[freels]

It is 3 transactions. Alice deposits money into her savings acount, then subsequently withdraws money from her checking account. In the meantime, some concurrent process transactionally reads the balances of both accounts.

Since there is no internal causality tying transactions 1 and 2 together, transaction 3 is allowed to arbitrarily order them without violating serializability.

I think the answer to your second question comes down to what anomalies you are trying to protect against. In order to prevent causal reverse, all read-write transactions must be strictly serializable, but it is safe to reorder read-only ones.

[mav3rick]

Yes this seems correct.