<p>I'm moving an Apache Mesos application, where my configurations are based in JSON, to Kubernetes, where my configurations are based in YAML. Would the JSON configuration files work as a YAML file since YAML is a superset of JSON, or would I need to write a new YAML file?</p>

<p>Yes, JSON works as well, it's just more pain than YAML to write it, manually. Also, you may be able to use <a href="https://github.com/micahhausler/container-transform" rel="nofollow noreferrer">micahhausler/container-transform</a> to convert your Marathon specs to Kubernetes specs.</p>
