XCM Pallet

XCM Pallet

Lesson goals:

• Understand what the interface of the pallet is and its implementation.
• How versioning discovery works.
• How receiving responses work.
• Understand how to craft XCM in FRAME pallets.

The XCM pallet

We have now learnt about the XCVM and FRAME.

The XCM pallet is the bridge between the XCVM subsystem and the FRAME subsystem.

It also allows us to send/execute XCM and interact with the XCM executor.

How XCM is expected to be used

XCM is not intended to be written by end-users.

Instead, developers write XCVM programs, and package them up into FRAME extrinsics.

Notes:

How do wallets wallet providers use XCM ?

We will see examples of XCM being built in the runtime when exploring teleport_assets and reserve_transfer_assets extrinsics.

Key roles of pallet-xcm

Notes:

• Even when the XCM pallet allows any FRAME origin to send XCMs, it distinguishes root calls vs any other origin calls. In the case of the latter, it appends the DescendOrigin instruction to make sure non-root origins cannot act on behalf of the parachain.

The XCM Pallet

pallet-xcm provides default implementations for many traits required by XcmConfig.

pallet-xcm also provides an interface containing 10 different extrinsics, which can be split into three categories:

• Primitive functions to locally execute or send an XCM.
• High-level functions for asset transfers between systems, e.g. teleportation and reserve asset transfers.
• Extrinsics aimed exclusively at version negotiation.

pallet-xcm Primitive extrinsics

• execute

  Direct access to the XCM executor. Executed on behalf of FRAME's signed origin.

Notes:

It checks the origin to ensure that the configured SendXcmOrigin filter is not blocking the execution. It executes the message locally and returns the outcome as an event.

---v

pallet-xcm Primitive extrinsics

• send

Sends a message to the provided destination.

Notes:

This extrinsic is a function to send a message to a destination. It checks the origin, the destination and the message. Then it lets the XcmRouter handle the forwarding of the message.

pallet-xcm Asset Transfer extrinsics

Notes:

We have already seen what teleports and reserve transfers mean in lesson 7.1; A quick reminder.

---v

pallet-xcm Asset Transfer extrinsics

limited_teleport_assets

This extrinsic allows the user to perform an asset teleport.

---v

limited_teleport_assets

1. pallet-xcm composes the following XCM and passes it to xcm‑executor

#![allow(unused)]
fn main() {
Xcm(vec![
  WithdrawAsset(assets),
  SetFeesMode {jit_withdraw: true},
  InitiateTeleport {assets: Wild(AllCounted(max_assets)), dest, xcm},
])
}

2. Parachain A then sends the following message to the trusted destination

#![allow(unused)]
fn main() {
Xcm(vec![
  ReceiveTeleportedAsset(assets),
  ClearOrigin,
  BuyExecution { fees, weight_limit },
  DepositAsset { assets: Wild(AllCounted(max_assets)), beneficiary },
])
}

---v

pallet-xcm Asset Transfer extrinsics

limited_reserve_transfer_assets

Allow the user to perform a reserve-backed transfer from the reserve chain to the destination.

---v

limited_reserve_transfer_assets

1. pallet-xcm composes the following XCM and passes it to xcm‑executor

#![allow(unused)]
fn main() {
Xcm(vec![
  SetFeesMode { jit_withdraw: true },
  TransferReserveAsset { assets, dest, xcm },
])
}

2. Reserve Chain then sends the following mesasge to the destination

#![allow(unused)]
fn main() {
Xcm(vec![
  ReserveAssetDeposited(assets),
  ClearOrigin,
  BuyExecution { fees, weight_limit },
  DepositAsset { assets: Wild(AllCounted(max_assets)), beneficiary },
])
}

🗣️ version negotiation with pallet-xcm

XCM is a versioned message format.

One version may contain more or different instructions than another, so for parties to communicate via XCM, it is important to know which version the other party is using.

The version subscription mechanism allows parties to subscribe to version updates from others.

#![allow(unused)]
fn main() {
pub enum VersionedXcm {
  V2(v2::Xcm),
  V3(v3::Xcm),
}
}

#![allow(unused)]
fn main() {
enum Instruction {
  // --snip--
  SubscribeVersion {
        query_id: QueryId,
        max_response_weight: u64,
  },
  QueryResponse {
        query_id: QueryId,
        response: Response,
        max_weight: u64,
  },
  // --snip--
}
}

#![allow(unused)]
fn main() {
impl Config for XcmConfig {
 // --snip--
 type ResponseHandler = PalletXcm;
 type SubscriptionService = PalletXcm;
}
}

#![allow(unused)]
fn main() {
pub struct QueryResponseInfo {
    pub destination: MultiLocation,
    pub query_id: QueryId,
    pub max_weight: Weight,
}
}

#![allow(unused)]
fn main() {
enum Instruction {
    // --snip--
    QueryResponse {
        query_id: QueryId,
        response: Response,
        max_weight: Weight,
        querier: Option<MultiLocation>,
    },
    // --snip--
}
}