Genesis Configuration

The first block produced by any blockchain is referred to as the genesis block. The hash associated with this block is the top-level parent of all blocks produced after that first block.

The Substrate node template provides the genesis configuration—the initial state—for a subset of pallets by default. As you add custom logic to the runtime—for example, by adding predefined or custom pallets—you might find that you want to modify the genesis configuration to include other storage items or set different initial values.

As you learned in Chain specification, the chain specification you use to start a node determines the genesis configuration for that node. However, the chain specification doesn't create the storage items that get initialized when you start a node. Instead, the storage items are defined in the pallets included in the runtime as described in Runtime storage.

After you create storage items for the runtime, you can choose whether they should be set to some initial value as part of the genesis configuration and included in the genesis block. To specify the storage items that you want to set an initial state for, Substrate provides two specialized FRAME attribute macros.

The macros you can use to initialize storage items as part of the genesis configuration for a chain are:

The #[pallet::genesis_config] macro defines the GenesisConfig data type and initializes storage items.
The #[pallet::genesis_build] macro builds the genesis configuration.

These macros are used in combination with the chain specification to define the initial state of the runtime.

Configure a simple storage value

The following example demonstrates adding a single storage value to the genesis configuration for the pallet_template. By default, the pallet_template has one storage item that isn't initialized in the genesis block. This example illustrates how you can use the #[pallet::genesis_config] and #[pallet::genesis_build] macros to set an initial value for the storage value as part of the genesis configuration for your chain.

Configure macros in the pallet

To initialize a storage item for the pallet_template:

Open a new terminal shell and navigate to the root directory for the node template.
Open the pallets/template/src/lib.rs file in a text editor.
Add the #[pallet::genesis_config] macro and add the storage value something as the GenesisConfig storage item for the pallet. For example, add the following macro to the file:
```
// Test Genesis Configuration
#[pallet::genesis_config]
#[derive(Default)]
 pub struct GenesisConfig {
       pub something: u32,
}
```


In this example, the `#[derive(Default)]` macro is required to satisfy a trait bound requirement in `frame_support::traits::GenesisBuild`.

4. Add the `#[pallet::genesis_build]` macro:

 ```rust
 #[pallet::genesis_build]
 impl<T: Config> GenesisBuild<T> for GenesisConfig {
    fn build(&self) { }
 }

In this example, there's no special handling for the build function to perform.

Save your changes and close the file.
Verify that the pallet compiles by running the following command:
```
cargo build --package pallet-template
```

Configure the chain specification

Now that you have configured the pallet to initialize a storage value in the genesis block, you can set an initial value for that storage item in the chain specification.

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

Add the TemplateModuleConfig to the node_template_runtime.

For example:

use node_template_runtime::{
   AccountId, AuraConfig, BalancesConfig, RuntimeGenesisConfig, GrandpaConfig, Signature, SudoConfig, SystemConfig, TemplateModuleConfig, WASM_BINARY,
};

Locate the GenesisConfig and set the initial value for the something storage item.

For example, in the node/src/chain_spec.rs file:

-> GenesisConfig {
     GenesisConfig {
             system: SystemConfig {
                     // Add Wasm runtime to storage.
                     code: wasm_binary.to_vec(),
             },

             template_module: TemplateModuleConfig {
                    something: 221u32,
             },

             transaction_payment: Default::default(),
     }
   }

Adding genesis configuration to the runtime

After you use the #[pallet::genesis_config] macro to add the GenesisConfig to each pallet where it's needed, you must include the Config trait for each pallet in the runtime to enable the runtime to initialize storage items in the genesis block.

All of the GenesisConfig types for the pallets that included in the construction of the runtime are then aggregated into a single RuntimeGenesisConfig type for that runtime.

The aggregated RuntimeGenesisConfig implements the BuildStorage trait to build all of the initial storage items for the runtime. For example, the node template runtime builds storage items for the following pallets that have a RuntimeGenesisConfig specified by default:

System pallet

#[pallet::genesis_config]
	pub struct GenesisConfig {
		#[serde(with = "sp_core::bytes")]
		pub code: Vec<u8>,
	}

Aura pallet

#[pallet::genesis_config]
	pub struct GenesisConfig<T: Config> {
		pub authorities: Vec<T::AuthorityId>,
	}

Grandpa pallet

#[pallet::genesis_config]
	pub struct GenesisConfig {
		pub authorities: AuthorityList,
	}

Balances pallet

#[pallet::genesis_config]
	pub struct GenesisConfig<T: Config<I>, I: 'static = ()> {
		pub balances: Vec<(T::AccountId, T::Balance)>,
	}

TransactionPayment pallet

#[pallet::genesis_config]
	pub struct GenesisConfig {
		pub multiplier: Multiplier,
	}

Sudo pallet

#[pallet::genesis_config]
	pub struct GenesisConfig<T: Config> {
		/// The `AccountId` of the sudo key.
		pub key: Option<T::AccountId>,
	}

Because these pallets include the #[pallet::genesis_config] macro with a GenesisConfig and have the Config trait defined in the runtime, they are aggregated into node_template_runtime::RuntimeGenesisConfig struct for the runtime:

pub struct RuntimeGenesisConfig {
    pub system: SystemConfig,
    pub aura: AuraConfig,
    pub grandpa: GrandpaConfig,
    pub balances: BalancesConfig,
    pub transaction_payment: TransactionPaymentConfig,
    pub sudo: SudoConfig,
}

Ultimately, the RuntimeGenesisConfig is exposed by way of the ChainSpec trait.

For a more complete example of genesis storage configuration for Substrate, see the chain specification that ships with the Substrate code base.

Initialize storage items within a pallet

You can use the #[pallet::genesis_build] macro to define the initial state of storage items within the pallet itself. Defining the genesis configuration within a pallet allows you to access the pallet's private functions.

The following example demonstrates using #[pallet::genesis_config] and #[pallet::genesis_build] to set the initial value of a storage item. In this example, there are two storage items:

A list of member account identifiers.
A specific account identifier that designates a member from the list to be the prime member.

The macros and data types for this example are defined in the my_pallet/src/lib.rs file:

#[pallet::genesis_config]
struct GenesisConfig {
    members: Vec<T::AccountId>,
    prime: T::AccountId,
}

#[pallet::genesis_build]
impl<T: Config> GenesisBuild<T> for GenesisConfig {
    fn build(&self) {
        Pallet::<T>::initialize_members(&self.members);
        SomeStorageItem::<T>::put(self.prime);
    }
}

The genesis configuration is defined in the node/src/chain_spec.rs file:

GenesisConfig {
    my_pallet: MyPalletConfig {
        members: LIST_OF_IDS,
        prime: ID,
    },
}

You can also use the genesis_build macro to define a GenesisConfig attribute that is not bound to a particular storage item. This can be useful if you want to invoke a private helper function within your pallet that sets several storage items, or to invoke a function defined in some other pallets included within your pallet. For example, using an imaginary private function called intitialize_members, the code might look like this:

In my_pallet/src/lib.rs:

#[pallet::genesis_config]
struct GenesisConfig {
    members: Vec<T::AccountId>,
    prime: T::AccountId,
}

#[pallet::genesis_build]
impl<T: Config> GenesisBuild<T> for GenesisConfig {
    fn build(&self) {
        Pallet::<T>::initialize_members(&config.members);
        SomeStorageItem::<T>::put(self.prime);
    }
}

In chain_spec.rs:

GenesisConfig {
    my_pallet: MyPalletConfig {
        members: LIST_OF_IDS,
        prime: ID,
    },
}

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