Module polkadot_sdk_docs::polkadot_sdk::xcm

Learn about XCM, the de-facto communication language between different consensus systems.

XCM

XCM, or Cross-Consensus Messaging, is a language to communicate intentions between consensus systems.

Overview

XCM is a standard, specification of which lives in the xcm format repo. It’s agnostic both in programming language and blockchain platform, which means it could be used in Rust in Polkadot, or in Go or C++ in any other platform like Cosmos or Ethereum.

It enables different consensus systems to communicate with each other in an expressive manner. Consensus systems include blockchains, smart contracts, and any other state machine that achieves consensus in some way.

XCM is executed on a virtual machine called the XCVM. Scripts can be written with the XCM language, which are often called XCMs, messages or XCM programs. Each program is a series of instructions, which get executed one after the other by the virtual machine. These instructions aim to encompass all major things users typically do in consensus systems. There are instructions on asset transferring, teleporting, locking, among others. New instructions are added and changes to the XCVM are made via the RFC process.

In Polkadot SDK

The Polkadot SDK allows for easily deploying sovereign blockchains from scratch, all very customizable. Dealing with many heterogeneous blockchains can be cumbersome. XCM allows all these blockchains to communicate with an agreed-upon language. As long as an implementation of the XCVM is implemented, the same XCM program can be executed in all blockchains and perform the same task.

Implementation

A ready-to-use Rust implementation lives in the polkadot-sdk repo, but will be moved to its own repo in the future.

Its main components are:

• xcm: The definition of the basic types and instructions.
• xcm_executor: An implementation of the virtual machine to execute instructions.
• pallet_xcm: A FRAME pallet for interacting with the executor.
• xcm_builder: A collection of types to configure the executor.
• xcm_simulator: A playground for trying out different XCM programs and executor configurations.

Example

To perform the very usual operation of transferring assets, the following XCM program can be used:

#[test]
fn example_transfer() {
	let _transfer_program = Xcm::<()>(vec![
		WithdrawAsset((Here, 100u128).into()),
		BuyExecution { fees: (Here, 100u128).into(), weight_limit: Unlimited },
		DepositAsset {
			assets: All.into(),
			beneficiary: AccountId32 { id: [0u8; 32].into(), network: None }.into(),
		},
	]);
}

Get started

To learn how it works and to get started, go to the XCM docs.