From 22d5b80d4486f2e9d53bf8ad1eb3efa11a4136af Mon Sep 17 00:00:00 2001 From: Vincent Geddes Date: Fri, 22 Mar 2024 13:52:51 +0200 Subject: [PATCH] Add a linear fee multiplier (#127) (#3790) Bridging fees are calculated using a static ETH/DOT exchange rate that can deviate significantly from the real-world exchange rate. We therefore need to add a safety margin to the fee so that users almost aways cover the cost of relaying. # FAQ > Why introduce a `multiplier` parameter instead of configuring an exchange rate which already has a safety factor applied? When converting from ETH to DOT, we need to _divide_ the multiplier by the exchange rate, and to convert from DOT to ETH we need to _multiply_ the multiplier by the exchange rate. > Other input parameters to the fee calculation can also deviate from real-world values. These include substrate weights, gas prices, and so on. Why does the multiplier introduced here not adjust those? A single scalar multiplier won't be able to accommodate the different volatilities efficiently. For example, gas prices are much more volatile than exchange rates, and substrate weights hardly ever change. So the pricing config relating to weights and gas prices should already have some appropriate safety margin pre-applied. # Detailed Changes: * Added `multiplier` field to `PricingParameters` * Outbound-queue fee is multiplied by `multiplier` * This `multiplier` is synced to the Ethereum side * Improved Runtime API for calculating outbound-queue fees. This API makes it much easier to for configure parts of the system in preparation for launch. * Improve and clarify code documentation Upstreamed from https://github.com/Snowfork/polkadot-sdk/pull/127 --------- Co-authored-by: Clara van Staden Co-authored-by: Adrian Catangiu --- .../pallets/inbound-queue/src/mock.rs | 3 +- .../outbound-queue/runtime-api/src/lib.rs | 11 +++-- .../pallets/outbound-queue/src/api.rs | 18 +++++--- .../pallets/outbound-queue/src/lib.rs | 32 +++++++++---- .../pallets/outbound-queue/src/mock.rs | 3 +- .../pallets/outbound-queue/src/test.rs | 46 +++++++++++-------- bridges/snowbridge/pallets/system/src/lib.rs | 2 + bridges/snowbridge/pallets/system/src/mock.rs | 3 +- .../primitives/core/src/outbound.rs | 13 ++++-- .../snowbridge/primitives/core/src/pricing.rs | 5 ++ .../bridge-hubs/bridge-hub-rococo/src/lib.rs | 11 +++-- 11 files changed, 97 insertions(+), 50 deletions(-) diff --git a/bridges/snowbridge/pallets/inbound-queue/src/mock.rs b/bridges/snowbridge/pallets/inbound-queue/src/mock.rs index 086b27cb82..39e9532ed3 100644 --- a/bridges/snowbridge/pallets/inbound-queue/src/mock.rs +++ b/bridges/snowbridge/pallets/inbound-queue/src/mock.rs @@ -173,7 +173,8 @@ parameter_types! { pub Parameters: PricingParameters = PricingParameters { exchange_rate: FixedU128::from_rational(1, 400), fee_per_gas: gwei(20), - rewards: Rewards { local: DOT, remote: meth(1) } + rewards: Rewards { local: DOT, remote: meth(1) }, + multiplier: FixedU128::from_rational(1, 1), }; } diff --git a/bridges/snowbridge/pallets/outbound-queue/runtime-api/src/lib.rs b/bridges/snowbridge/pallets/outbound-queue/runtime-api/src/lib.rs index 51f46a7b49..e6ddaa4393 100644 --- a/bridges/snowbridge/pallets/outbound-queue/runtime-api/src/lib.rs +++ b/bridges/snowbridge/pallets/outbound-queue/runtime-api/src/lib.rs @@ -3,7 +3,10 @@ #![cfg_attr(not(feature = "std"), no_std)] use frame_support::traits::tokens::Balance as BalanceT; -use