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Remote EVM Calls Through XCM

Introduction

The XCM Transactor Pallet provides a simple interface to perform remote cross-chain calls through XCM. However, this does not consider the possibility of doing remote calls to Moonbeam's EVM, only to Substrate specific pallets (functionalities).

Moonbeam's EVM is only accessible through the Ethereum Pallet. Among many other things, this pallet handles certain validations of transactions before getting them into the transaction pool. Then, it performs another validation step before inserting a transaction from the pool in a block. Lastly, it provides the interface through a transact function to execute a validated transaction. All these steps follow the same behavior as an Ethereum transaction in terms of structure and signature scheme.

However, calling the Ethereum Pallet directly through an XCM Transact is not feasible. Mainly because the dispatcher account for the remote EVM call (referred to as msg.sender in Ethereum) does not sign the XCM transaction on the Moonbeam side. The XCM extrinsic is signed in the origin chain, and the XCM executor dispatches the call, through the Transact instruction, from a known caller linked to the sender in the origin chain. In this context, the Ethereum Pallet will not be able to verify the signature and, ultimately, validate the transaction.

To this end, the Ethereum XCM Pallet was introduced. It acts as a middleware between the XCM Transact instruction and the Ethereum Pallet, as special considerations need to be made when performing EVM calls remotely through XCM. The pallet performs the necessary checks and validates the transaction. Next, the pallet calls the Ethereum Pallet to dispatch the transaction to the EVM. Due to how the EVM is accessed, there are some differences between regular and remote EVM calls.

The happy path for both regular and remote EVM calls through XCM is portrayed in the following diagram:

Happy path for regular and remote EVM calls through XCM

This guide will go through the differences between regular and remote EVM calls. In addition, it will show you how to perform remote EVM calls through the extrinsic exposed by the Ethereum XCM pallet.

Note

Remote EVM calls are done through the XCM Transactor Pallet. Therefore, it is recommended to get familiar with XCM Transactor concepts before trying to perform remote EVM calls through XCM.

Note that remote calls to Moonbeam's EVM through XCM are still being actively developed. In addition, developers must understand that sending incorrect XCM messages can result in the loss of funds. Consequently, it is essential to test XCM features on a TestNet before moving to a production environment.

Relevant XCM Definitions

  • Sovereign account — an account each chain in the ecosystem has, one for the relay chain and the other for other parachains. It is calculated as the blake2 hash of a specific word and parachain ID concatenated (blake2(para+ParachainID) for the Sovereign account in the relay chain, and blake2(sibl+ParachainID) for the Sovereign account in other parachains), truncating the hash to the correct length. The account is owned by root and can only be used through SUDO (if available) or governance (referenda). The Sovereign account typically signs XCM messages in other chains in the ecosystem

  • Multilocation — a way to specify a point in the entire relay chain/parachain ecosystem relative to a given origin. For example, it can be used to specify a specific parachain, asset, account, or even a pallet inside a parachain. In general terms, a multilocation is defined with a parents and an interior:

    • parents - refers to how many "hops" into a parent blockchain you need to take from a given origin
    • interior - refers to how many fields you need to define the target point.

    For example, to target a parachain with ID 1000 from another parachain, the multilocation would be { "parents": 1, "interior": { "X1": [{ "Parachain": 1000 }]}}

  • Multilocation-derivative account — an account derivated from the new origin set by the Descend Origin XCM instruction and the provided multilocation, which is typically the sovereign account from which the XCM originated. Derivative accounts are keyless (the private key is unknown). Consequently, derivative accounts related to XCM-specific use cases can only be accessed through XCM extrinsics. For Moonbeam-based networks, the derivation method is calculating the blake2 hash of the multilocation, which includes the origin parachain ID, and truncating the hash to the correct length (20 bytes for an Ethereum-styled account). The XCM call origin conversion happens when the Transact instruction gets executed. Consequently, each parachain can convert the origin with its own desired procedure, so the user who initiated the transaction might have a different derivative account per parachain. This derivative account pays for transaction fees, and it is set as the dispatcher of the call

  • Transact information — relates to extra weight and fee information for the XCM remote execution part of the XCM Transactor extrinsic. This is needed because the sovereign account pays the XCM transaction fee. Therefore, XCM Transactor calculates what the fee is and charges the sender of the XCM Transactor extrinsic the estimated amount in the corresponding XC-20 token to repay the sovereign account

Differences Between Regular and Remote EVM Calls Through XCM

As explained in the Introduction, the paths that regular and remote EVM calls take to get to the EVM are quite different. The main reason behind this difference is the dispatcher of the transaction.

