Difference between Azure App Deployment Slots & Environments
Ron Norman clarifies that Azure App Service Slots and deployment environments serve different purposes. Slots are ideal for previews, app warming, and pre-live checks but lack the full isolation—compute, network, and data—essential for true deployment environments. They share resources within the same App Service Plan, making them unsuitable for production and non-production environment replacement. Instead, slots should be seen as part of the production continuum, useful for staging, customer previews, and A/B testing within the production realm.
Many teams mistake Azure App Service Slots for real deployment environments. They are not the same. Azure App Service Slots excel for the purposes of product previews, warming up the app before going live, and sanity checks before going live. But they are by no means suited to replace production and non-production deployment environments.
Although an App Service Slot is a separate app with its own separate configurations and url, however, all slots share the same compute resources or, in other words, the same App Service Plan.
Why is that a potential problem? Well, right off the bat, it doesn't fit the definition of a deployment environment. One of the major premises of a deployment environment is the attribute of full isolation. Full isolation in all three areas: Compute, Network and Data.
App Service Slots do not provide any of those three levels of isolation. Not on the compute resources, not on the network, and certainly not on the data level.
For example, if developers, testers, and stakeholders are all banging away at their non-production app slots, they are actually competing with real production customers accessing the app. You might say well we can scale it up or scale it out. Yes, that's true but it's still not isolation because other issues can happen, especially with dev and test. You're still hitting the same VMs underneath and depriving production traffic and potential crashes.
So what are the Azure App Service Slots good for? Think of Azure App Service Slots as belonging to the production realm or continuum. The different slots are all different "shades" of production and within production.
For instance, point your release pipeline to a staging slot to warm it up and give it a quick sanity check then swap it to production. Or offer a preview access to customers to check out your new features before the full switch. Or do A/B testing and allow a percentage of live trafic to the staging slot to expose it to customer.
Notice that all the above examples have one thing in common: production. They're all tried closely tied to production or different exposures to it. Azure App Service Slot is an amazing features for "playing" with production. As you can see, deployment slots are not environments, they're a sandbox for production.
If you are staring out with a new product and have no need for true deployment isolation as of yet, then definitely you should start with Azure Service Slots for the different shades of production. Then, later, when the product grows in traffic and usage, add other App Service Plans that are dedicated to non-production environments. Those, do not need to use slots.
