HCS: Topic

Hello World sequence: Create a new topic Hedera Consensus Service (HCS), and publish a message to this topic.

What you will accomplish

• Create a new Topic on HCS

• Publish a new message to this Topic

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Prerequisites

Before you begin, you should have completed the following Hello World sequence:

• Create and fund account

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Get started: Set up project

To follow along, start with the main branch, which is the default branch of the repo. This gives you the initial state from which you can follow along with the steps as described in the tutorial.

You should already have this from the "Create and Fund Account" sequence. If you have not completed this, you are strongly encouraged to do so.

Alternatively, you may wish to create a .env file and populate it as required.

In the terminal, from the hello-future-world directory, enter the subdirectory for this sequence.

cd 06-hcs-topic/

Reuse the .env file by copying the one that you have previously created into the directory for this sequence.

cp ../00-create-fund-account/.env ./

Check that you have copied the `.env` file correctly

To do so, use the pwd command to check that you are indeed in the right subdirectory within the repo.

pwd

This should output a path that ends with /hello-future-world/06-hcs-topic. If not, you will need to start over.

/some/path/hello-future-world/06-hcs-topic

Next, use the ls command to check that the .env file has been copied into this subdirectory.

ls -a

The first few line of the output should look display .env. If not, you'll need to start over.

.
..
.env

Next, install the dependencies using npm.

npm install

Then open the script-hcs-topic.js file in a code editor, such as VS Code.

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Write the script

An almost complete script has already been prepared for you, script-hcs-topic.js, and you will only need to make a few modifications (outlined below) to run successfully.

Step 1: Create a topic

To create a new HCS topic, submit a TopicCreateTransaction, from the Hedera SDK. This transaction does not need any input parameters.

script-hcs-topic.js

    const topicCreateTx = await
        new TopicCreateTransaction()
        .freezeWith(client);

Note that once this transaction has been completed, it is important to extract the topicID from it, as you will need this to submit messages to it. This has already been done, and no modification is necessary.

script-hcs-topic.js

    const topicId = topicCreateTxReceipt.topicId;

Step 2: Publish message to topic

Once the topic has been registered, you're ready to submit messages to it. Messages may be any data up to 1024 bytes.

To do so, submit a TopicMessageSubmitTransaction from the Hedera SDK. This transaction needs 2 input parameters, topicId and message. Use the topicId obtained from the receipt of the TopicCreateTransaction submitted earlier. For message, use a string Hello HCS - followed by your name (or nickname). For example, if you used bguiz, the string should be Hello HCS - bguiz.

script-hcs-topic.js

    const topicMsgSubmitTx = await
        new TopicMessageSubmitTransaction({
            topicId: topicId,
            message: 'Hello HCS - bguiz',
        })
        .freezeWith(client);

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Run the script

In the terminal, run the script using the following command:

node script-hcs-topic.js

You should see output similar to the following:

Topic created. Waiting a few seconds for propagation...
accountId: 0.0.3643569
topicId: 0.0.3791978
topicExplorerUrl: https://hashscan.io/testnet/topic/0.0.3791978
topicCreateTxId: 0.0.3643569@1711530800.714890962
topicMsgSubmitTxId: 0.0.3643569@1711530810.343147411
topicMessageMirrorUrl: https://testnet.mirrornode.hedera.com/api/v1/topics/0.0.3791978/messages/1

To verify that both the TopicCreateTransaction and TopicMessageSubmitTransaction have worked, check Hashscan (the network explorer). To do so, open topicExplorerUrl in your browser and check that:

HCS topic in Hashscan, with annotated items to check.

• The topic exists, and its topic ID matches topicId output by the script. (1)

• There is one entry in the topic, and its message is Hello HCS - followed by your name/ nickname. (2)

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Complete

Congratulations, you have completed the Hedera Consensus Service Hello World sequence! 🎉🎉🎉

You have learned how to:

• Create a new Topic on HCS

• Publish a new message to this Topic

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Next Steps

Now that you have completed this Hello World sequence, you have interacted with Hedera Consensus Service (HCS). There are other Hello World sequences for Hedera Smart Contract Service (HSCS), Hedera File Service (HFS), and Hedera Token Service (HTS), which you may wish to check out next.

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Cheat sheet

Skip to final state

The repo, github.com/hedera-dev/hello-future-world, is intended to be used alongside this tutorial.

To skip ahead to the final state, use the completed branch. You may use this to compare your implementation to the completed steps of the tutorial.

git fetch origin completed:completed
git checkout completed

Alternatively, you may view the completed branch on Github: github.com/hedera-dev/hello-future-world/tree/completed/06-hcs-topic

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Writer: Brendan