Once you've tested your `green` deployment, you can *mirror* (or copy) a percentage of the live traffic to it. Traffic mirroring (also called shadowing) doesn't change the results returned to clients—requests still flow 100% to the `blue` deployment. The mirrored percentage of the traffic is copied and submitted to the `green` deployment so that you can gather metrics and logging without impacting your clients. Mirroring is useful when you want to validate a new deployment without impacting clients. For example, you can use mirroring to check if latency is within acceptable bounds or to check that there are no HTTP errors. Testing the new deployment with traffic mirroring/shadowing is also known as [shadow testing](https://microsoft.github.io/code-with-engineering-playbook/automated-testing/shadow-testing/). The deployment receiving the mirrored traffic (in this case, the `green` deployment) can also be called the *shadow deployment*.
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