Hi We are working on a project and have decided to go with the multitenant Saas architecture level 4 and the Shared database shared schema approach as shown on this page http://
My 2c on the shared schema approach...
This is my preferred approach(personal preference) but the way I see it is that the key differentiated factor is not the business logic but rather the retrieval of the data. This is one approach.
The business logic will and should operate the same way regardless of the data provided. In terms of code, the data layer should be responsible for pulling only the correct tenants data based on the tenant id. There may be some opinions that this filtering of data by tenant be also done in the business layer. One advantage is that you have the added benefit of making sure you are working with the correct tenant's data.
I don't think you will find a proper starter kit for a SAAS app. The best way in my opinion would be to read the code of a few completed projects to get you started.
On to some resources (since you have not specified what technology exactly, I am providing asp.net-mvc-related links)