Configurable Pallet Constants

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To declare constant values within a runtime, it is necessary to import the Get trait from frame_support

use frame_support::traits::Get;

Configurable constants are declared as associated types in the pallet's configuration trait using the Get<T> syntax for any type T.

pub trait Trait: system::Trait {
	type Event: From<Event> + Into<<Self as system::Trait>::Event>;

	/// Maximum amount added per invocation
	type MaxAddend: Get<u32>;

	/// Frequency with which the stored value is deleted
	type ClearFrequency: Get<Self::BlockNumber>;
}

In order to make these constants and their values appear in the runtime metadata, it is necessary to declare them with the const syntax in the decl_module! block. Usually constants are declared at the top of this block, right after fn deposit_event.

decl_module! {
	pub struct Module<T: Trait> for enum Call where origin: T::Origin {
		fn deposit_event() = default;

		const MaxAddend: u32 = T::MaxAddend::get();

		const ClearFrequency: T::BlockNumber = T::ClearFrequency::get();

		// --snip--
	}
}

This example manipulates a single value in storage declared as SingleValue.

decl_storage! {
	trait Store for Module<T: Trait> as Example {
		SingleValue get(fn single_value): u32;
	}
}

SingleValue is set to 0 every ClearFrequency number of blocks in the on_finalize function that runs at the end of blocks execution.

fn on_finalize(n: T::BlockNumber) {
	if (n % T::ClearFrequency::get()).is_zero() {
		let c_val = <SingleValue>::get();
		<SingleValue>::put(0u32);
		Self::deposit_event(Event::Cleared(c_val));
	}
}

Signed transactions may invoke the add_value runtime method to increase SingleValue as long as each call adds less than MaxAddend. There is no anti-sybil mechanism so a user could just split a larger request into multiple smaller requests to overcome the MaxAddend, but overflow is still handled appropriately.

fn add_value(origin, val_to_add: u32) -> DispatchResult {
	let _ = ensure_signed(origin)?;
	ensure!(val_to_add <= T::MaxAddend::get(), "value must be <= maximum add amount constant");

	// previous value got
	let c_val = <SingleValue>::get();

	// checks for overflow when new value added
	let result = match c_val.checked_add(val_to_add) {
		Some(r) => r,
		None => return Err(DispatchError::Other("Addition overflowed")),
	};
	<SingleValue>::put(result);
	Self::deposit_event(Event::Added(c_val, val_to_add, result));
	Ok(())
}

In more complex patterns, the constant value may be used as a static, base value that is scaled by a multiplier to incorporate stateful context for calculating some dynamic fee (i.e. floating transaction fees).

Supplying the Constant Value

When the pallet is included in a runtime, the runtime developer supplies the value of the constant using the parameter_types! macro. This pallet is included in the super-runtime where we see the following macro invocation and trait implementation.


#![allow(unused)]
fn main() {
parameter_types! {
	pub const MaxAddend: u32 = 1738;
	pub const ClearFrequency: u32 = 10;
}

impl constant_config::Trait for Runtime {
	type Event = Event;
	type MaxAddend = MaxAddend;
	type ClearFrequency = ClearFrequency;
}
}