Why SharePoint APP Model

SharePoint App  is one  of the new features that is been introduced with SharePoint 2013. When Developing any new functionality on SharePoint 2013 you have three options.

1. Fully Trusted Solution
2. Sand box Solution
3. SharePoint App Model

Solution Challenges

1. Most of the fully trusted custom solutions runs within the SharePoint host environment.
2. Managed code deployed using Sandbox solution runs within the SharePoint sandbox worker process.

Around 90% of cases, the primary cause for the instability of the SharePoint Farm is these Custom Solutions that runs within the SharePoint hosting environment.

Logically it makes more sense to move these custom solutions out of SharePoint hosting environment and convert these solution the App Model way, wherever possible. This would  minimizing the risk  and improving the stability of the SharePoint Farm.

The other advantage with the loosely coupled  App solutions would be that it would easily to upgrade an on-premises farm to a newer version.

SharePoint App is a standalone solution that are loosely integrated with SharePoint. The App Model is based on these main fundamental Design goals.

           -    App Code never runs within the SharePoint Host environment

           -    Apps must be supported in on-premises farm and in Office 365

           -    App permissions can be easily configurable independent to user permission

           -    Apps that are published in a app catalog , are easier to discover, install and upgrade