OpenAIRE assigns internal identifiers for each object it collects.
By default, the internal identifier is generated as `sourcePrefix::md5(localId)` where:
*`sourcePrefix` is a namespace prefix of 12 chars assigned to the data source at registration time
*`localid` is the identifier assigned to the object by the data source
After years of operation, we can say that:
*`localId` are unstable
* objects can disappear from sources
* PIDs provided by sources that are not PID agencies (authoritative sources for a specific type of PID) are often wrong (e.g. pre-print with the DOI of the published version, DOIs with typos)
Therefore, when the record is collected from an authoritative source:
* the identity of the record is forged using the PID, like `pidTypePrefix::md5(lowercase(doi))`
* the PID is added in a `pid` element of the data model
When the record is collected from a source which is not authoritative for any type of PID:
* the identity of the record is forged as usual using the local identifier
* the PID, if available, is added as `alternateIdentifier`
Currently, the following data sources are used as "PID authorities":
OpenAIRE also perform duplicate identification (see the [dedicated section for details](../../data-provision/deduplication/)).
All duplicates are **merged** together in a **representative record** which must be assigned a dedicated OpenAIRE identifier (i.e. it cannot have the identifier of one of the aggregated record).