snowbridge_core::outbound::Message; +use snowbridge_core::{ + outbound::{Command, Fee}, + PricingParameters, +}; use snowbridge_outbound_queue_merkle_tree::MerkleProof; sp_api::decl_runtime_apis! { @@ -11,10 +14,10 @@ sp_api::decl_runtime_apis! { { /// Generate a merkle proof for a committed message identified by `leaf_index`. /// The merkle root is stored in the block header as a - /// `\[`sp_runtime::generic::DigestItem::Other`\]` + /// `sp_runtime::generic::DigestItem::Other` fn prove_message(leaf_index: u64) -> Option; - /// Calculate the delivery fee for `message` - fn calculate_fee(message: Message) -> Option; + /// Calculate the delivery fee for `command` + fn calculate_fee(command: Command, parameters: Option>) -> Fee; } } diff --git a/bridges/snowbridge/pallets/outbound-queue/src/api.rs b/bridges/snowbridge/pallets/outbound-queue/src/api.rs index 44d63f1e2d..b904819b1b 100644 --- a/bridges/snowbridge/pallets/outbound-queue/src/api.rs +++ b/bridges/snowbridge/pallets/outbound-queue/src/api.rs @@ -4,8 +4,12 @@ use crate::{Config, MessageLeaves}; use frame_support::storage::StorageStreamIter; -use snowbridge_core::outbound::{Message, SendMessage}; +use snowbridge_core::{ + outbound::{Command, Fee, GasMeter}, + PricingParameters, +}; use snowbridge_outbound_queue_merkle_tree::{merkle_proof, MerkleProof}; +use sp_core::Get; pub fn prove_message(leaf_index: u64) -> Option where @@ -19,12 +23,14 @@ where Some(proof) } -pub fn calculate_fee(message: Message) -> Option +pub fn calculate_fee( + command: Command, + parameters: Option>, +) -> Fee where T: Config, { - match crate::Pallet::::validate(&message) { - Ok((_, fees)) => Some(fees.total()), - _ => None, - } + let gas_used_at_most = T::GasMeter::maximum_gas_used_at_most(&command); + let parameters = parameters.unwrap_or(T::PricingParameters::get()); + crate::Pallet::::calculate_fee(gas_used_at_most, parameters) } diff --git a/bridges/snowbridge/pallets/outbound-queue/src/lib.rs b/bridges/snowbridge/pallets/outbound-queue/src/lib.rs index 9e949a4791..9b9dbe854a 100644 --- a/bridges/snowbridge/pallets/outbound-queue/src/lib.rs +++ b/bridges/snowbridge/pallets/outbound-queue/src/lib.rs @@ -47,24 +47,37 @@ //! consume on Ethereum. Using this upper bound, a final fee can be calculated. //! //! The fee calculation also requires the following parameters: -//! * ETH/DOT exchange rate -//! * Ether fee per unit of gas +//! * Average ETH/DOT exchange rate over some period +//! * Max fee per unit of gas that bridge is willing to refund relayers for //! //! By design, it is expected that governance should manually update these //! parameters every few weeks using the `set_pricing_parameters` extrinsic in the //! system pallet. //! +//! This is an interim measure. Once ETH/DOT liquidity pools are available in the Polkadot network, +//! we'll use them as a source of pricing info, subject to certain safeguards. +//! //! ## Fee Computation Function //! //! ```text //! LocalFee(Message) = WeightToFee(ProcessMessageWeight(Message)) -//! RemoteFee(Message) = MaxGasRequired(Message) * FeePerGas + Reward -//! Fee(Message) = LocalFee(Message) + (RemoteFee(Message) / Ratio("ETH/DOT")) +//! RemoteFee(Message) = MaxGasRequired(Message) * Params.MaxFeePerGas + Params.Reward +//! RemoteFeeAdjusted(Message) = Params.Multiplier * (RemoteFee(Message) / Params.Ratio("ETH/DOT")) +//! Fee(Message) = LocalFee(Message) + RemoteFeeAdjusted(Message) //! ``` //! -//! By