Iroha 2

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Metadata ​

Metadata are key-value pairs that are attached to objects in the blockchain. The following can contain metadata:

  • domains
  • accounts
  • assets
  • asset definitions
  • triggers
  • transactions

The metadata can be of very different types, such as:

  • structures with named or unnamed fields
  • enums
  • integers
  • numbers with fixed decimal precision
  • strings
  • Boolean values
  • arrays
  • associative arrays
  • vectors
  • request results

The object's metadata can be transferred one by one, or in bulk via a WASM transaction. The Store asset type is used for working with metadata. Let's take a closer look at this asset type.

MetadataChanged ​

MetadataInserted or MetadataRemoved events are emitted when metadata is inserted or removed from accounts, domains, assets, or asset definitions. The emitted event also contains the data that was inserted or removed from the object. This data is stored in MetadataChanged in the form of a (key, value) pair.

Check data filters for details.

Store Asset ​

In Iroha 2 there is an asset called Store that was designed to be a package of data. You can use Store when you require a storage of key-value pairs. The SetKeyValue and RemoveKeyValue instructions are used with the Store asset type. Here is an example of SetKeyValue instruction:

rust
// Mouse's account
let mouse_id: AccountId = "mouse@wonderland".parse();

// Registering `Store` asset definition
let hat_definition_id: AssetDefinitionId =
    "hat#wonderland".parse();
let new_hat_definition = AssetDefinition::store(hat_definition_id);
let register_hat = RegisterBox::new(new_hat_definition);

let mouse_hat_id = AssetId::new(hat_definition_id, mouse_id);

// New Iroha Special Instruction for setting key-value pairs for Mouse's hats:
let set_hat_color = SetKeyValueBox::new(
    mouse_hat_id,
    Name::from_str("color"),
    "yellow".to_owned(),
);

Working with metadata ​

The following example showcases how to register and grant a role to access another account's metadata.

Example
rust
#[test]
fn register_and_grant_role_for_metadata_access() -> Result<()> {
    let (_rt, _peer, test_client) = <PeerBuilder>::new().start_with_runtime();
    wait_for_genesis_committed(&vec![test_client.clone()], 0);

    let alice_id = AccountId::from_str("alice@wonderland")?;
    let mouse_id = AccountId::from_str("mouse@wonderland")?;

    // Registering Mouse
    let mouse_key_pair = KeyPair::generate()?;
    let register_mouse = RegisterBox::new(Account::new(
        mouse_id.clone(),
        [mouse_key_pair.public_key().clone()],
    ));
    test_client.submit_blocking(register_mouse)?;

    // Registering role
    let role_id = <Role as Identifiable>::Id::from_str("ACCESS_TO_MOUSE_METADATA")?;
    let role = iroha_data_model::role::Role::new(role_id.clone())
        .add_permission(CanSetKeyValueInUserMetadata::new(mouse_id.clone()))
        .add_permission(CanRemoveKeyValueInUserMetadata::new(mouse_id.clone()));
    let register_role = RegisterBox::new(role);
    test_client.submit_blocking(register_role)?;

    // Mouse grants role to Alice
    let grant_role = GrantBox::new(role_id.clone(), alice_id.clone());
    let grant_role_tx = Transaction::new(mouse_id.clone(), vec![grant_role.into()].into(), 100_000)
        .sign(mouse_key_pair)?;
    test_client.submit_transaction_blocking(grant_role_tx)?;

    // Alice modifies Mouse's metadata
    let set_key_value = SetKeyValueBox::new(
        mouse_id,
        Name::from_str("key").expect("Valid"),
        Value::String("value".to_owned()),
    );
    test_client.submit_blocking(set_key_value)?;

    // Making request to find Alice's roles
    let found_role_ids = test_client.request(client::role::by_account_id(alice_id))?;
    assert!(found_role_ids.contains(&role_id));

    Ok(())
}

Queries ​

You can get the key value of an object metadata using queries:

Permissions ​

Permissions are tokens that allow accounts to set or remove key-values in Accounts, Assets, Asset Definitions, etc.

For more information on permissions, see the following topics: