A few months back I wrote a three part article about esxcli (part1,part2,part3) and mentioned that no APIs existed. I recently discovered while working on automating ESXi 4.1 installations that this was not the case. I noticed an interesting module called "EsxCLI" that was being loaded while watching the boot logs and that the format of the string match those of the managed object reference names found in vSphere API.
As you can see, all six current namespaces are implemented within the vSphere API with a new managed object called "EsxCLI" just like the esxcli utility:
These APIs of course are hidden and have not been exposed in any of the vSphere SDKs (vSphere SDK for Perl, PowerCLI, VI Java, etc), however, you can access them via the vSphere MOB. As far as I can tell, this is new with vSphere 4.1 and was not available with vSphere 4.0 (unconfirmed).
To access these API methods, you just need to open up a browser and use either an ESX or ESXi host and generate the following url (https://[esx_or_esxi_server]/mob/?moid=ha-cli-handler-vms-vm) and appending one of the above managed object path that begin with ha-
Here is an example of using esxcli's VM namespace:
We can invoke one of the two supported methods for this namespace. We'll query for any available running VMs by just clicking on the method which will open up a new window. You will now click on the "Invoke Method" as it does not require any additional paramters:
You can see this output matches that of esxcli:
I heard that esxcli API would eventually be exposed but may not be available for public consumption, it looks this is not entirely true. VMware, why are you holding out on us? Make your APIs public!