Logging in JSON Format

There are some use cases where it would desirable to have application messages formated as JSON documents. Most popular logging packages don't support this, but they are easily extended to handle different layouts. This repo has examples on how to do this for three commonly used logging framewoks: Log4J, Log4Net and Logback. Insructions are provided for adding an existing JSON layout to Log4J2. And lastyly, how to add application specific data is shown.

Extending Layouts

In all these examples, a Layout class is extended to format logging data as a JSON document. The Layout is then used with a rolling file appender. The three examples here are:

• Log4j
• Log4Net
• Logback

Using Existing Layouts

Beginning with version 2.14.0 of Log4J (referred to as Log4j2 since it's significantly different from pre 2.0 versions) there is a JSON layout available. An example of configuring it is here.

Adding Appication Specific Data

It may be that you'd like to add application or site specific data to the log messages. Like a application ID or enviroment name. It's easy enough to add a new property to the JsonLayout class and add it the configuration file. Like this for log4j:

log4j.appender.j=org.apache.log4j.DailyRollingFileAppender
log4j.appender.j.file=./logs/example.json
log4j.appender.j.datePattern='.'yyyy-MM-dd
log4j.appender.j.layout =net.remgant.log4j.JsonLayout
log4j.appender.j.layout.ApplicationID = 1234
log4j.appender.j.layout.EnvironmentName = DEV

Or this for log4net:

<appender name="Json" type="log4net.Appender.RollingFileAppender">
  <lockingModel type="log4net.Appender.FileAppender+MinimalLock" />
  <staticLogFileName value="false" />
  <file value="Logs\\example-" />
  <appendToFile value="true" />
  <rollingStyle value="Date" />
  <datePattern value="yyyy-MM-dd'.json'" />
  <layout type="Remgant.Log4Net.JsonLayout">
    <ApplicationID>1234</ApplicationID>
    <EnvironmentName>DEV</EnvironmentName>
  </layout>
</appender>

If you're using this for logstash, which is a likely use case for this, you could instead add filters to add fields to the document rather than adding properties to the Layout class. For instance, this logstash.conf file would accomplish the same thing:

input {
    file {
        path => "/home/user/app/logs/example*.json"
        codec => "json"
        start_position => "beginning"
    }
}
filter {
  mutate {
   add_field => {
      "applicationId" => "1234"
      "environmentName" => "DEV"
    }
 }
 output {
    stdout {
       codec => "json"
    }
 }