Salesforce is configured to call this API when a contact update modifies a relevant field (check the salesforce outbound message called CEX Address Change_Fulfilment for the specific triggering rules).
The incoming messages from salesforce are in XML format and contain a list of ids of recently modified contacts.
This app parses the received XML and for each contact Id it puts a json message in an sqs queue called salesforce-outbound-messages-[STAGE].
Messages in this queue will be picked up by membership workflow to update the related zuora and identity records accordingly.