[dkhenry]

but Spanner isn't Postgres compatible, they have their own transaction processing layer that is built for TrueTime. If this is using the Postgres frontend how are they incorporating TrueTime into the normal Postgres MVCC model that uses the monotonic xid.

edit: More details make it seem like this _isn't_ Postgres on the frontend, its just mostly postgres compatible, which would explain how they got away from the xid based MVCC. So it seems like this is an entirely new distributed database, rather then a modification to existing postgres.

[eightnoteight]

I think they are internally using postgres only. aurora uses a custom storage process on host which internally supports the custom storage engine (s3)

given the mentions of "log as a database", I believe that depending on the transaction the storage responds differently. like how mysql mvcc uses undolog to essentially rollback database internally so that transaction sees data consistency. they could be doing something similar i.e regardless of whatever postgres uses, if they can get a reference of transaction and its start time then they can use the custom storage engine to rollback the log and respond in that way

[eightnoteight]

remember it is aws, "leverage" is built into their DNA, I don't believe that they would go on the route of building a totally new SQL database like google did