Software as a Service (SaaS) is a model of software deployment whereby an Independent Software Vendor (ISV) licences an application to customers for use as a service on demand., over the Internet . A SaaS Vendor may host the application on their own web servers or download the application to the consumer device, disabling it after use or after the on demand contract expires.
Examples of SaaS vendors include SalesForce.com, NetSuite, Workday, RightNOW, SAP Business by Design which provide business applications online that are accessed from a web browser, while the software and data are stored on the servers.
The SaaS model has been well received by customers including SME's and Enterprises alike, mainly because it removes the burden of building and operating computer systems in order to run applications. The SaaS model is also favoured because of the agility it offers, both in terms of its on-demand nature and because it often avoids having to involve the IT department.
The SaaS model is also popular with customers because it is often a zero capital expenditure solution and avoids the up-front expense of software licence purchases, through less costly, on-demand pricing from hosting service providers.
A SaaS vendor will usually provide service in return for monthly subscriptions, based upon the number of users or and/or user consumption. This enables the vendor to establish a predictable, ongoing revenue stream.
There are a variety of SaaS architectures and Microsoft has defines four 'maturity' levels:
SaaS Level 1 - Ad-Hoc/Custom: At the first level of maturity, each customer has its own customized version of the hosted application and runs its own instance of the application on the host's servers. Migrating a traditional non-networked or client/server application to this level of SaaS typically requires the least development effort and reduces operating costs by consolidating server hardware and administration.
SaaS Level 2 - Configurable: The second maturity-level provides greater program flexibility through configurable, so that many customers can use separate instances of the same application code. This allows the vendor to meet the different needs of each customer through detailed configuration options, while simplifying maintenance and updating of a common code base.
SaaS Level 3 - Configurable, Multi-Tenant-Efficient: The third maturity level adds multi-tenancy to the second level, so that a single program instance serves all customers. This approach enables more efficient use of server resources without any apparent difference to the end user, but ultimately comes up against limits in scalability.
SaaS Level 4 - Scalable, Configurable, Multi-Tenant-Efficient: The fourth and final SaaS maturity level adds scalability through a multi-tier architecture supporting a load-balanced farm of identical application instances, running on a variable number of servers. The provider can increase or decrease the system's capacity to match demand by adding or removing servers, without the need for any further alteration of applications software architecture.
SaaS-Attack