To simplify the integration Barnebys has an XML Scheme (Download) to be used for validation of the feed.
xmllint --schema schema.xsd doc.xml
Add the XSI namespace to your XML and define the schemaLocation.
<?xml version="1.0" encoding="utf8" ?>
<barnebys version="2.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="https://dev.bbys.io/static/resources/xml-feed/schema.xsd" >
...
</barnebys>