Configure Genesis State

Genesis configuration defines the initial state for storage items such as accounts, balances, genesis for custom pallets, and more. This guide demonstrates how to configure the genesis state for a pallet with the following storage items:

• A SingleValue<T> type for a single StorageValue storage item.

• An AccountMap<T> type for simple single key StorageMap storage item.

Steps preview

1. Add the storage items in the pallet.

2. Add the genesis configuration macros in the pallet.

3. Set initial values in the chain specification.

Add storage items to the pallet

1. Open the src//lib.rs file for your pallet in a text editor.

2. Add the StorageValue storage item.

   For example:

   #[pallet::storage]
   #[pallet::getter(fn something)]
   pub type SingleValue<T: Config> = StorageValue<
      _, 
      T::Balance
   >;
   

3. Add the StorageMap storage item for a map that has enumerable entries.

   For example:

   #[pallet::storage]
   #[pallet::getter(fn accounts)]
   pub type AccountMap<T: Config> = StorageMap<
      _, 
      Blake2_128Concat, 
      T::AccountId, 
      T::Balance
   >;

Add genesis configuration macros

The GenesisConfig code should go after your storage items.

1. Add the #[pallet::genesis_config] attribute macro and define the GenesisConfig struct for the storage items to initialize.

   #[pallet::genesis_config]
   pub struct GenesisConfig<T: Config> {
   	pub single_value: T::Balance,
   	pub account_map: Vec<(T::AccountId, T::Balance)>,
   }

2. Set the default value for the GenesisConfig struct.

   #[cfg(feature = "std")]
   impl<T: Config> Default for GenesisConfig<T> {
   	fn default() -> Self {
   		Self { single_value: Default::default(), account_map: Default::default() }
   	}
   }

3. Add the #[pallet::genesis_build] attribute macro and implement the GenesisBuild trait.

   #[pallet::genesis_build]
   impl<T: Config> GenesisBuild<T> for GenesisConfig<T> {
   	fn build(&self) {
   		<SingleValue<T>>::put(&self.single_value);
   		for (a, b) in &self.account_map {
   			<AccountMap<T>>::insert(a, b);
   		}
   	}
   }

   The #[pallet::genesis_build] macro allows you to execute some logic to define how the GenesisConfig struct places something in storage.

Set initial values

After you configure the storage items and genesis configuration in your pallet, you can specify the values you want to set for the storage items in the genesis state of the chain. In this example, assume that the construct_runtime! macro for runtime refers to PalletSomething as the pallet name and pallet_something as the path to the pallet.

construct_runtime!(
   pub struct Runtime
   where
      Block = Block,
      NodeBlock = opaque::Block,
      UncheckedExtrinsic = UncheckedExtrinsic,
   {
      PalletSomething: pallet_something,
   }
)

1. Open the node/chain_spec.rs file in a text editor.

2. Verify the use node_template_runtime::BalanceConfig; imported at the top of our chain_spec.rs file.

3. Create a constant value of type T::Balance to be stored in <SingleValue<T>> (inside the testnet_genesis method).

   const VALUE: Balance = 235813;

4. Create a vector of accounts to initialize <AccountMap<T>> with inside the testnet_genesis method.

   let accounts_to_map: Vec<AccountId> =
   	vec![
   		get_account_id_from_seed::<sr25519::Public>("Alice"),
   		get_account_id_from_seed::<sr25519::Public>("Bob"),
   		get_account_id_from_seed::<sr25519::Public>("Charlie"),
   	];

5. Add the pallet to the GenesisConfig clause in the testnet_genesis function.

   The convention is to use lowercase spelling of the name of your pallet in runtime/src/lib.rs inside construct_runtime!. For pallets declared as CamelCase for example, the convention is to refer to it as camel_case in the testnet_genesis function.

   For this example pallet, the code looks like this:

   pallet_something: PalletSomethingConfig {
   	single_value: VALUE,
   	account_map: accounts_to_map.iter().cloned().map(|x| (x, VALUE)).collect(),
   }

   This sample code maps each account from accounts_to_map to an amount equal to VALUE. This pattern is very similar to the GenesisConfig for the Balances pallet.

Example

• Node template 'chain_spec.rs'

• Example pallet