Subscribing to New Blocks

In this section we'll go over how to set up a subscription to get events when their is a new block mined. First thing is we need an Ethereum provider that supports RPC over websockets. In this example we'll use the infura websocket endpoint.

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

Next we'll create a new channel that will be receiving the latest block headers.

headers := make(chan *types.Header)

Now we call the client's SubscribeNewHead method which takes in the headers channel we just created, which will return a subscription object.

sub, err := client.SubscribeNewHead(context.Background(), headers)
if err != nil {
  log.Fatal(err)
}

The subscription will push new block headers to our channel so we'll use a select statement to listen for new messages. The subscription object also contains an error channel that will send a message in case of a failure with the subscription.

for {
  select {
  case err := <-sub.Err():
    log.Fatal(err)
  case header := <-headers:
    fmt.Println(header.Hash().Hex()) // 0xbc10defa8dda384c96a17640d84de5578804945d347072e091b4e5f390ddea7f
  }
}

To get the full contents of the block, we can pass the block header hash to the client's BlockByHash function.

block, err := client.BlockByHash(context.Background(), header.Hash())
if err != nil {
  log.Fatal(err)
}

fmt.Println(block.Hash().Hex())        // 0xbc10defa8dda384c96a17640d84de5578804945d347072e091b4e5f390ddea7f
fmt.Println(block.Number().Uint64())   // 3477413
fmt.Println(block.Time())              // 1529525947
fmt.Println(block.Nonce())             // 130524141876765836
fmt.Println(len(block.Transactions())) // 7

As you can see, you can read the entire block's metadata fields, list of transactions, and much more.

Full code

block_subscribe.go

package main

import (
    "context"
    "fmt"
    "log"

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

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

    headers := make(chan *types.Header)
    sub, err := client.SubscribeNewHead(context.Background(), headers)
    if err != nil {
        log.Fatal(err)
    }

    for {
        select {
        case err := <-sub.Err():
            log.Fatal(err)
        case header := <-headers:
            fmt.Println(header.Hash().Hex()) // 0xbc10defa8dda384c96a17640d84de5578804945d347072e091b4e5f390ddea7f

            block, err := client.BlockByHash(context.Background(), header.Hash())
            if err != nil {
                log.Fatal(err)
            }

            fmt.Println(block.Hash().Hex())        // 0xbc10defa8dda384c96a17640d84de5578804945d347072e091b4e5f390ddea7f
            fmt.Println(block.Number().Uint64())   // 3477413
            fmt.Println(block.Time())              // 1529525947
            fmt.Println(block.Nonce())             // 130524141876765836
            fmt.Println(len(block.Transactions())) // 7
        }
    }
}

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