4. Contacts Introduction: In contacts we are handling bidirectional synchronization of contacts involves comparing any contacts which have changed since the last successful sync and pushing changes to the older contact record. Xero API supports the “If-modified-since” header parameter, which allows you to pull back only the records which are changed since the last successful sync. We establish a foreign key relationship between the contact in our application and the one in Xero. We are accomplish this by storing the…