title: Process Id in Logback Logging Pattern
body: <p>I have the following logback pattern:</p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;pattern&gt;
      {"hostname": "${HOSTNAME}", 
       "level": "%p", 
       "method": "%M", 
       "process_id": "${process}", 
       "thread_id": "%t", 
       "timestamp": "%d{Y-M-d}T%d{H:M:S.s}", 
       "mesg":"%msg"}%n
  &lt;/pattern&gt;
  </code></pre>
  
  <p>Unfortunately when the log messages are actually generated I see: <code>"process_id": "process_IS_UNDEFINED"</code></p>
  
  <p>Is there any automatically set variable for process id, such as there is for hostname? I am having a lot of trouble finding a documented list of such automatically set variables in the logback documentation, does anyone know of a better documentation source?</p>
  
  <p>Edit: I am aware of Mapped Diagnostic Contexts, but was hoping for a builtin solution that does not need such setup, much like how hostname works.</p>