GELF appender can add customFields to config for every message

lib/appenders/gelf.js

@@ -34,7 +34,7 @@ levelMapping[levels.FATAL] = LOG_CRIT;
  * @param hostname - hostname of the current host (default:os hostname)
  * @param facility - facility to log to (default:nodejs-server)
  */
-function gelfAppender (layout, host, port, hostname, facility) {
+function gelfAppender (layout, host, port, hostname, facility, customFields) {
   
   host = host || 'localhost';
   port = port || 12201;
@@ -42,6 +42,9 @@ function gelfAppender (layout, host, port, hostname, facility) {
   facility = facility || 'nodejs-server';
   layout = layout || layouts.messagePassThroughLayout;
 
+  var defaultCustomFields = customFields || {};
+  console.log(defaultCustomFields)
+  
   var client = dgram.createSocket("udp4");
   
   process.on('exit', function() {
@@ -55,9 +58,22 @@ function gelfAppender (layout, host, port, hostname, facility) {
    * @param msg
    */
   function addCustomFields(loggingEvent, msg){
+
+      /* append defaultCustomFields firsts */
+      var keys = Object.keys(defaultCustomFields);
+      for (var i in keys){
+          var key = keys[i];
+          // skip _id field for graylog2, skip keys not starts with UNDERSCORE
+          if (!key.match(/^_/) || key === "_id") continue; 
+          msg[key] = defaultCustomFields[key];
+      }
+
+
+      /* append custom fields per message */
       var data = loggingEvent.data;
       if (!Array.isArray(data) || data.length === 0) return;
       var firstData = data[0];
+
       if (!firstData['GELF']) return; // identify with GELF field defined
       var keys = Object.keys(firstData);
       for (var i in keys){
@@ -66,9 +82,9 @@ function gelfAppender (layout, host, port, hostname, facility) {
           if (!key.match(/^_/) || key === "_id") continue; 
           msg[key] = firstData[key];
       }
-      loggingEvent.data.shift(); // remove the first object
-      //console.log('=== msg ===');
-      //console.log(msg);
+
+      /* the custom field object should be removed, so it will not be looged by the later appenders */
+      loggingEvent.data.shift(); 
   }
  
   function preparePacket(loggingEvent) {
@@ -112,7 +128,7 @@ function configure(config) {
   if (config.layout) {
     layout = layouts.layout(config.layout.type, config.layout);
   }
-  return gelfAppender(layout, config.host, config.port, config.hostname, config.facility);
+  return gelfAppender(layout, config.host, config.port, config.hostname, config.facility, config.customFields);
 }
 
 exports.appender = gelfAppender;