👨‍🦱 💬 Application & data is always the top level of responsibility management. So-called SaaS. Runtime & middleware are next as those are the software layers used to host and support your application. The operating system is next, as it is the core component on which the runtime and the middleware run on. The operating system runs on virtual machines which is done by virtualization software. But in the end, everything runs on servers and servers need storage and networking to operate.

👨‍🦱 💬 Infrastructure as a Service means that the cloud provider manages physical infrastructure and virtualization, but customer must maintain everything else. When customer purchases virtual machines in Azure they don’t maintain any Infrastructure but they need to manage everything else, including system patches, middleware, etc.

👨‍🦱 💬 If you install SQL Server on a virtual machine manually or by using out of the box image, it is still a virtual machine. That means customers still need to manage all of the platform aspects. This includes the operating system settings, patches, middleware, SQL Server runtime settings, etc. As such this remains an Infrastructure as a Service. Azure has an option for Azure SQL Server on Virtual Machine where all the updates/backups are automated but even Microsoft on their website considers this option as IaaS. But the line is very thin here.

For more details check FAQ at https://azure.microsoft.com/en-us/services/virtual-machines/sql-server/

👨‍🦱 💬 Customers who purchase Azure SQL Database don’t need to maintain anything related to SQL platform because Microsoft manages that. They just use it by deploying their tables and data and consuming it with their applications. That’s why this is a Platform as a Service model. It’s not Software as a Service because they still need to maintain the database in order for their applications to work.

👨‍🦱 💬 If the customer owns the physical servers, that means they have their own data centers/server rooms. This is so-called on-premise environment.

👨‍🦱 💬 Software as a Service means cloud provided managing everything. Everything, including the application. So customers just buy the application/licenses and use it. They don’t have to maintain anything at all.

👨‍🦱 💬 Customers just buy the application/licenses and use it. That’s what Software as a Service is.

👨‍🦱 💬 Software layer consists the application (application code and set) & the application data.
Platform layer means all the supporting software and the operating system required to host the application.
Infrastructure layer consists hardware the infrastructure and virtualization required to host the platform.