All data is always available as long as the central identity vault is available. In a virtual directory implementation, some of the delegated data source may not be available and requests may return no or only incomplete data.
And, how do most vendors get around this problem? Well, in most cases you get the option to cache the data in case the delegated data source is not available. If you cache the data then what do you have?? Answer = a metadirectory.
There are use cases for virtualization but don't think that virtualization is that different than a metadirectory. Volker does mention one very important use case: politics - the 8th layer of the ISO stack. It's typically easier to implement a virtual directory than a metadirectory since virtual directories pull from data sources whereas a metadirectory usually is implemented to both push and pull data but you have to engineer the virtual directory against failure or incomplete responses. Architecturally, having to rely on the network - and the underlying directories or databases - for real-time response is difficult.
Either way, a rose by any other name...Technorati Tags:
Novell, identity management