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Receiving ZMQ Notifications

Overview

Receiving notifications from Dimecoin Core is important for a variety of use-cases. Although polling RPCs can be useful, in some scenarios it may be more desirable to have publish-subscribe functionality. Dimecoin Core’s built-in ZeroMQ (ZMQ) support provides the ability to subscribe to block, transaction, and governance related messages.

Further information regarding ZMQ support may be found in the ZMQ API Reference.

Enabling Dimecoin Core ZMQ Notifications

Note

This requires a Dimecoin Core full node or masternode

In the dimecoin.conf configuration file, add the following ZMQ notifications and assign the address that Dimecoin Core should listen on. The notifications selected here relate to transaction data.

## ZMQ
zmqpubrawblock=tcp://0.0.0.0:11391
zmqpubhashtx=tcp://0.0.0.0:11391
zmqpubhashblock=tcp://0.0.0.0:11391
zmqpubrawtx=tcp://0.0.0.0:11391

Restart the Dimecoin Core node once the configuration file has been updated.

JavaScript Example

Requires an installation of NodeJS

1. Install ZeroMq

The JavaScript zeromq package is available from npmjs.com and can be installed from the command line by running:

npm install zeromq@5

Attention

Version 5 of the zeromq package should be used for compatibility reasons.

2. Subscribe to ZeroMQ Messages

Create a file with the following contents. Then run it by typing node <your-filename.js> from the command line:

JAVASCRIPT

const zmq = require('zeromq');
const sock = zmq.socket('sub');
const zmqPort = 11391;

sock.connect('tcp://127.0.0.1:' + zmqPort);

// Subscribe to transaction notifications
sock.subscribe('hashtx'); // Note: this will subscribe to hashtxlock also

console.log('Subscriber connected to port %d', zmqPort);

sock.on('message', function(topic, message) {
  console.log(
    'Received',
    topic.toString().toUpperCase(),
    'containing:\n',
    message.toString('hex'),
    '\n'
  );
});

PYTHON

import binascii
import asyncio
import zmq
import zmq.asyncio
import signal

port = 11391

class ZMQHandler():
    def __init__(self):
        self.loop = asyncio.get_event_loop()
        self.zmqContext = zmq.asyncio.Context()

        self.zmqSubSocket = self.zmqContext.socket(zmq.SUB)
        self.zmqSubSocket.connect("tcp://127.0.0.1:%i" % port)

        ## Subscribe to transaction notifications
        self.zmqSubSocket.setsockopt_string(zmq.SUBSCRIBE, "hashtx")

        print('Subscriber connected to port {}'.format(port))

    @asyncio.coroutine
    def handle(self) :
        msg = yield from self.zmqSubSocket.recv_multipart()
        topic = msg[0]
        body = msg[1]
        sequence = "Unknown"

        print('Received {} containing:\n{}\n'.format(
            topic.decode("utf-8"), 
            binascii.hexlify(body).decode("utf-8")))

        ## schedule ourselves to receive the next message
        asyncio.ensure_future(self.handle())

    def start(self):
        self.loop.add_signal_handler(signal.SIGINT, self.stop)
        self.loop.create_task(self.handle())
        self.loop.run_forever()

    def stop(self):
        self.loop.stop()
        self.zmqContext.destroy()

daemon = ZMQHandler()
daemon.start()

Example Response

The following response demonstrates the notification provided by Dimecoin Core when it receives a transaction and then receives the associated messages. The four notifications represent:

1. The TXID of the transaction is received (HASHTX)

2. When a block is discovered or mined, the (HASHBLOCK) is sent over. This allows external applications or services listening to react almost instantly to new blocks.

3. When a transaction is seen by a node, the (RAWTX) sends the full raw transaction data to subscribers.

4. The (RAWBLOCK) data could decoded using the decoderawtransaction RPC for example)

Received HASHTX containing:
 b2e128661e3679c3d00cd081e32fdc9a12f44e486e083e6eab998bdfd6f64a9b

Received HASHBLOCK containing:
 b2e128661e3679c3d00cd081e32fdc9a12f44e486e083e6eab998bdfd6f64a9b

Received RAWTX containing:
 02000000025a4d18da609107e9ea3dc6 ... 5a32ea917a30147d6c9788ac6ea90400

Received RAWBLOCK containing:
 02000000025a4d18da609107e9ea3dc6 ... 9e889cee7ba48981ca002e6962a20236