Some time ago I had a blog post describing how to warm up an Azure Web App during deployment slots swap. In that post I explained the sequence of actions that happens during the swap. One important point in that explanation is that if a site has any app settings or connection strings that are marked as “Slot” then during slot swap those settings are read from target (e.g. Production) slot and applied to the site in the staging slot. That causes the restart of the site’s worker process so that those changes take effect and become visible as process environment variable.
The restart of the worker process is OK for the majority of the swaps. But sometimes it would be useful to pause right after the production settings were applied on the staging slot and before the actual swap of host names happens. Recently Azure Web Apps team has released a “Swap with Preview” feature that supports that use case. That feature will give you a chance to verify that the web site in the staging slot works fine with production settings. Also this will allow you to warm up/initialize the site in any way you want. For example you can generate some load on that site so that its cache is completely pre-populated. After you are satisfied with how the site works in staging slot you can complete the swap so that the site is moved to production slot and starts taking on production traffic. The site’s worker process will not be restarted during that step, which means that the worker process in production slot is exactly what you have tested in staging slot.
Note that currently this feature is in Preview meaning that it is ready to be tried but not yet recommended for use with business-critical applications.
To demonstrate how Swap with Preview works I’ve created a site and added a “Slot” setting to it with name “SlotName” and value “Production“. Continue Reading »