Identification rules are used to identify whether a CI that is populated in the CMDB is already existing in the CMDB. If it is already found to be present, the existing record will get updated. If not, a new CI will be created. An identification rule consists of a single CI identifier and one or more identifier entries and related entries, each with a different priority. The identification rules are considered only when the CMDB is populated by any of the methods which support identification & reconciliation engine (IRE) - like ServiceNow discovery, Service Graph Connectors for 3rd party integrations and integration hub ETL for manual data imports. Please note that IRE is NOT used by 3rd party integrations which are NOT using Service Graph Connectors and manual data imports which are NOT done via integration hub ETL. Hence these methods are usually not recommended for CMDB data population.
Consider the following identification rule of cmdb_ci_hardware for example. When a new CI is going to be populated to cmdb_ci_hardware using any method which follows IRE, the identification rule first checks for a match of the combination of serial number & serial number type (i.e. The first priority identifier entry). If a match is found, the data for that CI will be updated. Then the remaining identifier entries won't be checked. If a match is not found, next it will check for a matching serial number only (i.e. The next priority identifier entry). If a match is not found for that also, it will continue checking till the least priority identifier entry that is defined. As soon as a match is found based on any of the defined criteria, it will update the CI. If no match is found after evaluating all the defined identifier criteria in the order of priority, a new CI will be created in the CMDB.
Reconciliation rules determine which data sources can update CI attributes. Data sources with highest priority determine what an attribute value will be.
If you take the following reconciliation rule as example, it indicates the attributes which are allowed to be populated by ServiceNow discovery and by manual entry for cmdb_ci_hardware. For attributes which are defined as allowed by both the sources (Like Location, Name, Manufacturer), the data populated by ServiceNow discovery will be given more precedence. This means that the manual entry won't be able to override the data populated for these attributes by ServiceNow discovery.
