code: @SuppressWarnings("unchecked")
      public void addDoc(Map<String, Object> jsonFields, String collName,
              String docId) {
          if (!mCollections.containsKey(collName)) {
              // add new collection
              log.debug("Added collection " + collName);
              mCollections.put(collName, new ConcurrentHashMap<String, Map<String,Object>>());
          }
          Map<String, Map<String,Object>> collection = mCollections.get(collName);
  
          Map<String, Object> fields;
  
          if(jsonFields.get(DdpMessageField.FIELDS) == null) {
              fields = new ConcurrentHashMap<>();
          } else {
              fields = (Map<String, Object>) jsonFields.get(DdpMessageField.FIELDS);
          }
  
          collection.put(docId, fields);
  
          if (BuildConfig.DEBUG) {
              log.debug("Added docid " + docId + " to collection " + collName);
          }
      }
docs: Handles adding a document to collection.
  Override if you want to use your own collection data store.
  @param jsonFields fields for document
  @param collName collection name
  @param docId document ID