HCS: Submit your first message!


With the Hedera Consensus Service you can develop applications like stock markets, audit logs, stable coins, or new network services that require high throughput and decentralized trust. This is made possible by having direct access to the native speed, security, and fair ordering guarantees of the hashgraph consensus algorithm, with the full trust of the Hedera ledger.

Components of the Hedera Consensus Service/Terminology

  • Hedera client - a Hedera client sends transactions to a Hedera network node for consensus. The corresponding transaction types for the Hedera Consensus Service include create a topic, update a topic, submit messages, delete a topic, and get the info for a topic.

  • Hedera network node - receives transactions from a client and submits it to the Hedera network for consensus

  • Mirror node client - a mirror node client is used to subscribe to a topic and get messages that are in consensus order from a mirror node.

  • Mirror node: Mirror nodes receive information from Hedera network consensus nodes, but do not participate in consensus themselves. You can get more information about mirror nodes here.

  • Topic - a topic is the subject of information you would like to send messages to and what clients would subscribe to

  • Message - a message is the content published to the Hedera network

    to a topic which gets placed in consensus order

  • Subscriber - a client that subscribes to a desired topic in order to receive the appropriate messages

  • Publisher - publishes messages to a topic

HCS Flow

Create a topic by submitting a transaction from the Hedera client to a Hedera network node āž” Mirror node client subscribes to the topic from mirror node āž” Publish a message to a topic by submitting a transaction from the Hedera client to a Hedera network node for consensus āž” Mirror node client receives messages published to the topic from the mirror node

Getting Started with Hedera Consensus Service (Testnet)

What you will need to be successful with this tutorial:

šŸ’„ Testnet account ID and associated private key šŸ’„ Hedera Java SDK šŸ’„ Mirror node host:port šŸ’„ IDE of your choice

1. Get a testnet account

  • You will need a testnet account ID and private key to use HCS. If you do not already have one, visit the Hedera portal to create your profile and receive your testnet account ID. A friend already on testnet could also generously create one for you.

2. Get access to a mirror node

  • You will need a mirror node's host:port information so that you can subscribe to topics and receive the associated messages. If you do not have access to a mirror node, you can use the mirror node endpoints managed by Hedera here.

3. Create a new maven project in your favorite IDE

  • Add the following dependencies to your pom.xml file

<!-- SELECT ONE: -->
<!-- netty transport (for server or desktop applications) -->
<!-- netty transport, unshaded (if you have a matching Netty dependency already) -->
<!-- okhttp transport (for lighter-weight applications or Android) -->

4. Set-up your environment variables

  • Create a .env file in the projects root directory

  • Add the following:

    • OPERATOR_ID: Your testnet account ID goes here

    • OPERATOR_KEY: Your testnet account ID private key goes here

  • Add the following:

    • MIRROR_NODE_ADDRESS: Insert the mirror node host:port information here

  • Your environment set-up is now complete

Sample .env file:

# Operator ID and Key
# Mirror Node Address

5. Create a new HCS class

  • Create a new class and title it something like HederaConsensusService.java

6. Connect to the Hedera testnet

  • Here we are going to connect to the Hedera testnet and and set the operator information with your testnet account ID and private key. The operator is responsible to pay transaction fees and sign all transactions that will be generated in this tutorial. Luckily, this is testnet so you will have unlimited hbars to use in this development environment!

// Grab the OPERATOR_ID and OPERATOR_KEY from the .env file
private static final AccountId OPERATOR_ID = AccountId.fromString(Objects.requireNonNull(Dotenv.load().get("OPERATOR_ID")));
private static final Ed25519PrivateKey OPERATOR_KEY = Ed25519PrivateKey.fromString(Objects.requireNonNull(Dotenv.load().get("OPERATOR_KEY")));
// Build Hedera testnet client
Client client = Client.forTestnet();
// Set the operator account ID and operator private key
client.setOperator(OPERATOR_ID, OPERATOR_KEY);

7. Connect to a Hedera mirror node

// Grab the mirror node endpoint from the .env file
private static final String MIRROR_NODE_ADDRESS = Objects.requireNonNull(Dotenv.load().get("MIRROR_NODE_ADDRESS"));
// Build the mirror node client
final MirrorClient mirrorClient = new MirrorClient(MIRROR_NODE_ADDRESS);

8. Create your first topic!

  • To create you first topic, we will use the CreateTopicTransaction constructor, set its properties, and submit it to the Hedera network

  • You will want to grab the topic ID so later you can subscribe to that topic via the mirror node client

  • Topic IDs are in the following format: 0.0.10

//Create a new topic
final TransactionId transactionId = new ConsensusTopicCreateTransaction()
//Grab the newly generated topic ID
final ConsensusTopicId topicId = transactionId.getReceipt(client).getConsensusTopicId();
System.out.println("Your topic ID is: " +topicId);

9. Subscribe to a topic

  • Now we will shift our attention the mirror node client and subscribe to the topic created

  • We will achieve this by referencing the topics topicId

  • We will also print the consensus timestamp and message to the console

new MirrorConsensusTopicQuery()
.subscribe(mirrorClient, resp -> {
String messageAsString = new String(resp.message, StandardCharsets.UTF_8);
System.out.println(resp.consensusTimestamp + " received topic message: " + messageAsString);
// On gRPC error, print the stack trace

10. Submit a message to a topic

  • Now that we have created a topic and subscribed to that topic, we are ready to submit a message using the ConsensusSubmitTransaction constructor and submit to the Hedera network

  • The below example will submit a message with the message as "hello, HCS!"

//Submit a message to a topic
new ConsensusMessageSubmitTransaction()
.setMessage("hello, HCS! ")
  • If you have successfully followed the steps in this tutorial, you should see the following print to your console šŸ¤© :

2020-01-17T09:01:03.990648Z received topic message: hello, HCS!

NOTE: It may take 10-15 seconds before the message appears on your console from the mirror node.

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