How to subscribe to `newBlockHeaders` on local RSK node over websockets?

Question

I'm connecting to RSKj using the following endpoint:

ws://localhost:4444/

... However, I am unable to connect.

Note that the equivalent HTTP endpoint http://localhost:4444/ work for me, so I know that my RSKj node is running properly.

I need to listen for newBlockHeaders, so I prefer to use WebSockets (instead of HTTP).

How can I do this?

Answer 1

RSKj by default uses 4444 as the port for the HTTP transport; and 4445 as the port for the Websockets transport. Also note that the websockets endpoint is not at /, but rather at websocket. Therefore use ws://localhost:4445/websocket as your endpoint.

If you're using web3.js, you can create a web3 instance that connects over Websockets using the following:

const Web3 = require('web3');
const wsEndpoint = 'ws://localhost:4445/websocket';
const wsProvider =
  new Web3.providers.WebsocketProvider(wsEndpoint);
const web3 = new Web3(wsProvider);

The second part of your question can be done using eth_subscribe on newBlockHeaders. Using the web3 instance from above like so:

// eth_subscribe newBlockHeaders
web3.eth.subscribe('newBlockHeaders', function(error, blockHeader) {
  if (!error) {
    // TODO something with blockHeader
  } else {
    // TODO something with error
  }
});