.exit")  
  wlog.Fatalf("This is a fatal message %s", "that won't be printed")

The output would look similar to this

2019-03-10 20:57:04:280279 DBG This is a debug message
2019-03-10 20:57:04:280394 DBG This is a formatted debug message
2019-03-10 20:57:04:280402 NFO This is an info message
2019-03-10 20:57:04:280408 WRN This is a warning message
2019-03-10 20:57:04:280414 ERR This is an error message
2019-03-10 20:57:04:280421 FTL This is a fatal message that will call os.exit

Logging hooks

A logging hook is a function callback that can be used to perform common tasks when a logging event is triggered. You may install any number of hooks per logging level.

// Code left out for brevity

// Install a hook to catch all Info messages
wlog.InstallHook(wlog.Nfo, func(timestamp time.Time, l wlog.LogLevel, msg string){
  // Perform some action here for all Info log events

// Install a hook to catch all Error messages
wlog.InstallHook(wlog.Err, func(timestamp time.Time, l wlog.LogLevel, msg string){
  // Perform some action here for all Error log events

wlog.Info("This is an Info log entry. Hook installed")
wlog.Error("This is an Error log entry. Hook installed")
wlog.Warning("This is a Warning log entry. No hooks installed")

Structured logs

wlog provides support for structured logs allowing the addition of fields to a Logger global scope and also create scoped logger. See the code snippet below:

package main

import ""

func main() {

    wlog.SetGlobalFields(wlog.Fields{"userId": "dd18f2b6-35df-11ea-bb24-c0b88337ca26"})

    logger := wlog.WithScope(wlog.Fields{"field1": "field1_value"})

    logger.Info("This is a log entry")
    logger.Info("This is another log entry")


Here, wlog is set the log level INFO and after that we invoke the method SetGlobalFields passing wlog.Fields which is a alias type to map[string]interface{}. SetGlobalFields will attach Fields to the Logger global scope. Next, the WithScope method is called receiving Fields as arguments, this will return a new scope with Fields attached to it, this new scope will contain its own list of Fields, plus the fields previously added to the global scope. Child scopes use the JSONFormatter by default.

Running this code you should see the output below:

{"field1":"field1_value","level":"Info","msg":"This is a log entry","timestamp":"2020-01-23 09:57:54:157141","userId":"dd18f2b6-35df-11ea-bb24-c0b88337ca26"}
{"field1":"field1_value","level":"Info","msg":"This is another log entry","timestamp":"2020-01-23 09:57:54:157273","userId":"dd18f2b6-35df-11ea-bb24-c0b88337ca26"}

Note that we call the method Info two times with different messages, however, the field userId added to the global scope sticks with the logger and it is part of the log entry at all times. This behaviour was inspired by the great library logrus

The JsonFormatter has support for compact property names, this can be achieved by setting its property Compact to true, like so:

wlog.SetFormatter(wlog.JSONFormatter{Compact: true})
wlog.SetGlobalFields(wlog.Fields{"userId": "dd18f2b6-35df-11ea-bb24-c0b88337ca26"})

wlog.Info("This is a log entry")


{"@l":"Info","@m":"This is a log entry","@t":"2020-02-05 12:19:30:163927","userId":"dd18f2b6-35df-11ea-bb24-c0b88337ca26"}

Note that, the default fields level, timestamp and message are renamed to its first letter prefixed with a @ symbol. The Compact property in the JsonFormatter is optional and it is set to false by default.


go test
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