12.1 Logs

We want to build web applications that can keep track of events which have occurred throughout execution, combining them all into one place for easy access later on, when we inevitably need to perform debugging or optimization tasks. Go provides a simple log package which we can use to help us implement simple logging functionality. Logs can be printed using Go's fmt package, called inside error handling functions for general error logging. Go's standard package only contains basic functionality for logging, however. There are many third party logging tools that we can use to supplement it if your needs are more sophisticated (tools similar to log4j and log4cpp, if you've ever had to deal with logging in Java or C++). A popular and fully featured, open-source logging tool in Go is the seelog logging framework. Let's take a look at how we can use seelog to perform logging in our Go applications.

Introduction to seelog

Seelog is a logging framework for Go that provides some simple functionality for implementing logging tasks such as filtering and formatting. Its main features are as follows:

Dynamic configuration via XML; you can load configuration parameters dynamically without recompiling your program
Supports hot updates, the ability to dynamically change the configuration without the need to restart the application
Supports multi-output streams that can simultaneously pipe log output to multiple streams, such as a file stream, network flow, etc.
Support for different log outputs
- Command line output
- File Output
- Cached output
- Support log rotate
- SMTP Mail

The above is only a partial list of seelog's features. To fully take advantage of all of seelog's functionality, have a look at its official wiki which thoroughly documents what you can do with it. Let's see how we'd use seelog in our projects:

First install seelog:

go get -u github.com/cihub/seelog

Then let's write a simple example:

package main

import log "github.com/cihub/seelog"

func main() {
    defer log.Flush()
    log.Info("Hello from Seelog!")
}

Compile and run the program. If you see a Hello from seelog in your application log, seelog has been successfully installed and is running operating normally.

Custom log processing with seelog

Seelog supports custom log processing. The following code snippet is based on the its custom log processing part of its package:

package logs

import (
	"errors"
	"fmt"
	seelog "github.com/cihub/seelog"
	"io"
)

var Logger seelog.LoggerInterface

func loadAppConfig() {
	appConfig := `
<seelog minlevel="warn">
    <outputs formatid="common">
        <rollingfile type="size" filename="/data/logs/roll.log" maxsize="100000" maxrolls="5"/>
		<filter levels="critical">
            <file path="/data/logs/critical.log" formatid="critical"/>
            <smtp formatid="criticalemail" senderaddress="astaxie@gmail.com" sendername="ShortUrl API" hostname="smtp.gmail.com" hostport="587" username="mailusername" password="mailpassword">
                <recipient address="xiemengjun@gmail.com"/>
            </smtp>
        </filter>
    </outputs>
    <formats>
        <format id="common" format="%Date/%Time [%LEV] %Msg%n" />
	    <format id="critical" format="%File %FullPath %Func %Msg%n" />
	    <format id="criticalemail" format="Critical error on our server!\n    %Time %Date %RelFile %Func %Msg \nSent by Seelog"/>
    </formats>
</seelog>
`
	logger, err := seelog.LoggerFromConfigAsBytes([]byte(appConfig))
	if err != nil {
		fmt.Println(err)
		return
	}
	UseLogger(logger)
}

func init() {
	DisableLog()
	loadAppConfig()
}

// DisableLog disables all library log output
func DisableLog() {
	Logger = seelog.Disabled
}

// UseLogger uses a specified seelog.LoggerInterface to output library log.
// Use this func if you are using Seelog logging system in your app.
func UseLogger(newLogger seelog.LoggerInterface) {
	Logger = newLogger
}

The above implements the three main functions:

DisableLog

Initializes a global variable Logger with seelog disabled, mainly in order to prevent the logger from being repeatedly initialized

LoadAppConfig

Initializes the configuration settings of seelog according to a configuration file. In our example we are reading the configuration from an in-memory string, but of course, you can read it from an XML file also. Inside the configuration, we set up the following parameters:

Seelog

The minlevel parameter is optional. If configured, logging levels which are greater than or equal to the specified level will be recorded. The optional maxlevel parameter is similarly used to configure the maximum logging level desired.

Outputs

Configures the output destination. In our particular case, we channel our logging data into two output destinations. The first is a rolling log file where we continuously save the most recent window of logging data. The second destination is a filtered log which records only critical level errors. We additionally configure it to alert us via email when these types of errors occur.

Formats

Defines the various logging formats. You can use custom formatting, or predefined formatting -a full list of predefined formats can be found on seelog's wiki

UseLogger

Set the current logger as our log processor

Above, we've defined and configured a custom log processing package. The following code demonstrates how we'd use it:

package main

import (
	"net/http"
	"project/logs"
	"project/configs"
	"project/routes"
)

func main() {
	addr, _ := configs.MainConfig.String("server", "addr")
	logs.Logger.Info("Start server at:%v", addr)
	err := http.ListenAndServe(addr, routes.NewMux())
	logs.Logger.Critical("Server err:%v", err)
}

An error occurred mail

The above example explains how to set up email, we adopted the following smtp configured to send e-mail:

<smtp formatid="criticalemail" senderaddress="astaxie@gmail.com" sendername="ShortUrl API" hostname="smtp.gmail.com" hostport="587" username="mailusername" password="mailpassword">
	<recipient address="xiemengjun@gmail.com"/>
</smtp>

The format of the message through criticalemail configuration, and then send messages through other configuration server configuration, configured to receive mail through the recipient user, if there are multiple users can add one line.

To test this code is working correctly, you can add code similar to the following one false news. But remember that after should delete it, otherwise you will receive on-line after a lot of junk e-mail.

logs.Logger.Critical("test Critical message")

Now, as long as our application online record a Critical information that you will receive an e-mail Email, so that once the online system problems, you can immediately informed by e-mail, you can timely processing.

Using the Application log

For the application logs, each person's application scenarios may vary, and some people use to do data analysis application logs, some people use the application logs do performance analysis, analysis of user behavior that some people do, and some is pure record, to facilitate the application of a problem when the auxiliary find the problem.

As an example, we need to track a user attempts to log in the system operation. This will successful and unsuccessful attempts to record. Record the successful use of "Info" log level, rather than the successful use of "warn" level. If you want to find all unsuccessful landing, we can use the linux command tools like grep, as follows:

# cat /data/logs/roll.log | grep "failed login"
2012-12-11 11:12:00 WARN : failed login attempt from 11.22.33.44 username password

In this way we can easily find the appropriate information, this will help us to do something for the application log statistics and analysis. In addition, we also need to consider the size of the log, for a high- traffic Web applications, log growth is quite terrible, so we seelog configuration files which set up logrotate, so that we can ensure that the log file will not be changing large and lead us not enough disk space can cause problems.

Summary

On the face of seelog system and how it can be customized based on the learning log system, and now we can easily construct a suitable demand powerful log processing the system. Data analysis log processing system provides a reliable data source, such as through the log analysis, we can further optimize the system, or application problems arise easy to find location problem, another seelog also provides a log grading function, through the configuration for min-level, we can easily set the output test or release message level.

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