Since Beaker 0.15 for new client commands we have been adding "private"/undocumented HTTP APIs in the RESTful style, rather than continuing to add more XMLRPC methods. Now that those APIs have been stable for a few releases (and are beginning to be used in spite of their undocumented status) we should document them and make them officially stable.
I think the only ones we have so far in 0.17 are: POST /systems/<fqdn>/loans/ PUT /systems/<fdqn>/loans/+current GET,PUT /systems/<fqdn>/access-policy POST,DELETE /systems/<fqdn>/access-policy/rules/ GET /systems/<fdqn>/status There are a bunch more which will be added for new-system-page (bug 1014438) as well.
As folks are starting to use these now, it would be good to get them properly documented for Beaker 19.
http://gerrit.beaker-project.org/3940
New server API docs are visible here: https://beaker-project.org/docs-release-19/server-api/http.html
Beaker 19.3 has been released.