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Build status Coverage GoReportCard API documentation

Structured log interface

Package log provides the separation of the logging interface from its implementation and decouples the logger backend from your application. It defines a simple, lightweight and comprehensive Logger and Factory interfaces which can be used through your applications without any knowledge of the particular implemeting backend and can be configured at the application wiring point to bind a particular backend, such as Go's standard logger, apex/log, logrus, with ease.

To complement the facade, the package provides an implementation using the standard Go logger. The default log formatter for this implementation uses colour coding for log levels and logs the date leaving out the month and the year on the timestamp. However, the formatter is fully configurable.

Similarly, the package provides and implementation using the apex/log logger backend.

Interface details

The Logger interface defines a facade for a structured leveled log:

type Logger interface {
	Level(lvl LogLevel) Logger
	Field(k string, v interface{}) Logger
	Fields(data map[string]interface{}) Logger
	Error(err error) Logger
	Log(msg string) Tracer
	Logf(format string, v ...interface{}) Tracer

The Factory defines a facade for the creation of logger instances and setting the log output threshold for newly created instances:

type Factory interface {
	New() Logger
	Threshold(min LogLevel)

The package further defines three log levels differentiating between the (normally hidden) Debug, (default) Info and (erroneous) Error.


The log can be used both statically by binding a particular logger factory:

func init() {
	std.Use(os.Stderr, log.InfoLevel, std.DefaultFmtFun)

// elsewhere	
logger := log.Level(log.InfoLevel).Field("key", "value")

and dynamically by always going via a factory:

factory := std.NewFactory(os.Stderr, log.InfoLevel, std.DefaultFmtFun)
logger := factory.Level(log.InfoLevel).Field("key", "value")

By default a NoOp (no-operation) implementation is bound to the static factory.


To simplify debugging with execution time tracing, the Log and Logf methods return a tracer that can be used to measure and log the execution time:

logger := log.Level(log.DebugLevel).Field("key", "value")

defer logger.Log("start").Trace()
// code to trace the execution time of

The above code snippet would output two log entries (provided the threshold permits) the selected Debug level (her for the default formatter of the std logger):

08 16:31:42.023798 DBG start {key: value}
08 16:31:45.127619 DBG traced {duration: 3.103725832}, {key: value}

License and copyright

Copyright (c) 2017. Oleg Sklyar and MIT license applies. All rights reserved.