1. A system for providing a hosted application, the system comprising: at least two scale groups, each scale group being configured to support at least one tenant, wherein each scale group comprises a shared middle-tier and a shared database-tier for providing the hosted application to the tenants assigned thereto, and wherein the scale groups are configured to determine whether a request has been received for a tenant that has been moved to a different scale group, create a new temporary sub-domain corresponding to the different scale group for the tenant in response to determining that a request has been received for a tenant that has been moved to a different scale group, and redirect the request for the tenant to the new temporary sub-domain; a shared configuration database that is shared by tenants of the hosted application and configured to store mapping data to map an individual assignment of at least two of the tenants of the hosted application to one of the at least two scale groups and tenant customizations for the hosted application; and a site-wide server computer configured to receive a request for the hosted application from one of the at least two tenants, to search the shared configuration database for mapping data indicating the one of the at least two scale groups associated with the one of the at least two tenants, and to redirect the request to the one of the at least two scale groups associated with the one of the at least two tenants.