design, the computed fee is always going to conservative, to cover worst-case -//! costs of dispatch on Ethereum. In future iterations of the design, we will optimize -//! this, or provide a mechanism to asynchronously refund a portion of collected fees. +//! By design, the computed fee includes a safety factor (the `Multiplier`) to cover +//! unfavourable fluctuations in the ETH/DOT exchange rate. +//! +//! ## Fee Settlement +//! +//! On the remote side, in the gateway contract, the relayer accrues +//! +//! ```text +//! Min(GasPrice, Message.MaxFeePerGas) * GasUsed() + Message.Reward +//! ``` +//! Or in plain english, relayers are refunded for gas consumption, using a +//! price that is a minimum of the actual gas price, or `Message.MaxFeePerGas`. //! //! # Extrinsics //! @@ -106,7 +119,7 @@ pub use snowbridge_outbound_queue_merkle_tree::MerkleProof; use sp_core::{H256, U256}; use sp_runtime::{ traits::{CheckedDiv, Hash}, - DigestItem, + DigestItem, Saturating, }; use sp_std::prelude::*; pub use types::{CommittedMessage, ProcessMessageOriginOf}; @@ -366,8 +379,9 @@ pub mod pallet { // downcast to u128 let fee: u128 = fee.try_into().defensive_unwrap_or(u128::MAX); - // convert to local currency + // multiply by multiplier and convert to local currency let fee = FixedU128::from_inner(fee) + .saturating_mul(params.multiplier) .checked_div(¶ms.exchange_rate) .expect("exchange rate is not zero; qed") .into_inner(); diff --git a/bridges/snowbridge/pallets/outbound-queue/src/mock.rs b/bridges/snowbridge/pallets/outbound-queue/src/mock.rs index 850b13dcf3..67877a05c7 100644 --- a/bridges/snowbridge/pallets/outbound-queue/src/mock.rs +++ b/bridges/snowbridge/pallets/outbound-queue/src/mock.rs @@ -77,7 +77,8 @@ parameter_types! { pub Parameters: PricingParameters = PricingParameters { exchange_rate: FixedU128::from_rational(1, 400), fee_per_gas: gwei(20), - rewards: Rewards { local: DOT, remote: meth(1) } + rewards: Rewards { local: DOT, remote: meth(1) }, + multiplier: FixedU128::from_rational(4, 3), }; } diff --git a/bridges/snowbridge/pallets/outbound-queue/src/test.rs b/bridges/snowbridge/pallets/outbound-queue/src/test.rs index 8ed4a318d6..4e9ea36e24 100644 --- a/bridges/snowbridge/pallets/outbound-queue/src/test.rs +++ b/bridges/snowbridge/pallets/outbound-queue/src/test.rs @@ -268,28 +268,34 @@ fn encode_digest_item() { } #[test] -fn validate_messages_with_fees() { +fn test_calculate_fees_with_unit_multiplier() { new_tester().execute_with(|| { - let message = mock_message(1000); - let (_, fee) = OutboundQueue::validate(&message).unwrap(); + let gas_used: u64 = 250000; + let price_params: PricingParameters<::Balance> = PricingParameters { + exchange_rate: FixedU128::from_rational(1, 400), + fee_per_gas: 10000_u32.into(), + rewards: Rewards { local: 1_u32.into(), remote: 1_u32.into() }, + multiplier: FixedU128::from_rational(1, 1), + }; + let fee = OutboundQueue::calculate_fee(gas_used, price_params); assert_eq!(fee.local, 698000000); - assert_eq!(fee.remote, 2680000000000); + assert_eq!