Create a storage structure (struct)

Creating a struct of similarly grouped storage items is an orderly way to keep track of them. When grouped in this way, it is easier to reference them than it is if individual StorageValue items are kept separately. This can make testing and genesis configuration easier.

This guide shows you how to create a StorageValue storage item that holds a struct and is used in on_initialize. This struct does the following:

  • Keeps track of an initial amount (issuance)
  • Keeps track of the account that receives that amount (minter)
  • Keeps track of an account that can burn some amount (burner)
  • Is (partially) used in on_initialize

Before you begin

Make sure you have a working pallet to build your struct for. Use the template pallet if you don't have one you're already working on.

  1. Create a struct

    Name the struct MetaData and declare its different types:

    #[derive(Clone, Encode, Decode, Eq, PartialEq, RuntimeDebug, Default)]
    pub struct MetaData<AccountId, Balance> {
    	issuance: Balance,
    	minter: AccountId,
    	burner: AccountId,
  2. Declare the struct as a storage item

    Use StorageValue to declare the struct as a new single item in storage:

    #[pallet::getter(fn meta_data)]
    	pub(super) type MetaDataStore<T: Config> = StorageValue<_, MetaData<T::AccountId, T::Balance>, ValueQuery>;
  3. Configure GenesisConfig

    Use the #[pallet::genesis_config] attribute to initialize values from your MetaData struct.

    // Declare `admin` as type `T::AccountId`.
    pub struct GenesisConfig<T: Config> {
    	pub admin: T::AccountId,
    // Give it a default value.
    #[cfg(feature = "std")]
    impl<T: Config> Default for GenesisConfig<T> {
    	fn default() -> Self {
    		Self {
    			admin: Default::default(),

    This admin variable must correspond with the variable used in the node/ file inside fn testnet_genesis.

  4. Configure GenesisBuild

    Use the #[pallet::genesis_build] attribute to initialize the values of your struct, using admin to initialize the values of type T::AccountId:

    impl<T: Config> GenesisBuild<T> for GenesisConfig<T> {
    	fn build(&self) {
    		MetaDataStore::<T>::put(MetaData {
    			issuance: Zero::zero(),
    			minter: self.admin.clone(),
    			burner: self.admin.clone(),
  5. Use the struct in on_initialize()

    Assign an amount to the issuance field of MetaData to be initialized when the chain is launched:

    fn on_initialize(_n: T::BlockNumber) -> Weight {
    	// Create an alias for the StorageValue struct.
    	let mut meta = MetaDataStore::<T>::get();
    	// Add a value to the `issuance` field.
    	let value: T::Balance = 50u8.into();
    	meta.issuance = meta.issuance.saturating_add(value);
    	// Add the amount to the `minter` account in storage.
    	Accounts::<T>::mutate(&meta.minter, |bal| {
    		*bal = bal.saturating_add(value);

The on_initialize function ensures that the values for the specified items are written to storage when the chain is launched.