Modeling and Analysis of Availability in Multi-tenant SaaS

Software as a Service (SaaS) has become an important application development and service delivery model. Among different architectures, multi-tenant architecture (MTA) not only has advantage on maintenance, but also increases resource utilization by sharing instances. However, sharing instances brings challenges to the security of the service. As one of the three principal properties of the security, availability receives more and more attentions. Recently, there are extensive efforts on technical methods to implement a secure multi-tenant SaaS, but few works on the modeling and analysis of its availability. In this paper, we firstly present the availability issues of the multi-tenant SaaS. Two important mechanisms to implement the MTA SaaS are then introduced: network isolation and database sharing. After that, a stochastic Petri net (SPN) model is developed to analyze the availability. Specific metrics are proposed to measure the availability both from the aspects of the system and the tenant. To extend the SPN model for large scale analysis, we solve the state space explosion problem of SPN model based on the theory of Markov chain aggregation. Finally, numerical results are provided to demonstrate the effectiveness of the SPN model and the analysis is efficient.