Rehearse a Change and Retire an Old Tool

Two ways a change reaches production with less risk than a plain cut over. A new version can answer copies of real requests before it serves anyone, and an old tool call can be retired over a window instead of removed in one step. This course covers mirroring live requests, holding back the effects of a copy, checking a tool reply against the shape the caller depends on, and watching the callers of a deprecated tool until the list is empty.