堆栈层已经被服务组件替换掉了，由于有这么多的重用存在，使得这些堆栈几乎全部消失。

Stack layers have been replaced by service components and there is so much reuse that the stacks have all but disappeared.

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在这种时候，他们就解决问题、用适当的组件替换瓶颈，然后(大部分情况下)接下去做下一件事情。

And when that happens, they fix their problems, replace bottlenecks with scalable components, and (in most cases) move on to the next thing.

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这种灵活性使得你能够更加容易地进行单元测试，因为在测试期间，你可以将真实组件替换为Mock对象。

This flexibility also makes unit testing your code much, much easier, since you can swap out real components for mock objects during your tests.

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