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Extract dynamic fees calculation as separate feature
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@bkontur
bkontur committed Oct 28, 2024
1 parent 59c8b4b commit 501a5c0
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Showing 3 changed files with 54 additions and 104 deletions.
153 changes: 52 additions & 101 deletions bridges/modules/xcm-bridge-hub-router/src/lib.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -109,9 +109,12 @@ pub mod pallet {
type LocalXcmChannelManager: XcmChannelStatusProvider;

/// Additional fee that is paid for every byte of the outbound message.
/// See `calculate_fees` for more details.
type ByteFee: Get<u128>;
/// Asset that is used to paid bridge fee.
type FeeAsset: Get<AssetId>;
/// Asset used to pay the `ByteFee`.
/// If not specified, the `ByteFee` is ignored.
/// See `calculate_fees` for more details.
type FeeAsset: Get<Option<AssetId>>;
}

#[pallet::pallet]
Expand Down Expand Up @@ -216,6 +219,36 @@ pub mod pallet {
*f
});
}

/// Calculates final fees based on the configuration, supporting two optional features:
///
/// 1. Adds an (optional) fee for message size based on `T::ByteFee` and `T::FeeAsset`.
/// 2. Applies a dynamic fee factor `Self::delivery_fee_factor` to the `actual_cost` (useful for congestion handling).
///
/// Parameters:
/// - `message_size`
/// - `actual_cost`: the fee for sending a message from this chain to a child, sibling, or local bridge hub, determined by `Config::ToBridgeHubSender`.
pub(crate) fn calculate_fees(message_size: u32, mut actual_cost: Assets) -> Assets {
// Apply message size `T::ByteFee/T::FeeAsset` feature (if configured).
if let Some(asset_id) = T::FeeAsset::get() {
let message_fee = (message_size as u128).saturating_mul(T::ByteFee::get());
if message_fee > 0 {
// Keep in mind that this is only the additional fee for message size.
actual_cost.push((asset_id, message_fee).into());
}
}

// Apply dynamic fees feature - apply `fee_factor` to the `actual_cost`.
let fee_factor = Self::delivery_fee_factor();
let mut new_cost = Assets::new();
for mut a in actual_cost.into_inner() {
if let Fungibility::Fungible(ref mut amount) = a.fun {
*amount = fee_factor.saturating_mul_int(*amount);
}
new_cost.push(a);
}
new_cost
}
}

#[pallet::event]
Expand All @@ -234,97 +267,6 @@ pub mod pallet {
}
}

// This pallet acts as the `ExporterFor` for the `SovereignPaidRemoteExporter` to compute
// message fee using fee factor.
impl<T: Config<I>, I: 'static> ExporterFor for Pallet<T, I> {
fn exporter_for(
network: &NetworkId,
remote_location: &InteriorLocation,
message: &Xcm<()>,
) -> Option<(Location, Option<Asset>)> {
log::trace!(
target: LOG_TARGET,
"exporter_for - network: {network:?}, remote_location: {remote_location:?}, msg: {message:?}",
);
// ensure that the message is sent to the expected bridged network (if specified).
if let Some(bridged_network) = T::BridgedNetworkId::get() {
if *network != bridged_network {
log::trace!(
target: LOG_TARGET,
"Router with bridged_network_id {bridged_network:?} does not support bridging to network {network:?}!",
);
return None
}
}

// ensure that the message is sent to the expected bridged network and location.
let (bridge_hub_location, maybe_payment) = match T::Bridges::exporter_for(
network,
remote_location,
message,
) {
Some((bridge_hub_location, maybe_payment))
if bridge_hub_location.eq(&T::SiblingBridgeHubLocation::get()) =>
(bridge_hub_location, maybe_payment),
_ => {
log::trace!(
target: LOG_TARGET,
"Router configured with bridged_network_id {:?} and sibling_bridge_hub_location: {:?} does not support bridging to network {:?} and remote_location {:?}!",
T::BridgedNetworkId::get(),
T::SiblingBridgeHubLocation::get(),
network,
remote_location,
);
return None
},
};

// take `base_fee` from `T::Brides`, but it has to be the same `T::FeeAsset`
let base_fee = match maybe_payment {
Some(payment) => match payment {
Asset { fun: Fungible(amount), id } if id.eq(&T::FeeAsset::get()) => amount,
invalid_asset => {
log::error!(
target: LOG_TARGET,
"Router with bridged_network_id {:?} is configured for `T::FeeAsset` {:?} \
which is not compatible with {:?} for bridge_hub_location: {:?} for bridging to {:?}/{:?}!",
T::BridgedNetworkId::get(),
T::FeeAsset::get(),
invalid_asset,
bridge_hub_location,
network,
remote_location,
);
return None
},
},
None => 0,
};

