TxMetadata

Pre-requisites

Transaction Metadata

Since the Shelley era, the Cardano blockchain allows user-defined data to be associated with transactions.

The metadata hash is part of the transaction body, so is covered by all transaction signatures.

The cardano-wallet API server uses a JSON representation of transaction metadata, isomorphic to the binary encoding on chain.

⚠️ WARNING ⚠️

Please note that metadata provided in a transaction will be stored on the blockchain forever. Make sure not to include any sensitive data, in particular personally identifiable information (PII).

Metadata structure on chain

The top level is a map from metadata keys to metadata values.

Metadata keys are integers in the range 0 to 264 - 1.

Metadata values are one of three simple types or two compound types.

Simple types:

  • Integers in the range -(264 - 1) to 264 - 1

  • Strings (UTF-8 encoded)

  • Bytestrings

Compound types:

  • Lists of metadata values

  • Mappings from metadata values to metadata values

Note that lists and maps need not necessarily contain the same type of metadata value in each element.

Limits

  • Strings may be at most 64 bytes long when UTF-8 encoded.
  • Unencoded bytestrings may be at most 64 bytes long (i.e. at most 128 hex digits).
  • There are no limits to the number of metadata values, apart from the protocol limit on transaction size.

The string length limitation is explained in the Delegation Design Spec, section E.3:

The size of strings in the structured value is limited to mitigate the problem of unpleasant or illegal content being posted to the blockchain. It does not prevent this problem entirely, but it means that it is not as simple as posting large binary blobs.

JSON representation in cardano-wallet

The top level is a JSON object mapping metadata keys as decimal number strings to JSON objects for metadata values.

Every metadata value is tagged with its type, using a JSON object, like this:

  • Integers

    { "int": NUMBER }

  • Strings

    { "string": STRING }

  • Bytestrings

    { "bytes": HEX-STRING }

    The value must be base16-encoded (a hex string).

  • Lists of metadata values

    { "list": [ METADATA-VALUE, ... ] }

  • Mappings from metadata values to metadata values

    { "map": [{ "k": METADATA-VALUE, "v": METADATA-VALUE }, ... ] }

Examples

This is a transaction metadata which contains four values. 

{
  "64": {"string": "some text"},
  "32": {"int": 42},
  "16": {
    "map": [
      {
        "k": {"string": "numbers"},
        "v": {"list": [{"int": 1}, {"int": 2}, {"int": 4}, {"int": 8}]}
      },
      {
        "k": {"string": "alphabet"},
        "v": {
          "map": [
            {"k": {"string": "A"}, "v": {"int": 65}},
            {"k": {"string": "B"}, "v": {"int": 66}},
            {"k": {"string": "C"}, "v": {"int": 67}}
          ]
        }
      }
    ]
  },
  "8": { "bytes": "48656c6c6f2c2043617264616e6f21" }
}

Sample code: Converting from JavaScript objects

Use a function like this to translate arbitrary JavaScript values into metadata JSON format. If your application requires a more precise mapping, it can be modified to suit. Note that this code does not validate strings for length. 

#!/usr/bin/env node

function txMetadataValueFromJS(jsVal) {
  if (Array.isArray(jsVal)) {
    // compound type - List
    // (note that sparse arrays are not representable in JSON)
    return { list: jsVal.map(txMetadataValueFromJS) };
  } else if (typeof(jsVal) === 'object') {
    if (jsVal !== null ) {
      // compound type - Map with String keys
      return { map: Object.keys(jsVal).map(key => {
        return { k: { string: key }, v: txMetadataValueFromJS(jsVal[key]) };
      }) };
    } else {
      // null: convert to simple type - String
      return { string: 'null' };
    }
  } else if (Number.isInteger(jsVal)) {
    // simple type - Int
    return { int: jsVal };
  } else if (typeof(jsVal) === 'string') {
    // simple type - String
    return { string: jsVal };
  } else {
    // anything else: convert to simple type - String.
    // e.g. undefined, true, false, NaN, Infinity.
    // Some of these can't be represented in JSON anyway.
    // Floating point numbers: note there can be loss of precision when
    // representing floats as decimal numbers
    return { string: '' + jsVal };
  }
}

// Get JSON objects from stdin, one per line.
const jsVals = require('fs')
  .readFileSync(0, { encoding: 'utf8' })
  .toString()
  .split(/\r?\n/)
  .filter(line => !!line)
  .map(JSON.parse);

// Convert to transaction metadata JSON form
const txMetadataValues = jsVals.map(txMetadataValueFromJS);
const txMetadata = txMetadataValues
  .reduce((ob, val, i) => { ob['' + i] = val; return ob; }, {});

// Print JSON to stdout
console.log(JSON.stringify(txMetadata));

CLI

Metadata can be provided when creating transactions through the Command-Line Interface.

The JSON is provided directly as a command-line argument.

  • On Linux/MacOS JSON metadata can be put inside single quotes: 
--metadata '{ "0":{ "string":"cardano" } }'
  • On Windows it can be put in double quotes with double quotes inside JSON metadata escaped: 
--metadata "{ \"0\":{ \"string\":\"cardano\" } }"
Usage: cardano-wallet transaction create [--port INT] WALLET_ID
                                         --payment PAYMENT [--metadata JSON]
  Create and submit a new transaction.

Available options:
  -h,--help                Show this help text
  --port INT               port used for serving the wallet API. (default: 8090)
  --payment PAYMENT        address to send to and amount to send separated by @,
                           e.g. '<amount>@<address>'
  --metadata JSON          Application-specific transaction metadata as a JSON
                           object. The value must match the schema defined in
                           the cardano-wallet OpenAPI specification.

References

For a detailed explanation of the metadata design, and information about the transaction format, consult the following specifications.

The full JSON schema is specified in the OpenAPI 3.0 swagger.yaml.

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