https://www.youtube.com/watch?v=kx1K09lZF7I
https://qiita.com/kumagi/items/7dbb0e2a76484f6c522b
https://www.slideshare.net/kumagi/db-75506786
https://15445.courses.cs.cmu.edu/fall2019/slides/26-potpourri.pdf
https://www.slideshare.net/payberah/megastore-and-spanner
https://www.slideshare.net/harisankarh/google-spanner-our-understanding-of-concepts-and-implications
https://www.slideshare.net/kazuyukimiyake/azure-cosmos-db-101101090