Design a SaaS application is a big challenge. There are lots of dimension which we need to look upon while designing the application like maturity level, business goals, business & architecture principles, operational model etc.
According of Microsoft, SaaS application has a four maturity levels:
1. Ad Hoc/Custom: Each client or tenant has its own server instance and application code as modified to meet their requirements.
2. Configurable: In this level, each tenant or client hosts a separate instance of the application for each customer but the code-base remain same for each client i.e. application it quite configurable to support each customer.
3. Configurable, Multi-Tenant-Efficient: Each client or tenant runs on a single instance that serves every customer, with configurable metadata providing a unique user experience and feature set for each one. Only restriction of this level is that it can not be scalable across servers so performance and availability will be issue of this level.
4. Scalable, Configurable, Multi-Tenant-Efficient: This level multiple client or tenant are supported by the load-balanced farm of identical instances, with each tenant data/information kept separate, and with configurable metadata providing a unique user experience and feature set for each tenant/client.
As far as design and flexibility is concern, SaaS application should design for level 4.
Following are the design consideration of SaaS application:
1. Multi-Tenancy: Application should be design to handle multi-tenancy like data, UI, business rules etc should be separated per customer.
2. Security: Application design should ensure security of customer data is secure and there should be complete separation of data between customer/tenant. Also authentication should be "pluggable" to customer's enterprise identity management.
3. Availability: Application should be easily scalable and clusterable so that it is available anytime.
4. Scalable: Application design should be highly scalable and it should easily cope large organization and user base.
5. Data Model Extensibility: Database should be easily be tailor to the fulfill the need of each vendor without affecting the others.
6. Flexible Presentation: UI should be highly configurable and flexible so that it able be easily be customizable for each vendor without affecting the others.
7. Performance: Application should have less response time (for complex page also) and transactional processing time.
8. Configurable Business Rule/Process: Application should be design to support addition/customization of individual tenant/companies business rules/processes. Also application should seamlessly integrate with tenant's enterprise business processes.
9. Multi-language Support: Application should internationalization so that different tenant uses across the Internet and around the world.
10. Separation of Concern: Application should be design on concept of SOA and Separation of Concerns so that it can easily be deployed in cloud and cloud-bus can be plugged to communicate with different cloud and on-premises. Also business agility or time-to-market will be other advantage of this design pattern.
So, any application follow above design consideration then it can easily get the advantage of maturity "Level 4" and ported in cloud environment.
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