code: public void addSubscribedSource(Source source) {
          String sourceId = CollectorManagerUtils.getSourceId(source);
          //If present in pending subscription list remove it.
          pendingSubscriptions.remove(sourceId);
          subscribedSources.add(sourceId);
      }
docs: /*
  Method to add a subscribed source
  A source can either be in pending subscription list or the
  subscribed source list.