1. A method for real-time sharing of resources in a multi-tenant software-as-a-service (SaaS) system, the method comprising: receiving service requests from multiple users belonging to different tenants of the multi-tenant SaaS system; partitioning the resources in the SaaS system into different resource groups, each resource group handling a category of the service requests; estimating costs of the service requests; and dispatching the service requests to the resource groups according to the estimated costs, whereby the resources are shared, among the users, without impacting each other.