A release engineer swaps a newly tested staging slot into production for Cascade Regional Airlines' booking site. Immediately after the swap, the first wave of live users hits slow response times because the app has to finish its startup initialization under real traffic. What App Service feature should the engineer use before the next release to avoid this cold-start delay during swap?
Select an answer to reveal the explanation.
Short Explanation
A swap that happens before the app is actually warmed up just moves the cold start onto live traffic, which is exactly the problem here. App Service's swap-with-warm-up feature runs your app's startup requests against the staging slot first, so by the time the swap finishes, the app is already primed and ready.
Full Explanation
App Service supports configuring application initialization (auto-swap with warm-up) so that, before a slot swap actually completes, the platform sends warm-up requests to the staging slot's app, letting caches populate, connections establish, and just-in-time compilation finish while the slot is still isolated from live traffic. Only once those warm-up requests succeed does the swap route production traffic to the newly warmed instance, eliminating the cold-start hit users would otherwise experience. Increasing instance count after the swap does nothing to address the specific problem: the new instances would still need to go through the same uninitialized startup, and traffic has already arrived by that point. Disabling Always On is counterproductive, since Always On is what keeps the app loaded and prevents it from idling out and having to cold-start again; removing it would make cold starts more likely, not less. Adding a custom domain to the staging slot addresses reachability for manual testing but has nothing to do with startup performance during a swap. As an operational check, review the applicationInitialization section of the site's web.config or the equivalent App Service configuration and confirm the warm-up paths reflect real endpoints the app depends on at startup.