Sep 4, 2016

Asynchronous RPC server with GRPC

After a severe disappointment with the state of the Thrift RPC framework I was looking forward to the first stable release of GRPC. Of the original big three only the protobuf-based ecosystem seems to be still evolving enough to be a viable choice. With the arrival of v1.0 it is finally ready for production.

For better or worse despite using a binary transport protocol GRPC forces things with "HTTP" in their names upon us. That is unfortunate because a protocol designed for connecting to browsers outside of the data center will incur some penalties in complexity and probably latency on backend services not exposed to the public Internet. 

The question I want to investigate today is pretty much the same I had playing with Thrift last year. What does it take to configure and run a backend service exposing multiple end points with no blocking calls. Spoiler alert: it's so trivial I should probably double-check my understanding of the threading model to confirm GRPC is as reactive as it seems to be :)

To begin with I would like to illustrate a typical GRPC call. Let's assume we have a service API with one method. GRPC uses protobuf3 as its IDL and, other than not having required fields anymore, I find it hard to see any difference from protobuf2 or even Thrift. 

The first pleasant surprise is that a single maven plugin with a simple, mostly OS-specific configuration is all it takes to compile IDL into Java classes. For each XXX service you basically end up with two classes:
  • xxxProto - request and response classes
  • xxxGrpc - request handler base class and client-side factory method for different IO styles

The second pleasant surprise is how uniform server-side API is. When processing a request you are given an Observer with the same onNext/onComplete/onError methods used for all four currently supported RPC types. On the client side it's even simpler, all you need is a request instance and a guava callback instance.

  • a Channel is created for a TCP destination
  • a service stub using the new Channel is obtained
  • a callback instance is created to process server response
  • an RPC method is called on the stub
  • a mirror copy method is called on the request handler but with an observer instead of a callback
  • the handler calls either onNext/onSuccess or onError observer methods to finish processing
  • once the server response is received, GRPC invokes the callback on a thread from a pre-configured executor

Server-side initialization is equally terse. You give it a handler instance for each service end point, an executor, and a port to bind to. Four lines of code and you are in business.

In my example I created two nearly identical protobuf files to describe two service end points. There are two abstractions to establish a connection on the client-side and bootstrap the server on the server side. I followed the same CompletableFuture-based approach to implementing request handlers as discussed in the Thrift post. A unit test wires everything together and calls both end points in parallel.

So far so good. But what ointment would be without a fly? There are a couple of things that I either don't understand or need more time to sort out. And at least one of them is probably necessary for production deployments. 

A trivial note is that GRPC is only a second library I am aware of that depends on JUL (Parquet being the first but only temporarily).

One odd question I still have is how asynchronous Java GRPC implementation really is. What confuses me is that I can see an "asynchronous basics tutorial" for C++ only. Is it some special kind of asynchronicity attainable only by those who manage memory manually? Or is there some blocking still left in Java GRPC library?

A real question is the complexity of GRPC security. My example follows the lead of GRPC Java tutorial when it calls "usePlaintext()" on channel builder. At first glance I am not even sure if SSL/TLS is necessary for the traffic inside of the data center or whether AWS ELB could interfere with it. A topic for another day I guess.

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