Multi-tenant Software as a Service (SaaS) is the cloud computing delivery model that maximizes resource sharing up to the level of a single application instance, servicing many customer organizations (tenants) at once. Due to this scale of delivery, a SaaS offering, once successful, becomes difficult to upgrade and evolve without affecting service continuity, and this in turn limits its capabilities to respond to the reality of changing customer requirements. However, not all tenants are equal, and to some organizations such disruptions are more costly than to others. Supporting different quality trade-offs for different tenants is often a manual, error-prone task and far from trivial. This short paper outlines our middleware design for fine-grained, gradual and continuous evolution of multi-tenant SaaS applications, providing automated and systematic support for (i) tenant-aware upgrade enactment, and (ii) compensations that allow recovering from negative side-effects of the upgrade enactment.
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