I came across your question while reading the same book.

I think the issue to consider is that in the single leader scenario, there is a natural total ordering as it can apply a sequence number per transaction. It's easy to tell "which happened first". It always knows "which happened first". It's hard or impossible to do this correctly in multi leader due to issues like clock skew.

So I think it boils down to, does the system even know which happened first.

Your "write if" suggestion is mentioned/used many times and I think is agnostic to single/multi leader. Write if is application level (the user probably has to enter what they think the old value is) whereas single/multi leader is probably not exposed to the user. In single leader, the write if would unambiguously avoid conflicts; the total ordering determines which operation happens first, and whichever gets there second will safely fail due to the write if.

I think the advantage of multi is performance based; the advantage of single is "correctness/complexity" based.

I came across your question while reading the same book.

I think the issue to consider is that in the single leader scenario, there is a natural total ordering as it can apply a sequence number per transaction. It always knows "which happened first". It's hard or impossible to do this correctly in multi leader due to issues like clock skew.

So I think it boils down to, does the system even know which happened first.

Your "write if" suggestion is mentioned/used many times and I think is agnostic to single/multi leader. Write if is application level (the user probably has to enter what they think the old value is) whereas single/multi leader is probably not exposed to the user. In single leader, the write if would unambiguously avoid conflicts; the total ordering determines which operation happens first, and whichever gets there second will safely fail due to the write if.

I think the advantage of multi is performance based; the advantage of single is "correctness/complexity" based.