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How to create a metadata transaction using cardano-cli

note

This guide assumes that you have a basic understanding of cardano-wallet and cardano-cli, how to use it and that you have installed it into your system. Otherwise, we recommend reading Installing cardano-node, Running cardano-node, and Exploring Cardano Wallets guides first.

This guide also assumes that you have cardano-node and cardano-wallet running in the background and connected to the testnet network.

Overview​

This article will explore how we can utilize the Transaction Metadata feature of the Cardano blockchain. More specifically, how we can insert and retrieve metadata that we have stored in the blockchain for a decentralized application using Transaction Metadata.

Use case​

Let's imagine a decentralized To-Do List Application that stores and retrieve its metadata not from centralized servers or storage but instead from the Cardano blockchain. First, let's check the diagram below to see how something like this could theoretically work:

img

important

The diagram is heavily simplified and is not recommended to deploy in production.

Here, we see a front-end application the user interacts with and is responsible for inserting and retrieving the metadata to/from the back-end server. The back-end server is responsible for communicating to the cardano-node by creating transactions and querying the blockchain for the metadata information required by the front-end.

The front-end application is not necessarily tied to the specific back-end API and could switch to another API as long as it ultimately communicates to the Cardano network.

How do you actually create transaction metadata in the Cardano blockchain, you ask? We'll let's get our hands dirty!

Setup​

To create a transaction metadata using the cardano-cli, you must first create a payment key-pair and a wallet address if you haven't already.

Create Payment Keys 

cardano-cli address key-gen \
--verification-key-file payment.vkey \
--signing-key-file payment.skey

Create Wallet Address 

cardano-cli address build \
--payment-verification-key-file payment.vkey \
--out-file payment.addr \
--testnet-magic 1097911063

Now that you have a wallet address, you can now request some tAda funds from the testnet faucet.

Once you have some funds, we can now create the sample metadata that we want to store into the blockchain.

We start by creating a metadata.json file with the following content:

{
    "1337": {
        "name": "hello world",
        "completed": 0
    }
}
note

Based on our theoretical To-Do List application, this JSON shape could be a way to insert / update entries into our list. We choose an arbitrary number (1337) as the key; we are basically saying that all metadata that will be inserted with that key is related to the To-Do List application data. Although we don't have control over what will be inserted with that metadata key since Cardano is an open platform.

Now that we have our JSON data, we can create a transaction and embed the metadata into the transaction. Ultimately storing it into the Cardano blockchain forever.

Query UTXO​

The next step is to query the available UTXO from our wallet address:

cardano-cli query utxo --testnet-magic 1097911063 --address $(cat payment.addr)

You should see something like this:

                           TxHash                                 TxIx        Amount
--------------------------------------------------------------------------------------
dfb99f8f103e56a856e04e087255dbaf402f3801acb71a6baf423a1054d3ccd5     0        1749651926 lovelace

Here we can see that our wallet address contains some spendable lovelace with the TxHash: dfb99f8f103e56a856e04e087255dbaf402f3801acb71a6baf423a1054d3ccd5 and TxIndex: 0. We can then use it to pay for the transaction fee when we store our data on the blockchain.

Submit to blockchain​

Next, we create a draft transaction with the metadata embedded into it using the TxHash and TxIndex result from our last query.

cardano-cli transaction build-raw \
--tx-in dfb99f8f103e56a856e04e087255dbaf402f3801acb71a6baf423a1054d3ccd5#0 \
--tx-out $(cat payment.addr)+0 \
--metadata-json-file metadata.json \
--fee 0 \
--out-file tx.draft

Then we calculate the transaction fee like so:

cardano-cli transaction calculate-min-fee \
--tx-body-file tx.draft \
--tx-in-count 1 \
--tx-out-count 1 \
--witness-count 1 \
--byron-witness-count 0 \
--testnet-magic 1097911063 \
--protocol-params-file protocol.json

You should see something like this:

171793 Lovelace

With that, We build the final transaction with the total amount of our wallet minus the calculated fee as the total output amount. 1749651926 - 171793 = 1749480133

cardano-cli transaction build-raw \
--tx-in dfb99f8f103e56a856e04e087255dbaf402f3801acb71a6baf423a1054d3ccd5#0 \
--tx-out $(cat payment.addr)+1749480133 \
--metadata-json-file metadata.json \
--fee 171793 \
--out-file tx.draft

We then sign the transaction using our payment signing key:

cardano-cli transaction sign \             
--tx-body-file tx.draft \
--signing-key-file payment.skey \
--testnet-magic 1097911063 \
--out-file tx.signed

Finally, we submit the signed transaction to the blockchain:

cardano-cli transaction submit \
--tx-file tx.signed \    
--testnet-magic 1097911063

Congratulations, you are now able to submit Cardano transactions with metadata embedded into them. 🎉🎉🎉

Up next, we discuss how to retrieve metadata that we have stored in the Cardano blockchain. @TODO