A regular EVM call has an apparent sender who signs the Ethereum transaction with its private key. The signature, of ECDSA type, can be verified with the signed message and the r-s values that are produced from the signing algorithm. Ethereum signatures use an additional variable, called v, which is the recovery identifier.

With remote EVM calls, the signer signed an XCM transaction in another chain. Moonbeam receives that XCM message which must be constructed with the following instructions:

The first instruction, DescendOrigin, will mutate the origin of the XCM call on the Moonbeam side to a keyless account through the multilocation-derivative account mechanism described in the Relevant XCM Definitions section. The remote EVM call is dispatched from that keyless account (or a related proxy). Therefore, because the transaction is not signed, it does not have the real v-r-s values of the signature, but 0x1 instead.

Because a remote EVM call does not have the actual v-r-s values of the signature, there could be collision problems of the EVM transaction hash, as it is calculated as the keccak256 hash of the signed transaction blob. In consequence, if two accounts with the same nonce submit the same transaction object, they will end up with the same EVM transaction hash. Therefore, all remote EVM transactions use a global nonce that is attached to the Ethereum XCM pallet.

Another significant difference is in terms of the gas price. The fee for remote EVM calls is charged at an XCM execution level. Consequently, the gas price at an EVM level is zero, and the EVM will not charge for the execution itself. This can also be seen in the receipt of a remote EVM call transaction. Accordingly, the XCM message must be configured so that the BuyExecution buys enough weight to cover the gas cost.

The last difference is in terms of gas limit. Ethereum uses a gas-metered system to moderate the amount of execution that can be done in a block. On the contrary, Moonbeam uses a weight-based system, in which each call is characterized by the time it takes to execute in a block. Each unit of weight corresponds to one picosecond of execution time.

The configuration of the XCM queue suggests that XCM messages should be executable within 20,000,000,000 weight units (that is, 0.02 seconds of block execution time). Suppose the XCM message can't be executed due to the lack of execution time in a given block, and the weight requirement is over 20,000,000,000. In that case, the XCM message will be marked as overweight and would only be executable through democracy.

The 20,000,000,000 weight limit per XCM message constrains the gas limit available for remote EVM calls through XCM. For all Moonbeam-based networks, there is a ratio of 25,000 units of gas per unit of weight (WEIGHT_REF_TIME_PER_SECOND / GAS_PER_SECOND). Considering that you need some of the XCM message weight to execute the XCM instructions themselves. Therefore, a remote EVM call might have around 18,000,000,000 weight left, which is 720,000 gas units. Consequently, the maximum gas limit you can provide for a remote EVM call is around 720,000 gas units. Note that this might change in the future.

In summary, these are the main differences between regular and remote EVM calls:

  • Remote EVM calls use a global nonce (owned by the Ethereum-XCM pallet) instead of a nonce per account
  • The v-r-s values of the signature for remote EVM calls are 0x1. The sender can't be retrieved from the signature through standard methods (for example, through ECRECOVER). Nevertheless, the from is included in both the transaction receipt and when getting the transaction by hash (using the Ethereum JSON-RPC)
  • The gas price for all remote EVM calls is zero. The EVM execution is charged at an XCM execution level and not at an EVM level
  • The current maximum gas limit you can set for a remote EVM call is 720,000 gas units

Ethereum XCM Pallet Interface

Extrinsics

The Ethereum XCM pallet provides the following extrinsics (functions) that can be called by the Transact instruction to access Moonbeam's EVM through XCM:

  • transact(xcmTransaction) — function to remotely call the EVM through XCM. Only callable through the execution of an XCM message
  • transactThroughProxy(transactAs, xcmTransaction) — similar to the transact extrinsic, but with transactAs as an additional field. This function allows the remote EVM call to be dispatched from a given account with known keys (the msg.sender). This account needs to have set the multilocation-derivative account as a proxy of type any on Moonbeam. On the contrary, the dispatch of the remote EVM call will fail. Transaction fees are still paid by the multilocation-derivative account

Where the inputs that need to be provided can be defined as:

  • xcmTransaction — contains the Ethereum transaction details of the call that will be dispatched. This includes the call data, msg.value and gas limit
  • transactAs — account from which the remote EVM call will be dispatched (the msg.sender). The account set in this field needs to have set the multilocation-derivative account as a proxy of type any on Moonbeam. Transaction fees are still paid by the multilocation-derivative account

Building a Remote EVM call through XCM

This guide covers building an XCM message for remote EVM calls using the XCM Pallet from the relay chain to Moonbase Alpha. More specifically, it will use the transact function. The steps to use the transactThroughProxy function are identical. However, you'll need to provide the transactAs account and ensure that this account has set the multilocation-derivative account as a proxy of type any on Moonbase Alpha.

