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With the Single-Instance Multi-Tenancy (SIMT) model for composite Software-as-a-Service (SaaS) applications, a single composite application instance can host multiple tenants, yielding the benefits of better service and resource utilization, and reduced operational cost for the SaaS provider. An SIMT application needs to share services and their aggregation (the application) among its tenants while supporting variations in the functional and performance requirements of the tenants. The SaaS provider requires a middleware environment that can deploy, enact and manage a designed SIMT application, to achieve the varied requirements of the different tenants in a controlled manner. This paper presents the SDSN@RT (Software-Defined Service Networks @ RunTime) middleware environment that can meet the aforementioned requirements. SDSN@RT represents an SIMT composite cloud application as a multi-tenant service network, where the same service network simultaneously hosts a set of virtual service networks (VSNs), one for each tenant. A service network connects a set of services, and coordinates the interactions between them. A VSN realizes the requirements for a specific tenant and can be deployed, configured, and logically isolated in the service network at runtime. SDSN@RT also supports the monitoring and runtime changes of the deployed multi-tenant service networks. We show the feasibility of SDSN@RT with a prototype implementation, and demonstrate its capabilities to host SIMT applications and support their changes with a case study. The performance study of the prototype implementation shows that the runtime capabilities of our middleware incur little overhead.