Why Deploy a Discovery Service?
The A discovery service is a solution to the IdPDiscovery problem. Briefly summarized, it seeks to solve the problem that facing an SP has when a user first approaches it:
"Where can I direct this user to go to acquire the credentials I need to allow them access to the pages they are want to visit"
If the SP knows that only one IdP will ever provide authentication then it can immediately send the user to that IdP. However, an SP will usually be able to handle users authenticated by multiple IdPs and at that point it has no option but to ask the user to select an IdP, this work which is usually done by a DiscoveryService Discovery Service.
Although this solution is deceptively simple it is complicated by several factors:
...