private void writeObject(java.io.ObjectOutputStream out) throws IOException {
    out.defaultWriteObject();
    // transform roles to string names which are serializable...
    Set fr = failedRoles;
    if (fr == null) {
      fr = Collections.EMPTY_SET;
    }
    Set roleNames = new HashSet(fr.size());
    for (final Object o : fr) {
      String name = ((Role) o).getName();
      roleNames.add(name);
    }
    out.writeObject(roleNames);
  }
Returns the required roles that caused this exception. One or more roles failed to receive a
cache distribution message or acknowledge receipt of that message.

@return the required roles that caused this exception
geode-core/src/main/java/org/apache/geode/cache/RegionDistributionException.java