pointer of checkpoint in FlinK

https://ci.apache.org/projects/flink/flink-docs-release-1.1/apis/streaming/savepoints.html

https://info.lightbend.com/rs/558-NCX-702/images/preview-apache-flink.pdf

https://github.com/apache/flink/blob/master/flink-runtime/src/main/java/org/apache/flink/runtime/checkpoint/Checkpoints.java

https://github.com/apache/flink/blob/master/flink-runtime/src/main/java/org/apache/flink/runtime/state/StateBackend.java

https://www.confluent.io/blog/real-time-stream-processing-the-next-step-for-apache-flink/

http://jcconf.tw/2016/assets/workshop/flink.pdf