The PRH Handler is used in the 5G PNF Plug and Play use case.

https://docs.onap.org/projects/onap-integration/en/latest/docs_5g_pnf_pnp.html 

In 5G RAN, there can be multiple PNFs (RUs/DUs) inside a GNB service model. When this service is instantiated by SO Macro Flow, the PNFs must send the PNF Registration event to PRH in the same order as specified in the SO instantiation request, otherwise their registration event is dropped. 

This is because PRH is stateless, and when it receives a PNF Registration event , it checks if the PNF of the same source name as event has been registered by SO in AAI. If not found, it drops the event.

This presents a problem because in a GNB, DU/RUs cannot be enforced to start up in a particular order. There should be a way that a PNF can send the event before SO registers the PNF in AAI, and the event is stored by PRH until the SO registration is done. An age limit can be set on the event so that it is not stored forever.

The detailed problem and solution is explained in the attachment.