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 tenant businesses profoundly. However, not all tenants are equal, and to some organizations such disruptions are more costly than to others. To account for such tenant-specific requirements, middleware for upgrading SaaS applications should support tenant-specific enactment of upgrades that allow for a customizable schedule and type of enactment in accordance to the tenant SLA. In this paper, we present our design and implementation of a SaaS middleware that enables run-time adaptation by means of a gradual tenant-by-tenant activation of upgrades. The adaptation mechanism is multi-staged, i.e. supports configuration based on the inputs of the tenant administrator and other stakeholders, and is maximally automated. We have validated the middleware in an OSGi-based prototype implementation and evaluated this prototype, showing negligible performance overhead of the middleware and yet clearly showcasing service continuity improvements in realistic upgrade scenarios.
