@Override
  public void restore(ChangelogSSPIterator messagesToRestore) {
    while (messagesToRestore.hasNext()) {
      incomingMessageEnvelopes.add(messagesToRestore.next());
    }
  }
A mock StorageEngine that stores what it receives from the StorageEngine.
Those variables/values can be retrieved directly by variable names.
samza-core/src/test/java/org/apache/samza/storage/MockStorageEngine.java