What are the macros #[pallet::genesis_config] and #[pallet::genesis_build] doing on the sudo pallet?

Question

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

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

    #[pallet::genesis_build]
    impl<T: Config> GenesisBuild<T> for GenesisConfig<T> {
        fn build(&self) {
            <Key<T>>::put(&self.key);
        }
    }

What is happening here with genesis_config and genesis_build macros here? Some reading seemed to suggest that implement the trait GenesisBuild<T,I = ()> but I am confused as the docs read: "T and I are placeholder for pallet trait and pallet instance."

What are pallet instances? I also assume pallet trait means the Config trait for the sudo pallet. Is it some how related to configuring a unique StorageValue for the Genesis block so that each instance of a running node can verify that the same Account is Root? Can someone help break it down for me.

Answer 1

#[pallet::genesis_build] implements the trait sp_runtime::BuildModuleGenesisStorage which puts the initial state into the pallet's storage at genesis time.
(The pallet can put state into child storage also at that point.)

The pub enum/struct with #[pallet::genesis_config] holds the fields that genesis_build reads in order to set up the storage. For example there may be investors at genesis that need a vesting schedule set up (See the vesting pallet: https://github.com/paritytech/substrate/blob/3cdb30e1ecbafe8a866317d4550c921b4d686869/frame/vesting/src/lib.rs#L231 ). So for the sudo pallet, the genesis_config struct holds the root key which is set here:

GenesisConfig {
        sudo: SudoConfig {
            // Assign network admin rights.
            key: root_key,
        },
        ...
}

( https://github.com/paritytech/substrate/blob/3cdb30e1ecbafe8a866317d4550c921b4d686869/bin/node-template/node/src/chain_spec.rs#L150 )

Docs for genesis_config are here: https://docs.substrate.io/rustdocs/latest/frame_support/attr.pallet.html#genesis-config-palletgenesis_config-optional