Thanks for the link, not much familiar with CockroachDB but I worked on a similar system that wanted to provide strong consistency when reading from many databases, some of which where Spanner but not all of them.

Eventually the system implemented something similar to "find transaction timestamp at execution time" + optimistic locking, as the article states you lose global linearizability and have to do some re-reads for writes.

What I missed the most from Spanner compared to the system above is that its tricky to do client-side transaction chaining where there are many clients and the order is important.