# Bifrost SALP Contribution and Confirmation Mechanism
###### tags: `External`

## SALP Contribution & Confirmation Workflow


**Step 1**: Users use XMP-DMP cross-in their KSM to Bifrost Parachian.

**Step 2**: Users call the SALP-contribute to a Crowdloan, the contribution will be executed by XCM-UMP transact.

**Step 3**: Subql Multisig nodes comfrim the contributed events on Kusama Relay chain, and send the result back to Bifrost parachain.

**Step 4**: Bifrost SALP receive the result, issue/or not issue derivatives vsKSM & vsBond for users.

## Contribute-UMP Transact

　Crowdloan contribution is proxy called by Bifrost Parachain account with UMP Transact

### Encode Call

- pallet&call index should match runtime definition in dest chain
https://github.com/paritytech/polkadot/blob/c406f7b7193709a4aa6f1003d2e6c2c8aa7ce220/runtime/kusama/src/lib.rs#L1517

#[derive(Encode, Decode)]
pub enum CrowdloanContributeCall<BalanceOf> {
	#[codec(index = 73)]
	CrowdloanContribute(ContributeCall<BalanceOf>),
}

#[derive(Encode, Decode)]
pub enum ContributeCall<BalanceOf> {
	#[codec(index = 1)]
	Contribute(Contribution<BalanceOf>),
}

#[derive(Debug, PartialEq, Encode, Decode)]
pub struct Contribution<BalanceOf> {
	#[codec(compact)]
	pub index: ParaId,
	#[codec(compact)]
	pub value: BalanceOf,
	pub signature: Option<MultiSignature>,
}

let call = CrowdloanContributeCall::CrowdloanContribute(
    ContributeCall::Contribute(Contribution { index, value, signature: None }),
)
.encode()
.into();

### Construct Transact

let message = Xcm::WithdrawAsset {
    assets: vec![MultiAsset::ConcreteFungible {
        id: MultiLocation::Null,
        amount: WeightToFee::calc(transact_weight+transact_debt)),
    }],
    effects: vec![Order::BuyExecution {
        fees: MultiAsset::All,
        weight: transact_weight,
        debt: transact_debt,
        halt_on_error: true,
        xcm: vec![Xcm::Transact {
            origin_type: OriginKind::SovereignAccount,
            require_weight_at_most: Unlimited,
            call:call,
        }],
    }],
};

### Weight Calculation

- Shallow weight & Deep weight
https://github.com/paritytech/polkadot/blob/7a9f624777ad2d2adb3a1e6797a31f9d653c9587/xcm/xcm-builder/src/weight.rs#L30-L83

- Debt(cover shallow weight)
https://github.com/paritytech/polkadot/blob/release-v0.9.9/xcm/xcm-builder/src/barriers.rs#L59

- Inner weight(cover deep weight)
https://github.com/paritytech/polkadot/blob/7a9f624777ad2d2adb3a1e6797a31f9d653c9587/xcm/xcm-builder/src/barriers.rs#L35

### Fee Calculation&Compensation

- use weightToFee from relaychain

(https://github.com/paritytech/polkadot/blob/7a9f624777ad2d2adb3a1e6797a31f9d653c9587/runtime/kusama/src/constants.rs#L71-L86)

- extend pallet_transaction_payment with OnChargeTransaction

impl pallet_transaction_payment::Config for Runtime {
	type OnChargeTransaction = FlexibleFee;
	...
}

T::MiscFeeHandler::deduct_fee(who, &T::TreasuryAccount::get(), call).map_err(
    |_| TransactionValidityError::Invalid(InvalidTransaction::Custom(77u8)),
)?;
			

### XCM Confirmation
　　XCM essentially is asynchronous&asymetric and in current XCM version you can not tell if the remote call is successful or not in relaychain. To achieve it we deply `salp-confirm-service` to confirm the contribution with multisig mechanism

- generate xcm messageId

https://github.com/bifrost-finance/bifrost/blob/bc11527ff2a35f275b41815803b149cff37f345c/xcm-support/src/lib.rs#L133-L135

- confirm service
https://github1s.com/bifrost-finance/salp-confirm-service

- events

https://polkadot.js.org/apps/?rpc=wss%3A%2F%2Fbifrost-parachain.api.onfinality.io%2Fpublic-ws#/explorer/query/426657

https://polkadot.js.org/apps/?rpc=wss%3A%2F%2Fkusama-rpc.polkadot.io#/explorer/query/9345308

https://polkadot.js.org/apps/?rpc=wss%3A%2F%2Fbifrost-parachain.api.onfinality.io%2Fpublic-ws#/explorer/query/426668

## Challenges&Issues

https://github.com/paritytech/polkadot/issues/3500

https://github.com/paritytech/polkadot/blob/65313050a8416ea358005a0f0905a0db87f9290f/node/service/src/chain_spec.rs#L185

https://github.com/paritytech/cumulus/issues/598

## TODO

- Migrate to V2 and remove confirm-service with fully decentralized Xcm notify&query

https://github.com/paritytech/polkadot/blob/65313050a8416ea358005a0f0905a0db87f9290f/xcm/pallet-xcm/src/lib.rs#L979

https://github.com/paritytech/xcm-format/pull/22