Database Management and Platform Security · Topic 4 of 5
CMDB and CSDM
ServiceNow CMDB and CSDM for the CSA exam: configuration items, classes, relationships, Discovery, identification, reconciliation, health and CSDM.
- 4 min
- A CI is a record that extends the base Configuration Item table; its type is its class.
- Relationships link CIs and carry a direction.
- IRE uses identification rules to avoid duplicates and reconciliation rules to protect data from non-authoritative sources.
What it is
The Configuration Management Database (CMDB) stores the things IT manages and how they connect. Each thing is a configuration item (CI): a server, a switch, a business application, a database. The CMDB also stores relationships between CIs, such as which application runs on which server.
The Common Service Data Model (CSDM) is the ServiceNow framework that says how to organize CMDB and service data so that every process uses the same structure. The CMDB is the data; CSDM is the model that tells you what to put where.
How it works
CIs, classes and relationships
CIs are records in tables that extend the base Configuration Item table (cmdb_ci). Each kind of CI is a class, such as Server, Linux Server or Network Gear. Classes form a hierarchy, so a Linux Server has the fields of a Server and of the base CI table.
Relationships link two CIs, and they have a direction and a type, for example Runs on::Runs or Depends on::Used by. Relationship data is what lets you see the impact of a failing server on the services above it.
CIs connect to daily work. An incident, change or problem can name the affected CI, and the CI's relationships show what else may be involved.
Getting data in
CIs can be added by hand, imported, or populated by tools:
| Source | Role |
|---|---|
| Discovery | Finds devices and applications on the network and creates or updates CIs |
| Service Mapping | Maps the CIs that make up an application service |
| Import sets | Bring CI data in from another system or file |
| Integrations | Feed CIs from other tools such as cloud platforms |
Identification and reconciliation
Many sources can describe the same server. The Identification and Reconciliation Engine (IRE) keeps the CMDB from filling with duplicates.
Identification rules decide whether an incoming record is a CI that already exists. They compare key attributes, and the rules are inherited down the class hierarchy, so a child class can use its parent's rule.
Reconciliation rules decide which source is allowed to set which attribute. A source that is not authoritative for an attribute cannot overwrite it. This keeps a weak source from replacing good data.
CMDB health
The CMDB Health dashboard scores the data on three measures:
| Measure | Question it answers |
|---|---|
| Completeness | Are the expected fields filled in? |
| Correctness | Is the data free of duplicates and orphans, and does it follow rules? |
| Compliance | Do CIs meet the expected audit and policy rules? |
CSDM
CSDM groups the data into domains. Foundation holds the shared data such as locations, groups and users. Other domains cover how services are designed, built and delivered, for example business applications and application services, up to the service offerings a business consumes.
The practical point: model services in the CMDB the way CSDM describes, so a business service links down through applications to the servers that run it. Teams that do this get dependable impact analysis.
Think of CSDM as the blueprint and the CMDB as the building.
How the exam asks about this
- "Two tools report the same server and the CMDB now has a duplicate." The IRE identification rules prevent this.
- "A less trusted source keeps overwriting a CI's data." Reconciliation rules set which source is authoritative.
- "Where is the impact of a server outage visible?" In the CMDB relationships to dependent services.
What to remember for the exam
- A CI is a record that extends the base Configuration Item table; its type is its class.
- Relationships link CIs and carry a direction.
- IRE uses identification rules to avoid duplicates and reconciliation rules to protect data from non-authoritative sources.
- Identification rules are inherited down the class hierarchy.
- CMDB Health measures completeness, correctness and compliance.
- CSDM is a data model framework, organized in domains; it is not a tool that finds CIs.
Discovery is not the CMDB. Discovery populates it, and the IRE is what keeps that data clean.
Common mistakes
Mixing up CMDB and CSDM. The CMDB holds the data. CSDM is the guidance on how to structure it.
Believing more sources always means better data. Without reconciliation rules, a weaker source can overwrite a stronger one.
Treating relationships as optional. Without them the CMDB is a list of devices, and impact analysis does not work.
Check yourself
What does the Identification and Reconciliation Engine prevent?
It prevents duplicate CIs through identification rules and stops non-authoritative sources from overwriting attributes through reconciliation rules.
Name the three CMDB Health measures.
Completeness, correctness and compliance.
What is CSDM?
A data model framework, organized into domains, that standardizes how CMDB and service data is structured.
Topic 4 of 5 in Database Management and Platform Security
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