GET是应该没有副作用的操作，即所谓的等幂性属性。
GET is an operation that should be free of side effects, a property also known as idempotence.
刷新缓存时会损失一些服务器加载数据和一些带宽；这并不违背基本的等幂性。
All you lose in flushing the cache is a little server load and some bandwidth; it does not violate underlying idempotency.
基于进入系统的时间，写操作具有幂等性(不管操作多少次结果都不变的性质，比如取绝对值的函数就具有幂等性)和交换性(操作顺序不影响结果，比如加法就具有交换性)。
Write operations are idempotent and commutative, based on the time they enter the system.
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