@Override
    public void serialize(CheckpointId checkpointId, JsonGenerator gen, SerializerProvider serializers) throws IOException {
      gen.writeString(checkpointId.serialize());
    }
@return Returns a new ObjectMapper that's been configured to (de)serialize
        Samza's job data model, and simple data types such as TaskName,
        Partition, Config, and SystemStreamPartition.
samza-core/src/main/java/org/apache/samza/serializers/model/SamzaObjectMapper.java