in flume-morphline-solr-sink/src/main/java/org/apache/flume/sink/solr/morphline/UUIDInterceptor.java [65:73]
public Event intercept(Event event) {
Map<String, String> headers = event.getHeaders();
if (preserveExisting && headers.containsKey(headerName)) {
// we must preserve the existing id
} else if (isMatch(event)) {
headers.put(headerName, generateUUID());
}
return event;
}