Thou Shalt Not Reassign Identifiers
08 Jan 2010
The Web and especially OpenID has yet to learn important lesson: nothing is permanent. Will Norris mentions it in his post. To make his long story short, the problem is that OpenID relies on DNS and DNS names can be reassigned. With change of control of DNS name the control of associated OpenID identifier is changed as well. Therefore a user may be required to pay for a domain that he does not want any longer just to avoid losing control over the OpenID identifier. The root of the problem is that DNS is not really an identification mechanism, but rather an addressing mechanism. OpenID design does not account for that.
The purpose of address is to locate an object, therefore it contains information about object's location - directly or indirectly. Address must change if the location of the object changes. DNS is using a level of indirection to reduce the number of changes needed if object location changes, but it does not reduce them to zero. You may be forced to pay for a domain forever if you want to make DNS name a permanent identifier - assuming you can do that at all. For example the rules for sk top-level domain will force you to yield your domain in case someone registers a trademark that is the same as your existing domain name. Therefore making DNS name persistent may be quite costly. DNS domain is an address. Get over it.
The purpose of identifier is to distinguish the object from other similar objects. Well-designed identifiers does not need to change. The identifier may identify an object that does not exist any longer, but it should never identify a different object. Think of ANS.1 OIDs, ISBNs or similar identifiers. For identifiers to be efficient their assignment should be very cheap and maintenance must be extremely cheap or entirely free.
It is not wrong per se to use address in your system. But it is a mistake to use an address and assume that it has properties of identifier. It is a failure to assume that address will not change - almost as serious a mistake as assumption that identifier can always be resolved.