What's the difference between IaaS, PaaS and SaaS? There still seems to be confusion especially about PaaS. I hope this will help.
Consider what lies behind using a software application, like email. (I will be simplifying and generalising below, to get the point across, so no need to point out eg that some languages are interpreted, that some programs can be run directly without installation, and that PaaS applications may need to be coded to integrate with the specific PaaS provider’s libraries!).
- The application is coded – someone writes the application in a programming language like C++, Python etc.
- The application is compiled – the code is converted into a form that can be run on a particular operating system eg Windows, Mac, Linux, Android, iPhone (iOS) etc.
- The application is acquired – eg downloaded from a website, obtained on DVD.
- The application is installed on the operating system – eg doubleclicking an .msi file in Windows.
- The application is run and used by the user – eg doubleclicking on the program filename.
Non-cloud – the end user of the application typically only takes steps 3-5, or even just 5 on a corporate network where the IT department has already taken care of 3 and 4.
SaaS – the cloud user only takes step 5, typically by logging into the SaaS service over the Internet (or company network) to access the application, instead of clicking on a local program name; the SaaS provider takes care of all the rest.
IaaS – the cloud user must take care of ALL of steps 1-5. In addition (consider this a step 3.5!) it must also manage its own VMs including creating VMs and installing the operating systems on its VMs (though it can use snapshots). But it could use someone else’s code (eg open source software) rather than writing the code itself (in which case it skips step 3). Or it could use someone else’s application, go straight to step 3 and install the application in its cloud VM on top of the operating system it installed, assuming the application licence allows installations in VMs. In step 5 the individual end users could be the employees of the cloud user organisation, or its customers, or both.
PaaS – the cloud user only takes care of step 1, again writing its own code (normally using an SDK or software development kit downloadable from the provider) or obtaining code from elsewhere. The PaaS provider handles steps 2-4. Step 1 can be and is often done locally, then the code is uploaded to the PaaS provider. Again, in step 5 the end users could be employees of the cloud user organisation or its customers. Hence startups offering new services over the web, eg mobile applications, like using IaaS or PaaS because they don’t have to buy equipment to service their customers, they can just focus on running their systems (in IaaS) and coding (in both). With PaaS, they don’t even have to manage IT systems - they can concentrate just on coding. Hence the ‘platform’ in PaaS – it provides a ‘platform’ for PaaS users to code their applications, deploy their applications (to servers provided by the PaaS provider) and host their applications (on servers provided by the PaaS provider), so that the applications are available for use by their end users over the Internet or corporate network.