Testing Log Output in Go with logrus

Published: November 15, 2020

Tags:

• Go
• Golang

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logrus provides a nice facility for testing logging, which is documented in the README. While the README gives you a general idea of the offering, it doesn’t provide any opinions on how to structure your project to support log testing, leaving you on your own to decide on a strategy. Here I wanted to show you the strategy I came up with and am currently using on my project dbanon.

Creating logger.go

After some consideration, the best solution I came up with was to implement a file logger.go. It looks like this:

import (
	"github.com/sirupsen/logrus"
)

var logger *logrus.Logger

func init() {
	logger = logrus.New()
}

// GetLogger returns the logger instance.
// This instance is the entry point for all logging
func GetLogger() *logrus.Logger {
	return logger
}

// SetLogger sets the logger instance
// This is useful in testing as the logger can be overridden
// with a test logger
func SetLogger(l *logrus.Logger) {
	logger = l
}

Example: https://github.com/mpchadwick/dbanon/blob/v0.6.0/src/logger.go

You’ll see the reasoning for this file in the coming examples.

Using logger.go in application

In the application the idea is to do any logging through the *logrus.Logger instance the GetLogger function defined logger.go returns.

logger := GetLogger()
if !someCheck() {
	logger.Error("Something failed")
}

Example: https://github.com/mpchadwick/dbanon/blob/v0.6.0/src/provider.go#L111-L118

Any configuration should also be done against the *logrus.Logger instance returned by GetLogger

logger := GetLogger()
logger.SetLevel(level)

Example: https://github.com/mpchadwick/dbanon/blob/v0.6.0/main.go#L67-L86

Using logger.go in testing

In your tests, you use the SetLogger() function defined in logger.go to replace the actual logger with test logger.

testLogger, hook := test.NewNullLogger()
SetLogger(testLogger)

Example: https://github.com/mpchadwick/dbanon/blob/v0.6.0/src/provider_test.go#L9-L10

Now you can call methods with inputs that are expected to generate log messages and check the hook.

_ := DoSomething("Bad input")
	if hook.LastEntry().Message != "That input was bad" {
		t.Errorf("Bad input handled incorrectly")
	}

Example: https://github.com/mpchadwick/dbanon/blob/v0.6.0/src/provider_test.go#L39-L42