Calculate fees
This guides steps through the process of customizing WeightToFee
for your runtime's implementation of pallet_transaction_payment
.
Fees are broken down into three components:
- Byte fee - A fee proportional to the transaction's length in bytes. The proportionality constant is a parameter in the Transaction Payment Pallet.
- Weight fee - A fee calculated from the transaction weight. The conversion doesn't need to be linear, although it often is. The same conversion function is applied across all transactions for all pallets in the runtime.
- Fee Multiplier - A multiplier for the computed fee, that can change as the chain progresses.
FRAME provides the Transaction Payment Pallet for calculating and collecting fees for executing transactions. It can be useful to modify the way fees are calculated to more accurately charge fees.
Goal
Customize WeightToFee
to modify how fees are calculated for your runtime.
Use Cases
Modify the way fees are calculated, instead of using IdentityFee
which maps one unit of fee to one unit of weight.
Steps
1. Write the LinearWeightToFee
struct
In runtime/src/lib.rs
, create the struct called LinearWeightToFee
that implements WeightToFeePolynomial
.
It must returns a smallvec of WeightToFeeCoefficient
integers.
runtime/src/lib.rs
pub struct LinearWeightToFee<C>(sp_std::marker::PhantomData<C>);
impl<C> WeightToFeePolynomial for LinearWeightToFee<C>
where
C: Get<Balance>,
{
type Balance = Balance;
fn polynomial() -> WeightToFeeCoefficients<Self::Balance> {
let coefficient = WeightToFeeCoefficient {
coeff_integer: C::get(),
coeff_frac: Perbill::zero(),
negative: false,
degree: 1,
};
smallvec!(coefficient)
}
}
2. Configure pallet_transaction_payment
in your runtime
Convert the dispatch weight
type WeightToFee
to the chargeable fee LinearWeightToFee
(replacing IdentityFee<Balance>;
):
runtime/src/lib.rs
parameter_types! {
// Used with LinearWeightToFee conversion.
pub const FeeWeightRatio: u128 = 1_000;
// Establish the byte-fee. It is used in all configurations.
pub const TransactionByteFee: u128 = 1;
}
impl pallet_transaction_payment::Config for Runtime {
type OnChargeTransaction = CurrencyAdapter<Balances, ()>;
type TransactionByteFee = TransactionByteFee;
// Convert dispatch weight to a chargeable fee.
type WeightToFee = LinearWeightToFee<FeeWeightRatio>;
type FeeMultiplierUpdate = ();
}