This is something like my third attempt to read the spanner paper - I get how it helps ordering of transactions but I am confused if it is used in making transactions atomic across machines ?
> I get how it helps ordering of transactions but I am confused if it is used in making transactions atomic across machines ?
AIUI, you cannot quiet think of it like a regular database where a particular row has a particular value which would necessitate only one writer doing (atomic) updates at a time.
Rather it is a MVCC-like database and a bit like an append-only log: as many writers as needed can write and there are 'multiple values' for each row. The "actual" value of the row is the one with the highest transaction ID / timestamp. So updates can happening without (atomic) locking by just adding to the value(s) that already exist.
When reading, applications just generally get served the value with the highest-value timestamp, and since time is synchronized to such a tiny interval, it is a reasonably sure bet that the highest value is the most recent transaction.
This is similar in concept to a vector clock (see also Lamport):
