An implementation of Twirp in Crystal.

1. Add the dependency to your shard.yml:

   dependencies:
     twirp:
       github: mloughran/twirp.cr

2. Run shards install

To see an example proto definition, generator, server, and client, see the included example.

Includes a protoc plugin for generating Twirp server and client implementations from a protobuf service definition.

protoc --twirp_crystal_out=src --plugin=bin/protoc-gen-twirp_crystal example.proto

Typically you will also want to generate protobuf message definitions:

protoc --crystal_out=src --plugin=bin/protoc-gen-crystal example.proto

For example:

require "./example.twirp.cr"
require "./example.pb.cr"

client = ExampleService::Client.new("localhost", 8080)

req = HelloWorldRequest.new(name: "twirp")
resp = client.hello_world(req)
puts resp.greeting

I've elected to raise an error (which will be a subclass of Twirp::Error) if a Twirp error response is received, rather that take the ruby approach of returning a result object. Exceptions will also be raised by the HTTP::Client in case of transport errors.

Implement a handler (subclassing the generated abstract class):

class ExampleHandler < ExampleService
  def hello_world(req : HelloWorldRequest) : HelloWorldResponse
    HelloWorldResponse.new(greeting: "Hello #{req.name}")
  end
end

Pass this to a Twirp server (which is itself a HTTP::Handler):

require "twirp/server"

twirp_server = Twirp::Server.new(ExampleHandler.new)

Expose this via a HTTP::Server:

server = HTTP::Server.new(twirp_server)
address = server.bind_tcp 8080
puts "Listening on http://#{address}"
server.listen

To return a Twirp error, raise the appropriate Twirp::Error from your handler. For example:

class ExampleHandler < ExampleService
  def hello_world(req : HelloWorldRequest) : HelloWorldResponse
    raise Twirp::Error::Unauthenticated.new("Sorry!")
  end
end

Any other errors raised by handler code will be caught, logged, and an internal Twirp error will be returned to the client. Exceptions can be captured by passing an exception_handler proc to Twirp::Server.new.

Twirp::Server produces log output when handling handling requests. This can be customised, e.g.:

Twirp::Log.level = Log::Severity::Warn

Install shards and build

shards install
shards build

Verify that example generates cleanly

cd example && make clean generate && cd -

Run server and client:

crystal run example/server.cr
crystal run example/client.cr

There are currently no specs, sorry.

• Martyn Loughran - creator and maintainer

With thanks to https://github.com/jgaskins/grpc for the generator (which is more or less verbatim), and for inspiring the double-macro approach for the service DSL (since I'm still wrapping my head around crystal macros).