Quotes
Virtual directories are similar in concept to metadirectories in that they create a single directory view from multiple independent directories. They differ in the means used to accomplish this goal. Whereas metadirectories typically use software agents to replicate and synchronize data from various directories in what might be thought of as batch processes, virtual directories create a single view of multiple directories using real-time queries based on mappings from fields in the virtual schema to fields in the physical schemas of the real directories.
(Windley, 2008, p. 148)
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