Subscribing to Event Logs

First thing we need to do in order to subscribe to event logs is dial to a websocket enabled Ethereum client. Fortunately for us, Infura supports websockets.

client, err := ethclient.Dial("wss://rinkeby.infura.io/ws")
if err != nil {
  log.Fatal(err)
}

The next step is to create a filter query. In this example we'll be reading all events coming from the example contract that we've created in the previous lessons.

contractAddress := common.HexToAddress("0x147B8eb97fD247D06C4006D269c90C1908Fb5D54")
query := ethereum.FilterQuery{
  Addresses: []common.Address{contractAddress},
}

The way we'll be receiving events is through a Go channel. Let's create one with type of Log from the go-ethereum core/types package.

logs := make(chan types.Log)

Now all we have to do is subscribe by calling SubscribeFilterLogs from the client, which takes in the query options and the output channel. This will return a subscription struct containing unsubscribe and error methods.

sub, err := client.SubscribeFilterLogs(context.Background(), query, logs)
if err != nil {
  log.Fatal(err)
}

Finally all we have to do is setup an continous loop with a select statement to read in either new log events or the subscription error.

for {
  select {
  case err := <-sub.Err():
    log.Fatal(err)
  case vLog := <-logs:
    fmt.Println(vLog) // pointer to event log
  }
}

You'll have to parse the log entries, which we'll learn how to do in the next section.

Full code

Commands

solc --abi Store.sol
solc --bin Store.sol
abigen --bin=Store_sol_Store.bin --abi=Store_sol_Store.abi --pkg=store --out=Store.go

Store.sol

pragma solidity ^0.4.24;

contract Store {
  event ItemSet(bytes32 key, bytes32 value);

  string public version;
  mapping (bytes32 => bytes32) public items;

  constructor(string _version) public {
    version = _version;
  }

  function setItem(bytes32 key, bytes32 value) external {
    items[key] = value;
    emit ItemSet(key, value);
  }
}

event_subscribe.go

package main

import (
    "context"
    "fmt"
    "log"

    "github.com/ethereum/go-ethereum"
    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/core/types"
    "github.com/ethereum/go-ethereum/ethclient"
)

func main() {
    client, err := ethclient.Dial("wss://rinkeby.infura.io/ws")
    if err != nil {
        log.Fatal(err)
    }

    contractAddress := common.HexToAddress("0x147B8eb97fD247D06C4006D269c90C1908Fb5D54")
    query := ethereum.FilterQuery{
        Addresses: []common.Address{contractAddress},
    }

    logs := make(chan types.Log)
    sub, err := client.SubscribeFilterLogs(context.Background(), query, logs)
    if err != nil {
        log.Fatal(err)
    }

    for {
        select {
        case err := <-sub.Err():
            log.Fatal(err)
        case vLog := <-logs:
            fmt.Println(vLog) // pointer to event log
        }
    }
}

$ solc --version
0.4.24+commit.e67f0147.Emscripten.clang