On librados and libcephfs command functions

Both librados and libcephfs have functions that are named like *_command that generally take json formatted input "commands". Across all the functions the json `{"prefix: "get_command_descriptions"}` appears to work everywhere, returning a json formatted listing of the available commands. Related to this I have a few questions that I have not been able to answer myself just by looking at the sources: There appears to be a distinction between 'tell' commands and those that are not 'tell' commands but I couldn't quite figure it all out on my own. What's the difference and how can I determine which commands in the listing are tell commands or not? Sometimes certain commands appear in more than one listing. An example that is relevant to some upcoming work for the ceph-csi project is subvolume creation for cephfs. By tracing/instrumenting the `ceph` cli tool I can see that (on my system) it sends the command to create the subvolume via mon_command, but the command ("fs subvolume create") appears for both mon_command and mgr_command. I would like some advice about which command function we should prefer to use even if both work. I tend to prefer doing what the cli does, as I assume that gets a lot of real work testing. However, if someone on the list who understands the fundamentals of this system better could provide some advice I'd greatly appreciate it. Somewhat related to the previous item, is it safe to "hard code" the json structure or should we always prefer to dynmically fetch the parameters of the commands via get_command_descriptions like the cli does? What are the risks if a code base were to "hard code" the json for a command? When I was testing the rados_mon_command_target function, I noticed some varying behavior when the server side was running binaries based on the octopus release vs. client libraries from nautilus. When I use nautilus libs to query the octopus-based server things seem to look like many of the other results of "get_command_descriptions". However, using octopus client libs to query the octopus-based server I see output that looks a bit different. The json returned has extra white space and there are odd signatures like "0" and "1". Example: "cmd000": { "sig": [ "0" ], "help": "" }, Poking around I saw that this might be related to the following items: https://github.com/ceph/ceph/pull/30217 https://github.com/ceph/ceph/pull/30859 https://github.com/ceph/ceph/pull/31138 But I couldn't say for sure. The formatting of the JSON does not bother me but the "0", "1", etc signatures don't look right and I'm thinking that it might be a bug but didn't want to file something without a hint that this isn't desired behavior. As a relative newcomer to ceph I'm doing more research into how to use this family of functions works (is expected to work) versus having absorbed institutional knowledge. So, I'd be happy to record some of what I'm learning somewhere more permanent for future folks in my current situation. If I were to contribute documentation about these functions what's the preferred place to do so? I could see this going into the comments in librados.h or somewhere in the sphinx based docs but I'd appreciate a few pointers before I make patches/PRs. FWIW, If there's already docs for it, I've managed to overlook it despite looking a number of times for such a thing so apologies if it is noise. Lastly, and possibly leastly, does the team have a better name/label for this collection of functions/general approach? I've been calling them *_command functions or command json functions. I would be happy to more widely known name for them if one exists. :-) Thanks in advance! --John M.