How to write a Ruby gRPC client for the Lightning Network Daemon

This section enumerates what you need to do to write a client that communicates with lnd in Ruby.

Introduction

lnd uses the gRPC protocol for communication with clients like lncli.

gRPC is based on protocol buffers and as such, you will need to compile the lnd proto file in Ruby before you can use it to communicate with lnd.

Setup

Install gRPC rubygems:

$ gem install grpc
$ gem install grpc-tools

Clone the Google APIs repository:

$ git clone https://github.com/googleapis/googleapis.git

Fetch the rpc.proto file (or copy it from your local source directory):

$ curl -o rpc.proto -s https://raw.githubusercontent.com/lightningnetwork/lnd/master/lnrpc/rpc.proto

Compile the proto file:

$ grpc_tools_ruby_protoc --proto_path googleapis:. --ruby_out=. --grpc_out=. rpc.proto

Two files will be generated in the current directory:

rpc_pb.rb
rpc_services_pb.rb

Examples

Simple client to display wallet balance

Every time you use the Ruby gRPC you need to require the rpc_services_pb file.

We assume that lnd runs on the default localhost:10009.

We further assume you run lnd with --no-macaroons.

#!/usr/bin/env ruby

$:.unshift(File.dirname(__FILE__))

require 'grpc'
require 'rpc_services_pb'

# Due to updated ECDSA generated tls.cert we need to let gprc know that
# we need to use that cipher suite otherwise there will be a handhsake
# error when we communicate with the lnd rpc server.
ENV['GRPC_SSL_CIPHER_SUITES'] = "HIGH+ECDSA"

certificate = File.read(File.expand_path("~/.lnd/tls.cert"))
credentials = GRPC::Core::ChannelCredentials.new(certificate)
stub = Lnrpc::Lightning::Stub.new('127.0.0.1:10009', credentials)

response = stub.wallet_balance(Lnrpc::WalletBalanceRequest.new())
puts "Total balance: #{response.total_balance}"

This will show the total_balance of the wallet.

Streaming client for invoice payment updates

#!/usr/bin/env ruby

$:.unshift(File.dirname(__FILE__))

require 'grpc'
require 'rpc_services_pb'

ENV['GRPC_SSL_CIPHER_SUITES'] = "HIGH+ECDSA"

certificate = File.read(File.expand_path("~/.lnd/tls.cert"))
credentials = GRPC::Core::ChannelCredentials.new(certificate)
stub = Lnrpc::Lightning::Stub.new('127.0.0.1:10009', credentials)

stub.subscribe_invoices(Lnrpc::InvoiceSubscription.new) do |invoice|
  puts invoice.inspect
end

Now, create an invoice on your node:

$ lncli addinvoice --amt=590
{
	"r_hash": <R_HASH>,
	"pay_req": <PAY_REQ>
}

Next send a payment to it from another node:

$ lncli sendpayment --pay_req=<PAY_REQ>

You should now see the details of the settled invoice appear.

Using Macaroons

To authenticate using macaroons you need to include the macaroon in the metadata of the request.

# Lnd admin macaroon is at ~/.lnd/data/chain/bitcoin/simnet/admin.macaroon on Linux and
# ~/Library/Application Support/Lnd/data/chain/bitcoin/simnet/admin.macaroon on Mac
macaroon_binary = File.read(File.expand_path("~/.lnd/data/chain/bitcoin/simnet/admin.macaroon"))
macaroon = macaroon_binary.each_byte.map { |b| b.to_s(16).rjust(2,'0') }.join

The simplest approach to use the macaroon is to include the metadata in each request as shown below.

stub.get_info(Lnrpc::GetInfoRequest.new, metadata: {macaroon: macaroon})

However, this can get tiresome to do for each request. We can use gRPC interceptors to add this metadata to each request automatically. Our interceptor class would look like this.

class MacaroonInterceptor < GRPC::ClientInterceptor
  attr_reader :macaroon

  def initialize(macaroon)
    @macaroon = macaroon
    super
  end

  def request_response(request:, call:, method:, metadata:)
    metadata['macaroon'] = macaroon
    yield
  end

  def server_streamer(request:, call:, method:, metadata:)
    metadata['macaroon'] = macaroon
    yield
  end
end

And then we would include it when we create our stub like so.

certificate = File.read(File.expand_path("~/.lnd/tls.cert"))
credentials = GRPC::Core::ChannelCredentials.new(certificate)
macaroon_binary = File.read(File.expand_path("~/.lnd/data/chain/bitcoin/simnet/admin.macaroon"))
macaroon = macaroon_binary.each_byte.map { |b| b.to_s(16).rjust(2,'0') }.join

stub = Lnrpc::Lightning::Stub.new(
	'localhost:10009',
	credentials,
	interceptors: [MacaroonInterceptor.new(macaroon)]
)

# Now we don't need to pass the metadata on a request level
p stub.get_info(Lnrpc::GetInfoRequest.new)

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