Remote Procedure Call

2017/10/11

Message Exchange

So a server written in Go and destined to provide authentication information must be accessible by some way. Otherwise it won’t be particularly useful. We could use HTTP and REST requests to push and pull JSON to the server, but this has a lot of problems. For one it requires a JSON parser for any language that wants to talk to NetAuth, and it requires the server to understand JSON. I’m not particularly fond of JSON and its not the easiest interchange format to do things with.

So if JSON is out, what other formats are available? Well, there’s always XML, but as mentioned in the first post the goal is to build something that feels like its from the Sun days, and XML does not feel like that at all.

There’s always the option of a totally custom format, or of a custom binary format such as a binary packed string. But these options are frustrating to implement and take away from what we’re building which is an auth server. If I wanted to build an interchange format I would go do that seperately. Go has excellent support for Protocol Buffers. Protocol Buffers, or protobuf as they’re more widely known, are Google’s data interchange format and support a few very useful things.

First, protobuf supports reading older message versions transparently. Second, it supports serialization to a binary format for line transmission efficiency. For those that must have JSON, protobuf encodes the required information to be able to get to JSON. Perhaps most importantly, protobuf can generate headers for a myriad of languages so that the serialization and deserialization of the messages is done consistently.

RPC

The messages shall be protobuf, but how shall they be transferred? Go has excellent web support, so they could be sent as the body of POST requests to a remote server. This would allow everything to run over HTTP and pass over annoyingly oversensitive firewalls, but this isn’t really a clean way to do it. Really, what we want to do is ask the server to run some arbitrary function with some data that we have, then return some more data back to us. Basically we want to remotely run functions on the server.

Fortunately, Sun had this figured out quite some time ago and built NFS on it. Remote Procedure Call, RPC, is a design paradigm where the caller is on one system and through a well defined and agreed upon standard will send a request to a server with some packed data for execution, and will then get results back as a result from the call.

Golang has an RPC system within the net/rpc package, but this requires doing a lot of manual processing and futzing around with setting up a server to handle the requests. It would be much easier if there was just a library that did this already so we could get on with the business of writing an authentication server.

gRPC

Well that was easy. We’ll use gRPC. gRPC is an RPC implementation designed by Google and using protobuf as the interface defenition language (IDL). Unlike Sun’s RPC though, gRPC uses HTTP/2 as the transport. For those not experienced with SunRPC, this is means that the port number of the server is a fixed value. In Sun’s system, the port was a randomly allocated value that you needed to query another service to find. In addition to being annoying, this made it virtually impossible to write a clean and easy to understand firewall ruleset for Sun RPC servers. Using HTTP/2 as the transport also means that we get the firewall crossing capabilities of a well understood and deployed protocol for free!

Perhaps one of the more important things that gRPC buys though is that since it generates stubs for the service like protobuf generates stubs for the types, it can generate stubs for languages other than Go. There is no intent right now to build a client library in anything other than Go, but the fact that it is available as an option is important and should not be overlooked.