How both of these can be true?

> Data in cache is not durable, which means that sometimes version information that is important for conflict resolution can get evicted.

> We also added a special flag to the query that Polaris sends to the cache server. So, in the reply, Polaris would know whether the target cache server has seen and processed the cache invalidation event.

To make special flag work, cache server need to track not only current version state, but also past versions. If it tracks past versions, then conflicts can be resolved at the cache server level, but whole premise of article is that cache servers can't resolve conflicts by themselves.