🪝 FRAME/Pallet Hooks 🪝

Hooks: All In One

#[pallet::hooks]
impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {
  fn on_runtime_upgrade() -> Weight {}
  fn on_initialize() -> Weight {}
  fn on_finalize() {}
  fn on_idle(remaining_weight: Weight) -> Weight {}
  fn offchain_worker() {}
  fn integrity_test() {}
  #[cfg(feature = "try-runtime")]
  fn try_state() -> Result<(), &'static str> {}
}

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

Hooks: On_Initialize

• Mandatory Hooks should really be lightweight and predictable, with a bounded complexity.

fn on_initialize() -> Weight {
  // any user can create one entry in `MyMap` 😱🔫.
  <MyMap<T>>::iter().for_each(do_stuff);
}

Hooks: On_Initialize

• ­ Question: If you have 3 pallets, in which order their on_initialize are called?
• ­ Question: If your runtime panics on_initialize, how can you recover from it?
• ­ Question: If your on_initialize consumes more than the maximum block weight?

Hooks: on_finalize

Generally, avoid using it unless if something REALLY needs to be happen at the end of the block.

The Future: Moving Away From Mandatory Hooks

• on_initialize -> poll
• on_finalize -> on_idle
• New primitives for multi-block migrations
• New primitives for optional service work via extrinsics.

Hooks: genesis_build

#[pallet::genesis_build]
pub struct GenesisConfig<T: Config> {
  pub foo: Option<u32>,
  pub bar: Vec<u8>,
}

impl<T: Config> Default for GenesisConfig<T> {
  fn default() -> Self {
    // snip
  }
}

#[pallet::genesis_build]
impl<T: Config> GenesisBuild<T> for GenesisConfig<T> {
  fn build(&self) {
    // use self.foo, self.bar etc.
  }
}

Hooks: genesis_build

• GenesisConfig is a composite/amalgamated item at the top level runtime.

construct_runtime!(
  pub enum Runtime where {
    System: frame_system,
    Balances: pallet_balances,
  }
);

struct RuntimeGenesisConfig {
  SystemConfig: pallet_system::GenesisConfig,
  PalletAConfig: pallet_a::GenesisConfig,
}

Hooks: genesis_build

• Recent changes moving genesis_build to be used over a runtime API, rather than native runtime.
• #[cfg(feature = "std")] in pallets will go away.

Hooks: offchain_worker

• Execution entirely up to the client.
• Has a totally separate thread pool than the normal execution.

--offchain-worker <ENABLED>
    Possible values:
    - always:
    - never:
    - when-authority

--execution-offchain-worker <STRATEGY>
    Possible values:
    - native:
    - wasm:
    - both:
    - native-else-wasm:

Hooks: offchain_worker

• Threads can overlap, each is reading the state of its corresponding block

Hooks: offchain_worker

• ­Offchain workers have their own special host functions: http, dedicated storage, time, etc.

• ­Offchain workers have the same execution limits as Wasm (limited memory, custom allocator).

• ­Source of confusion, why OCWs cannot write to state.