如果所依赖服务的API是幂等的，那就意味着可以安全地对失败请求进行重试。
If the API of a dependent service is idempotent, that means it is safe to retry failed requests.
从设计的角度看，这也意味着消息处理的次数会幂等增加。
From a design standpoint this means that message processing has to be idempotent.
第二个问题是以幂等方式处理序列消息，这样这些消息被执行一次，且一次。
A second problem was handling sequential messages in an idempotent fashion, such that they are executed once and only once.
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