Relationship (ITILv3): A connection or interaction between two people or things. In Business Relationship Management it is the interaction between the IT Service Provider and the Business. In Configuration Management it is a link between two Configuration Items that identifies a dependency or connection between them. For example Applications may be linked to the Servers they run on. IT Service may have many links to all the CIs that contribute to them.
See also: TBD
Relationship (ITILv2): Describes the dependency or connectivity between Configuration Items (CIs), using terms such as 'used by', 'part of'', 'connected to', 'resides on'. Relationships not only provide a means of assessing the potential full impact of a proposed Change but can also indicate the potential damage likely to be caused by a single component failure. Relationships are described as a 'primary' if they are hierarchical and 'secondary' if not hierarchical.
See also: [ITIL] Availability Management, [ITIL] Change Management, [ITIL] Configuration Management
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Thoughts
I like particularly the phrase "potential damage likely to be caused by a single component failure." We often focus on the inventory and asset management aspects of relationship when we construct a CMDB. As a result, we do not give due attention to the reason we go through this effort.
Why do we seek to identify relationships?
To understand the impact on availability when a component fails.