// compute fee amount. Keep in mind that this is only the bridge fee. The fee for sending
// message from this chain to child/sibling bridge hub is determined by the
// `Config::ToBridgeHubSender`
let message_size = message.encoded_size();
let message_fee = (message_size as u128).saturating_mul(T::ByteFee::get());
let fee_sum = base_fee.saturating_add(message_fee);

let fee_factor = Self::delivery_fee_factor();
let fee = fee_factor.saturating_mul_int(fee_sum);
let fee = if fee > 0 { Some((T::FeeAsset::get(), fee).into()) } else { None };

log::info!(
target: LOG_TARGET,
"Going to send message to {:?} ({} bytes) over bridge. Computed bridge fee {:?} using fee factor {}",
(network, remote_location),
message_size,
fee,
fee_factor,
);

Some((bridge_hub_location, fee))
}
}

// This pallet acts as the `SendXcm` to the sibling/child bridge hub instead of regular
// XCMP/DMP transport. This allows injecting dynamic message fees into XCM programs that
// are going to the bridged network.
Expand Down Expand Up @@ -363,7 +305,7 @@ impl<T: Config<I>, I: 'static> SendXcm for Pallet<T, I> {

// The bridge doesn't support oversized or overweight messages. Therefore, it's
// better to drop such messages here rather than at the bridge hub. Let's check the
// message size."
// message size.
if message_size > HARD_MESSAGE_SIZE_LIMIT {
return Err(SendError::ExceedsMaxMessageSize)
}
Expand All @@ -381,6 +323,14 @@ impl<T: Config<I>, I: 'static> SendXcm for Pallet<T, I> {
.into_version(destination_version)
.map_err(|()| SendError::DestinationUnsupported)?;

// now let's apply fees features
let cost = Self::calculate_fees(message_size, cost);

log::info!(
target: LOG_TARGET,
"Going to send message to {dest_clone:?} ({message_size:?} bytes) with actual cost: {cost:?}"
);

Ok(((message_size, ticket), cost))
},
Err(e) => {
Expand Down Expand Up @@ -566,8 +516,11 @@ mod tests {
let xcm: Xcm<()> = vec![ClearOrigin].into();
let msg_size = xcm.encoded_size();

// initially the base fee is used: `BASE_FEE + BYTE_FEE * msg_size + HRMP_FEE`
let expected_fee = BASE_FEE + BYTE_FEE * (msg_size as u128) + HRMP_FEE;
// `BASE_FEE + BYTE_FEE * msg_size + HRMP_FEE`
let base_cost_formula = || BASE_FEE + BYTE_FEE * (msg_size as u128) + HRMP_FEE;

// initially the base fee is used
let expected_fee = base_cost_formula();
assert_eq!(
XcmBridgeHubRouter::validate(&mut Some(dest.clone()), &mut Some(xcm.clone()))
.unwrap()
Expand All @@ -577,14 +530,12 @@ mod tests {
);

// but when factor is larger than one, it increases the fee, so it becomes:
// `(BASE_FEE + BYTE_FEE * msg_size) * F + HRMP_FEE`
// `base_cost_formula() * F`
let factor = FixedU128::from_rational(125, 100);
DeliveryFeeFactor::<TestRuntime, ()>::put(factor);
let expected_fee =
(FixedU128::saturating_from_integer(BASE_FEE + BYTE_FEE * (msg_size as u128)) *
factor)
.into_inner() / FixedU128::DIV +
HRMP_FEE;
(FixedU128::saturating_from_integer(base_cost_formula()) * factor)
.into_inner() / FixedU128::DIV;
assert_eq!(
XcmBridgeHubRouter::validate(&mut Some(dest), &mut Some(xcm)).unwrap().1.get(0),
Some(&(BridgeFeeAsset::get(), expected_fee).into()),
Expand Down
3 changes: 1 addition & 2 deletions bridges/modules/xcm-bridge-hub-router/src/mock.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -83,8 +83,7 @@ impl pallet_xcm_bridge_hub_router::Config<()> for TestRuntime {
LatestOrNoneForLocationVersionChecker<Equals<UnknownXcmVersionForRoutableLocation>>;

type ToBridgeHubSender = SovereignPaidRemoteExporter<
// use pallet itself as `ExportFor` provider.
XcmBridgeHubRouter,
NetworkExportTable<BridgeTable>,
TestToBridgeHubSender,
Self::UniversalLocation,
>;
Expand Down
2 changes: 1 addition & 1 deletion bridges/modules/xcm-bridge-hub/src/mock.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -231,7 +231,7 @@ impl pallet_xcm_bridge_hub_router::Config<()> for TestRuntime {
// produced by `pallet_xcm_bridge_hub_router` is compatible with the `ExportXcm` implementation
// of `pallet_xcm_bridge_hub`.
type ToBridgeHubSender = SovereignPaidRemoteExporter<
XcmOverBridgeWrappedWithExportMessageRouter,
NetworkExportTable<BridgeTable>,
// **Note**: The crucial part is that `ExportMessage` is processed by `XcmExecutor`, which
// calls the `ExportXcm` implementation of `pallet_xcm_bridge_hub` as the
// `MessageExporter`.
Expand Down

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