1. Apparatus for providing identity provider services in association with an identity provider instance discovery service distinct from and external to the apparatus, comprising: a processor; computer memory holding computer program instructions executed by the processor to carry out a set of operations comprising: as requests for identity provider instances are processed by the identity provider instance discovery service, receiving and storing at the apparatus data identifying the identity provider instances assigned by the identity provider instance discovery service; receiving at the apparatus an update concerning a resource associated with the identity provider instance discovery service, the update received from the identity provider instance discovery service via a topic-based publish-subscribe notification service; based on the update received at the apparatus, modifying the data; and upon receipt of a new request for an identity provider instance, and in lieu of forwarding the new request from the apparatus to the identity provider instance discovery service for handling, using the modified data to identify an identity provider instance for use in responding to the new request.