Unless care is taken, a coherence problem can arise if multiple actors (e.g., multiple cores) have access to multiple copies of a datum (e.g., in multiple caches) and at least one access is a write.
比如日历的同步，网页的同步等都可以算作考虑coherency的场景。
coherence/consistency 都为一致性，但略有区别
Related
- extracted from: cache coherence