How to communicate with the JSON RPC server programmatically using Go?

Question

Is there a way to interact with the JSON RPC server directly in Go?

I tried the following:

1. Starting the RPC Server with "geth --rpc"

2. Running the following code:

   package main
   
   import (
       "fmt"
   
       "github.com/ethereum/go-ethereum/rpc"
       "github.com/ethereum/go-ethereum/rpc/comms"
   )
   
   func main() {
   
       client, err := comms.ClientFromEndpoint("rpc:127.0.0.1", 0)
       if err != nil {
           panic(err)
       }
   
       xeth := rpc.NewXeth(client)
   
       type p interface {
       }
   
       response := make(map[string]interface{}, 0)
       var params []interface{}
       response, err = xeth.Call("eth_gasPrice", params)
   
       if err != nil {
           panic(err)
       }
   
       fmt.Println(response)
   }
   

The connection seems to work, but I get the following error:

panic: interface conversion: interface {} is string, not map[string]interface {}

goroutine 1 [running]:
panic(0x48c3d00, 0xc820098380)
    /usr/local/go/src/runtime/panic.go:481 +0x3e6
github.com/ethereum/go-ethereum/rpc.(*Xeth).Call(0xc8201bdc80, 0x49ee5f0, 0xc, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
    /Users/tim/Documents/go_workspace/src/github.com/ethereum/go-ethereum/rpc/xeth.go:72 +0x4dc
main.main()
    /Users/tim/Documents/go_workspace/src/eth_test/main.go:23 +0x12e
exit status 2

If I change the "Call" method here: https://github.com/ethereum/go-ethereum/blob/master/rpc/xeth.go, I can change the type assertion to the right type and I get the correct result back.

Apart from that, the comments in the linked file (xeth.go) tell that it is an interface to a remote node, I however want an interface to a local node.

I figured that it must be possible to first start the RPC server (that is otherwise started with "geth --rpc" on the console) purely programmatically since the console command will call Go methods after all and second it should be possible to send RPC requests using GO.

What am I doing wrong here?

Answer 1

You might take a look at a simplistic approach we took for etherapis: https://github.com/etherapis/etherapis/blob/master/etherapis/geth/api.go

However there's an RPC client in the works that should already support subscriptions too. Not sure when Felix will open his PR with it though.

Answer 2

I don't have the solution for your code because I haven't looked into using the go-ethereum internals to do RPC calls but I've made a very small rpc client library you can find on my github that uses gorequest and simplejson ( I'm no go expert ;) )

There's also the "main" file where I started to wrap the calls by going to a more well defined api, you can check how the simplest method is implemented here, I basically just call the mini-lib with the right method and arguments, in this case none, cast the value and return it.

Also this will get you the code and work straight away if your geth has the --rpc enabled and its on the default port:

go get github.com/makevoid/web3_go

cd into the directory and then run:

go run web3_go.go

Hope this helps, tell me if you have problems in running it.