how GAE DataStore support transaction?

Question

As I known, DataStore is implemented based on bigtable, and transaction only support in single entity group or maximum 5 cross entity groups, but IMHO bigtable only support single row transaction, Entities in the same entity group will be inserted in multiple row, how DataStore entity group archive all-or-nothing transaction with the restriction of bigtable.

Answer 1

A bit dated, but http://googleappengine.blogspot.com/2009/09/migration-to-better-datastore.html might provide some answers. There are more details on Megastore in http://www.cidrdb.org/cidr2011/Papers/CIDR11_Paper32.pdf

Answer 2

Next generation Datastore, known as Firestore uses an architecture similar to Spanner, utilizing similar technology to achieve multi-row transactions.

More information here and here.