ServiceNow CMDB Guide: CIs, Relationships, IRE & CSDM

Introduction: The Single Source of Truth

The Configuration Management Database (CMDB) is the shared source of truth about your IT estate: what you have, and how it all connects. It quietly powers the impact analysis behind Incident, Problem, and Change — which is why a healthy CMDB is a prerequisite for mature ITSM.

Configuration Items and Classes

The things in the CMDB are Configuration Items (CIs) — servers, applications, databases, services. All technical CIs extend a base cmdb_ci table, and specific classes extend it further to add class-specific attributes:

Concept	Meaning
cmdb_ci	The base table all technical CIs extend
cmdb_ci_server, cmdb_ci_service, …	Specific classes with their own attributes
Principal class	Once set, filters the Configuration item field on incident/problem/change to that class
Dynamic CI group	A group of CIs by common criteria (default max 10,000), for applying schedules/policies

Choosing the right class matters because attributes and identification rules are class-specific.

Relationships

Connections between CIs are stored in the cmdb_rel_ci table and are directional — "Runs on", "Depends on", "Used by". Direction matters because it determines how impact flows: if a database a service depends on goes down, the CMDB shows every affected service. These relationships drive dependency views / service maps and impact analysis.

The Identification and Reconciliation Engine (IRE)

A CMDB is only useful if it is trustworthy, and the IRE keeps it clean:

Rule type	Job
Identification rules	Match incoming data to an existing CI so the same asset is not duplicated
Reconciliation rules	Decide which data source is authoritative for each attribute, so a trusted source is not overwritten by a less trusted one

Every data source that writes to the CMDB (Discovery, integrations, imports) should go through the IRE.

CSDM — the Standard Blueprint

The Common Service Data Model (CSDM) is ServiceNow's prescriptive guidance for structuring CMDB and service data. The CMDB is the instantiation; CSDM is the map that keeps it consistent, reportable, and aligned across business and technical views.

Key Roles

itil, itil_admin, cmdb_read, the CMDB administrator roles, plus system-level access for Discovery.

Frequently Asked Questions

What is a Configuration Item (CI)? Any component tracked in the CMDB — a server, application, database, or service. Technical CIs all extend the cmdb_ci base table.

Which table stores CI relationships? cmdb_rel_ci. Relationships are directional (Runs on, Depends on, Used by), which is what makes impact analysis possible.

What does the IRE do? The Identification and Reconciliation Engine prevents duplicate CIs (identification rules) and protects trusted data from being overwritten (reconciliation rules).

What is the difference between the CMDB and CSDM? The CMDB is the actual database of CIs and relationships. CSDM is the prescriptive framework describing how to structure that data correctly.

Why does the CMDB matter for Incident, Problem, and Change? Its relationships power impact analysis — showing which services and CIs are affected — so those processes can assess and prioritize correctly.

Key Takeaways

• CIs extend cmdb_ci; the right class drives attributes and identification.
• Relationships live in cmdb_rel_ci and are directional — that is what enables impact analysis.
• The IRE = identification (no duplicates) + reconciliation (authoritative source wins).
• CSDM is the standard blueprint; the CMDB is its instantiation.
• A healthy CMDB underpins Incident, Problem, and Change.

Where to go next

• See how the data is used end-to-end: ServiceNow ITSM Modules Explained
• Start from the top: What is ServiceNow ITSM?
• Practise: free practice test