Note

When using transactThroughProxy, the EVM call is dispatched by the transactAs account you provide, acting as the msg.sender, as long as this account has set the the multilocation-derivative account as a proxy of type any in the Moonbeam-based network you are using. However, transaction fees are still paid by the multilocation-derivative account, so you need to ensure it has enough funds to cover them.

Checking Prerequisites

To be able to send the call from the relay chain, you need to have the following:

  • An account on the relay chain with funds (UNIT) to pay for the transaction fees. You can acquire some xcUNIT by swapping for DEV tokens (Moonbase Alpha's native token) on Moonbeam-Swap, a demo Uniswap-V2 clone on Moonbase Alpha, and then send them to the relay chain. Additionally, you can contact us to get some UNIT tokens directly

  • Fund the multilocation-derivative account, which you can obtain by following the steps in the next section. The account must have enough DEV tokens (or GLMR/MOVR for Moonbeam/Moonriver) to cover the cost of the XCM execution of the remote EVM call. Note that this is the account from which the remote EVM call will be dispatched (the msg.sender). Consequently, the account must satisfy whatever conditions are required for the EVM call to be executed correctly. For example, hold any relevant ERC-20 token if you are doing an ERC-20 transfer

Note

Suppose you are using the transactThroughProxy function. In that case, the transactAs account must satisfy whatever conditions are required for the EVM call to be executed correctly, as it acts as the msg.sender. However, the multilocation-derivative account is the one that needs to hold the DEV tokens (or GLMR/MOVR for Moonbeam/Moonriver) to cover the cost of the XCM execution of the remote EVM call.

Calculating the Multilocation-Derivative Account

As mentioned before, a remote EVM call is dispatched from an account called the multilocation-derivative account. This is calculated using the information provided by the Descend Origin instruction. Consequently, the computed account depends directly on how the instruction is constructed.

For example, from the relay chain, the DescendOrigin instruction is natively injected by the XCM Pallet. In the case of Moonbase Alpha's relay chain (based on Westend), is with the following format (a multilocation junction):

{
  DescendOrigin: {
    X1: {
      AccountId32: {
        network: { westend: null },
        id: decodedAddress,
      },
    },
  },
}

Where the decodedAddress corresponds to the address of the account who signed the transaction on the relay chain (in a decoded 32 bytes format). You can make sure that your address is properly decoded by using the following snippet, which will decode an address if needed and ignore it if not:

import { decodeAddress } from '@polkadot/util-crypto';
const decodedAddress = decodeAddress('INSERT_ADDRESS');

When the XCM instruction gets executed in Moonbeam (Moonbase Alpha in this example), the origin will have mutated to the following multilocation:

{
  DescendOrigin: {
    parents: 1,
    interior: {
      X1: {
        AccountId32: {
          network: { westend: null },
          id: decodedAddress,
        },
      },
    },
  },
}

Copy the account of your existing or newly created account on the Moonbase relay chain. You're going to need it to calculate the corresponding multilocation-derivative account, which is a special type of account that’s keyless (the private key is unknown). Transactions from a multilocation-derivative account can be initiated only via valid XCM instructions from the corresponding account on the relay chain. In other words, you are the only one who can initiate transactions on your multilocation-derivative account, and if you lose access to your Moonbase relay account, you’ll also lose access to your multilocation-derivative account.

