CMDB — core concepts, tables, governance

Definitive questions

• +What is the CMDB in ServiceNow?
• +What is the parent table of every CI class?
• +What is the CI Class Manager?
• +What is CMDB Data Manager?

Scenario drills

1. Extending a CI class without breaking upgrades

   “You need to add a 'lease_expiry' field on Windows servers. Do you extend cmdb_ci_win_server or create a custom table?”

   Approach

   • Add columns to the existing OOB class via CI Class Manager; this preserves identification rules and inheritance.
   • Never create a parallel u_windows_server table — you'd lose OOB identification, reconciliation, and Discovery mapping.
   • Prefix custom columns with u_ so upgrades don't clash.

   // Add column via sys_dictionary (or CI Class Manager UI)
   column.name = 'u_lease_expiry';
   column.table = 'cmdb_ci_win_server';
   column.type = 'glide_date';

   Alternate approach

   Alternate: create a scoped application table that references the OOB CI (cmdb_ci_win_server) via a reference field — good when the data is app-specific rather than a CI property.

   Pitfall

   Extending too deep in the tree forces every subclass to inherit the column. Extend at the lowest class that needs it.