From the documentation:
> Nomad HCL is parsed in the command line and sent to Nomad in JSON format via the HTTP API.
So why not just JSON or even JSON at all and not MsgPack or just straight up HCL because that's over and over introduced as being machine readable and human friendly both at the same time?
JSON is fine for serialization, but I hate typing it out. There are too many quotes and commas - all keys and values have to be quoted. The last item in a list or object can't have a trailing comma, which makes re-ordering items a pain. Comments aren't supported (IMO the biggest issue).
YAML is too whitespace dependent. This is fiddly and makes copy-pasting a pain. I'm also not sure how it affects serialization, like if you want to send yaml over the network, do you also have to send all of the whitespace? That sounds like a pain. OTOH I like that quotes are usually optional and that all valid json is also valid YAML. Comments are a godsend.
HCL has the same basic structure of objects and lists, but it's much more user friendly. Keys don't need to be quoted. Commas aren't usually needed unless you compress items onto the same line. It supports functions, list/object comprehensions, interpolations, and variable references which all lead to more powerful and DRY configuration. Granted I'm not sure if these are specific to TF's HCL implementation, but I hope not.
For serialization, HCL doesn't have any advantage over JSON. Sure it's machine-readable but probably much harder to write code that works on HCL specifically than to convert to JSON and use one of the zillions of JSON libraries out there.
HCL requires a lot more code to parse and many more resources to keep in memory vs JSON. I think it completely makes sense to do it this way. K8s is the same. On the server it does everything in JSON. Your YAML gets converted prior to sending to K8s.
This means you get to build upon what you already have, instead of doing everything from scratch again because your new infrastructure-layer service needs to integrate with 5 different APIs that have slightly different structure.
We already had something that just ran containers and that was Mesos. They had the opinion that all the stuff like service discovery could be implemented and handled by other services like Marathon. But it did not work well. Most people that was to deploy containers in an orchestrated manner want service discovery.
At least these parts are being provided by the same company (Hashicorp) so it probably won't suffer the same lack of coordination between separate projects that Mesos did.
The benefit to the kitchen sink opinionated framework that K8s does is that your deployments and descriptors are not very difficult to understand and can be shared widely. I do not think the comparison of massively sharded NoSQL to Postgres is the same because most people will not need massive sharding, but almost everyone is going to need the service discovery and other things like secrets management that K8s provides.