(fee.remote, 1000000); }); } #[test] -fn test_calculate_fees() { +fn test_calculate_fees_with_multiplier() { new_tester().execute_with(|| { let gas_used: u64 = 250000; - let illegal_price_params: PricingParameters<::Balance> = - PricingParameters { - exchange_rate: FixedU128::from_rational(1, 400), - fee_per_gas: 10000_u32.into(), - rewards: Rewards { local: 1_u32.into(), remote: 1_u32.into() }, - }; - let fee = OutboundQueue::calculate_fee(gas_used, illegal_price_params); + let price_params: PricingParameters<::Balance> = PricingParameters { + exchange_rate: FixedU128::from_rational(1, 400), + fee_per_gas: 10000_u32.into(), + rewards: Rewards { local: 1_u32.into(), remote: 1_u32.into() }, + multiplier: FixedU128::from_rational(4, 3), + }; + let fee = OutboundQueue::calculate_fee(gas_used, price_params); assert_eq!(fee.local, 698000000); - assert_eq!(fee.remote, 1000000); + assert_eq!(fee.remote, 1333333); }); } @@ -297,13 +303,13 @@ fn test_calculate_fees() { fn test_calculate_fees_with_valid_exchange_rate_but_remote_fee_calculated_as_zero() { new_tester().execute_with(|| { let gas_used: u64 = 250000; - let illegal_price_params: PricingParameters<::Balance> = - PricingParameters { - exchange_rate: FixedU128::from_rational(1, 1), - fee_per_gas: 1_u32.into(), - rewards: Rewards { local: 1_u32.into(), remote: 1_u32.into() }, - }; - let fee = OutboundQueue::calculate_fee(gas_used, illegal_price_params.clone()); + let price_params: PricingParameters<::Balance> = PricingParameters { + exchange_rate: FixedU128::from_rational(1, 1), + fee_per_gas: 1_u32.into(), + rewards: Rewards { local: 1_u32.into(), remote: 1_u32.into() }, + multiplier: FixedU128::from_rational(1, 1), + }; + let fee = OutboundQueue::calculate_fee(gas_used, price_params.clone()); assert_eq!(fee.local, 698000000); // Though none zero pricing params the remote fee calculated here is invalid // which should be avoided diff --git a/bridges/snowbridge/pallets/system/src/lib.rs b/bridges/snowbridge/pallets/system/src/lib.rs index 6e5ceb5e9b..39c73e3630 100644 --- a/bridges/snowbridge/pallets/system/src/lib.rs +++ b/bridges/snowbridge/pallets/system/src/lib.rs @@ -159,6 +159,7 @@ pub mod pallet { type DefaultPricingParameters: Get>; /// Cost of delivering a message from Ethereum + #[pallet::constant] type InboundDeliveryCost: Get>; type WeightInfo: WeightInfo; @@ -334,6 +335,7 @@ pub mod pallet { let command = Command::SetPricingParameters { exchange_rate: params.exchange_rate.into(), delivery_cost: T::InboundDeliveryCost::get().saturated_into::(), + multiplier: params.multiplier.into(), }; Self::send(PRIMARY_GOVERNANCE_CHANNEL, command, PaysFee::::No)?; diff --git a/bridges/snowbridge/pallets/system/src/mock.rs b/bridges/snowbridge/pallets/system/src/mock.rs index a711eab5d3..0312456c98 100644 --- a/bridges/snowbridge/pallets/system/src/mock.rs +++ b/bridges/snowbridge/pallets/system/src/mock.rs @@ -193,7 +193,8 @@ parameter_types! { pub Parameters: PricingParameters = PricingParameters { exchange_rate: FixedU128::from_rational(1, 400), fee_per_gas: gwei(20), - rewards: Rewards { local: DOT, remote: meth(1) } + rewards: Rewards { local: DOT, remote: meth(1) }, + multiplier: FixedU128::from_rational(4, 3) }; pub const InboundDeliveryCost: u128 = 1_000_000_000; diff --git a/bridges/snowbridge/primitives/core/src/outbound.rs b/bridges/snowbridge/primitives/core/src/outbound.rs index bce123878d..0ba0fdb610 100644 --- a/bridges/snowbridge/primitives/core/src/outbound.rs +++ b/bridges/snowbridge/primitives/core/src/outbound.rs @@ -136,6 +136,8 @@ mod v1 { exchange_rate: UD60x18, // Cost of delivering a message from Ethereum to BridgeHub, in ROC/KSM/DOT delivery_cost: u128, + // Fee multiplier + multiplier: UD60x18, }, } @@ -203,10 +205,11 @@ mod v1 { Token::Uint(U256::from(*transfer_asset_xcm)), Token::Uint(*register_token), ])]), - Command::SetPricingParameters { exchange_rate, delivery_cost } => + Command::SetPricingParameters { exchange_rate, delivery_cost, multiplier } => ethabi::encode(&[Token::Tuple(vec![ Token::Uint(exchange_rate.clone().into_inner()), Token::Uint(U256::from(*delivery_cost)), + Token::Uint(multiplier.clone().into_inner()), ])]), } } @@ -273,7 +276,8 @@ mod v1 { } } -#[cfg_attr(feature = "std", derive(PartialEq, Debug))] +#[derive(Clone, Encode, Decode, RuntimeDebug, TypeInfo)] +#[cfg_attr(feature = "std", derive(PartialEq))] /// Fee for delivering message pub struct Fee where @@ -346,12 +350,13 @@ pub trait GasMeter { /// the command within the message const MAXIMUM_BASE_GAS: u64; + /// Total gas consumed at most, including verification & dispatch fn maximum_gas_used_at_most(command: &Command) -> u64 { Self::MAXIMUM_BASE_GAS + Self::maximum_dispatch_gas_used_at_most(command) } - /// Measures the maximum amount of gas a command payload will require to dispatch, AFTER - /// validation & verification. + /// Measures the maximum amount of gas a command payload will require to *dispatch*, NOT + /// including validation & verification. fn maximum_dispatch_gas_used_at_most(command: &Command) -> u64; } diff --git a/bridges/snowbridge/primitives/core/src/pricing.rs b/bridges/snowbridge/primitives/core/src/pricing.rs index 33aeda6d15..0f392c7ad4 100644 --- a/bridges/snowbridge/primitives/core/src/pricing.rs +++ b/bridges/snowbridge/primitives/core/src/pricing.rs @@ -13,6 +13,8 @@ pub struct PricingParameters { pub rewards: Rewards, /// Ether (wei) fee per gas unit pub fee_per_gas: U256, + /// Fee multiplier + pub multiplier: FixedU128, } #[derive(Clone, Encode, Decode, PartialEq, RuntimeDebug, MaxEncodedLen, TypeInfo)] @@ -43,6 +45,9 @@ where if self.rewards.remote.is_zero() { return Err(InvalidPricingParameters) } + if self.multiplier == FixedU128::zero() { + return Err(InvalidPricingParameters) + } Ok(()) } } diff --git a/cumulus/parachains/runtimes/bridge-hubs/bridge-hub-rococo/src/lib.rs b/cumulus/parachains/runtimes/bridge-hubs/bridge-hub-rococo/src/lib.rs index bf7483179f..3980fa0d50 100644 --- a/cumulus/parachains/runtimes/bridge-hubs/bridge-hub-rococo/src/lib.rs +++ b/cumulus/parachains/runtimes/bridge-hubs/bridge-hub-rococo/src/lib.rs @@ -38,7 +38,9 @@ pub mod xcm_config; use cumulus_pallet_parachain_system::RelayNumberMonotonicallyIncreases; use snowbridge_beacon_primitives::{Fork, ForkVersions}; use snowbridge_core::{ - gwei, meth, outbound::Message, AgentId, AllowSiblingsOnly, PricingParameters, Rewards, + gwei, meth, + outbound::{Command, Fee}, + AgentId, AllowSiblingsOnly, PricingParameters, Rewards, }; use snowbridge_router_primitives::inbound::MessageToXcm; use sp_api::impl_runtime_apis; @@ -503,7 +505,8 @@ parameter_types! { pub Parameters: PricingParameters = PricingParameters { exchange_rate: FixedU128::from_rational(1, 400), fee_per_gas: gwei(20), - rewards: Rewards { local: 1 * UNITS, remote: meth(1) } + rewards: Rewards { local: 1 * UNITS, remote: meth(1) }, + multiplier: FixedU128::from_rational(1, 1), }; } @@ -1022,8 +1025,8 @@ impl_runtime_apis! { snowbridge_pallet_outbound_queue::api::prove_message::(leaf_index) } - fn calculate_fee(message: Message) -> Option { - snowbridge_pallet_outbound_queue::api::calculate_fee::(message) + fn calculate_fee(command: Command, parameters: Option>) -> Fee { + snowbridge_pallet_outbound_queue::api::calculate_fee::(command, parameters) } }