Comparison With Relational Databases
There are a number of things that distinguish a traditional directory service from a typical relational database. Of course there are exceptions, but in general:
- directory information is read more often than it is written; this makes features related to transactions and rollback less important.
- data can be redundant if it helps performance.
Directory schemas are defined as object classes, attributes, name bindings and knowledge (namespaces), where an object class has:
- Must - attributes that each of its instances must have
- May - attributes that can be defined for an instance, but can be omitted with the absence treated somewhat like NULL in a relational database
- Attributes are sometimes multi-valued allowing multiple naming attributes at one level such as machine type and serial number concatenated or multiple phone numbers for "work phone".
- Attributes and object classes are standardized throughout the industry and formally registered with the IANA for their object ID. Therefore directory applications seek to reuse much of the standard classes and attributes to maximize the benefit of existing directory server software.
- Object instances are slotted into namespaces. That is, each object class inherits from its parent object class (and ultimately from the root of the hierarchy) adding attributes to the must/may list.
- Directory services are often a central component in the security design of an IT system and have a correspondingly fine granularity regarding access control: who may operate in which manner on what information. Also see: ACLs
Read more about this topic: Directory Service
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