Unlike other Web applications, corporate portals reckon to provide an integration space for corporate services. Mashups contribute to this goal by bringing a relevant customization technique whereby portal users can supplement portal services with their own data needs. The challenge is to find a balance between portal reliability and mashup freedom. Our approach is to split responsibilities between service providers and portal users. Providers decide on how services can be mashuped, portal users determine the supplemented content, and finally, the portal engine mediates between the two. This permits portal services to be reliably customized through user mashups. The approach is realized for
as the portal engine, portlets as the realization of portal services, and XBL as the integration technology.