There are four main models for running an application in the cloud:
- Infrastructure as a Service (IaaS): The cloud provider is in charge of the infrastructure, but you are responsible for configuring it and the application that runs on it.
- Platform as a Service (PaaS): The cloud provider fully manages the infrastructure, and you are responsible for the application and the configuration of running it on the infrastructure.
- Function as a Service (FaaS): The cloud provider manages the infrastructure and platform, and based on some trigger (HTTP call, message in a queue, a scheduled time, and so on), activates your application; you are only responsible for writing the single function code that will be run when triggered.
- Software as a Service (SaaS): The infrastructure, platform, and the application itself is managed for you; you only configure and adjust it to your needs. Users use the application without installing anything on their machines.
Another way to look at the different models is to compare them to eating a pizza:
- When you host your application on premises, it's the same as making your own pizza at home: you control the ingredients, the making process, and the eating itself
- IaaS is similar to buying a half-baked pizza base: you still need to put the toppings and bake it, but a lot of the infrastructure has already been supplied to you
- PaaS is like ordering pizza: the entire pizza making process was done for you, but you still need to take care of setting the table and cleaning the dishes
- FaaS is like buying pizza at a counter: you can buy as many slices as you like and pay accordingly
- SaaS is like eating pizza in a restaurant where you get the full service: you can select the style of pizza and special additions, you sit at a nice table, and you don't need to worry about the mess—you just pay for it
The following diagram summarizes the differences between the cloud service models:
