create-a-topic.md

Create a topic

ConsensusTopicCreateTransaction() creates a new topic recognized by the Hedera network. The newly generated topic can be referenced by its topicId. The topicId is used to identify a specific topic to submit messages to. All messages within a topic are sequenced with respect to one another and are provided sequence number.

You can also create a private topic where only authorized parties can submit messages to that topic. To create a private topic you would need to set the submitKey property of the transaction. The submitKey value is then shared with the authorized parties and is required to successfully submit messages to the private topic.

Constructor	Description
ConsensusTopicCreateTransaction()	Initializes the ConsensusTopicCreateTransaction object

new ConsensusTopicCreateTransaction()

Example

{% tabs %} {% tab title="Java" %}

 final TransactionId transactionId = new ConsensusTopicCreateTransaction()
     .execute(client);

{% endtab %}

{% tab title="JavaScript" %}

const transactionId = await new ConsensusTopicCreateTransaction()
    .execute(client);

{% endtab %} {% endtabs %}

Advanced

The adminKey and submitKey on the topic can be any of the following key structures:

• Simple Key: one key has the authority to modify the topic (adminKey) or to submit a message to that topic (submitKey)
• Key List: A list of keys that are all required to sign transactions to modify the properties of a topic (adminKey) or to submit a message to that topic (submitKey)
• Threshold Keys: Requires a minimum of x number of signatures from a total of y signatures to modify the properties of a topic (adminKey) or to submit a message to that topic (submitKey)

Method	Type	Description
setAdminKey(<key>)	PublicKey	The key that has the ability to update or delete the topic. expirationTime can be modified by anyone. If no adminKey is specified, updateTopic may only be used to extend the expirationTime, and deleteTopic is disallowed.
setSubmitKey(<key>)	PublicKey	Access control for submitMessage. If this property is not set, no access control is performed on ConsensusService.submitMessage (all submissions allowed).
setTopicMemo(<memo>)	String	Short publicly visible memo about the topic. No guarantee of uniqueness.
setAutoRenewPeriod(<autoRenewPeriod>)	Duration	The initial lifetime of the topic and the amount of time to attempt to extend the topic's lifetime by (once autoRenew functionality is supported by HAPI)
setAutoRenewAccountId(<autoRenewAccountId>)	AccountId	Optional account to be used at the topic's expirationTime to extend the life of the topic (once autoRenew functionality is supported by HAPI)

Create a topic with a submitKey

Creating a topic with a submitKey creates a private topic. In order to submit messages to a private topic, you will need the submitKey. The submitKey can be a simple key that is shared amongst those who have been granted permission to submit messages to that topic. The submitKey can also be a KeyList or ThresholdKey.

Example

{% tabs %} {% tab title="Java" %}

// Generate a Ed25519 private, public key pair
submitKey = Ed25519PrivateKey.generate();
Ed25519PublicKey submitPublicKey = submitKey.publicKey;

final TransactionId transactionId = new ConsensusTopicCreateTransaction()
    .setMaxTransactionFee(20000000)
    .setTopicMemo("HCS topic with submit key")
    .setSubmitKey(submitPublicKey)// The key that is assigned to the topic and required when submitting messages to this topic
    .execute(hapiClient);

topicId = transactionId.getReceipt(hapiClient).getConsensusTopicId();
System.out.println("Created new topic " + topicId + " with ED25519 submitKey of " + submitKey);

{% endtab %}

{% tab title="JavaScript" %}

const { Client, ConsensusTopicCreateTransaction, Ed25519PrivateKey, Ed25519PublicKey} = require("@hashgraph/sdk");
require("dotenv").config();

async function main() {
    const operatorPrivateKey = process.env.OPERATOR_KEY;
    const operatorAccount = process.env.OPERATOR_ID;

    if (operatorPrivateKey == null || operatorAccount == null) {
        throw new Error("environment variables OPERATOR_KEY and OPERATOR_ID must be present");
    }

    const client = Client.forTestnet();

    client.setOperator(operatorAccount, operatorPrivateKey);

    const submitKey = await Ed25519PrivateKey.generate();
    const submitPublicKey = submitKey.publicKey;

    const transactionId = await new ConsensusTopicCreateTransaction()
        .setTopicMemo("HCS topic with submit key")
        .setSubmitKey(submitPublicKey)
        .execute(client);
    
    const receipt = await transactionId.getReceipt(client); 
    const topicId = receipt.getConsensusTopicId(); 

    console.log(`Created new topic ${topicId} with ED25519 submitKey of ${submitKey}`)

}

main();

{% endtab %} {% endtabs %}