To generate the multilocation-derivative account, first clone the xcm-tools repo. Run yarn to install the necessary packages, and then run:

yarn calculate-multilocation-derivative-account \
--ws-provider wss://wss.api.moonbase.moonbeam.network \
--address INSERT_MOONBASE_RELAY_ACCOUNT \
--para-id INSERT_ORIGIN_PARACHAIN_ID_IF_APPLIES \
--parents INSERT_PARENTS_VALUE_IF_APPLIES

Let's review the parameters passed along with this command:

  • The --ws-provider or -w flag corresponds to the endpoint we’re using to fetch this information
  • The --address or -a flag corresponds to your Moonbase relay chain address
  • The --para-id or -p flag corresponds to the parachain ID of the origin chain (if applicable). If you are sending the XCM from the relay chain, you don't need to provide this parameter
  • The -parents flag corresponds to the parents value of the origin chain in relation to the destination chain. If you're deriving a multi-location derivative account on a parachain destination from a relay chain origin, this value would be 1. If left out, the parents value defaults to 0

For example, for Alice's relay chain account of 5DV1dYwnQ27gKCKwhikaw1rz1bYdvZZUuFkuduB4hEK3FgDT, you can calculate her Moonbase Alpha multilocation-derivative account by running:

yarn calculate-multilocation-derivative-account \
--ws-provider wss://wss.api.moonbase.moonbeam.network \
--address 5DV1dYwnQ27gKCKwhikaw1rz1bYdvZZUuFkuduB4hEK3FgDT \
--parents 1

The relevant values for this calculation are summarized in the following table:

Name Value
Origin Chain Encoded Address 5DV1dYwnQ27gKCKwhikaw1rz1bYdvZZUuFkuduB4hEK3FgDT
Origin Chain Decoded Address 0x3ec5f48ad0567c752275d87787954fef72f557b8bfa5eefc88665fa0beb89a56
Multilocation Received in Destination Chain {"parents":1,"interior":{"x1":{"accountId32":{"network": {"westend":null},"id":"0xdd2399f3b5ca0fc584c4637283cda4d73f6f87c0afb2e78fdbbbf4ce26c2556c"}}}}
Multilocation-Derivative Account (32 bytes) 0xdd2399f3b5ca0fc584c4637283cda4d73f6f87c0afb2e78fdbbbf4ce26c2556c
Multilocation-Derivative Account (20 bytes) 0xdd2399f3b5ca0fc584c4637283cda4d73f6f87c0

Consequently, for this example, the multilocation-derivative account for Moonbase Alpha is 0xdd2399f3b5ca0fc584c4637283cda4d73f6f87c0. Note that Alice is the only person who can access this account through a remote transact from the relay chain, as she is the owner of its private keys and the multilocation-derivative account is keyless.

Ethereum XCM Transact Call Data

Before you send the XCM message from the relay chain to Moonbase Alpha, you need to get the encoded call data that will be dispatched through the execution of the Transact XCM instruction.

In this example, you'll be interacting with the transact function of the Ethereum XCM Pallet, which accepts an xcmTransaction as a parameter.

The xcmTransaction parameter requires you to define:

  • A gas limit
  • The action to be executed, which provides two options: Call and Create. The current implementation of the Ethereum XCM pallet does not support the CREATE operation. Therefore, you can't deploy a smart contract through remote EVM calls. For Call, you'll need to specify the contract address you're interacting with
  • The value of native tokens to send
  • The input, which is the encoded call data of the contract interaction

For the action to be executed, you'll be performing a contract interaction with a simple incrementer contract, which is located at 0xa72f549a1a12b9b49f30a7f3aeb1f4e96389c5d8. You'll be calling the increment function, which has no input argument and will increase the value of the number by one. Also, it will store the block's timestamp in which the function is executed to the timestamp variable.

The encoded call data of the interaction with the increment function is 0xd09de08a, which is the first eight hexadecimal characters (or 4 bytes) of the keccak256 hash of increment(). If you choose to interact with a function that has input parameters, they also need to be encoded. The easiest way to get the encoded call data is to emulate a transaction either in Remix or Moonscan. Next, in Metamask, check the HEX DATA: 4 BYTES selector under the HEX tab before signing it. You don't need to sign the transaction.

Now that you have the encoded contract interaction data, you can determine the gas limit for this call using the eth_estimateGas JSON-RPC method. For this example, you can set the gas limit to 155000.

For the value, you can set it to 0 since this particular interaction does not need DEV (or GLMR/MOVR for Moonbeam/Moonriver). For an interaction that requires DEV, you'll need to modify this value accordingly.

Now that you have all of the components required for the xcmTransaction parameter, you can build it:

const xcmTransaction = {
  V2: {
    gasLimit: 155000,
    action: { Call: '0xa72f549a1a12b9b49f30a7f3aeb1f4e96389c5d8' }, // Call the incrementer contract
    value: 0,
    input: '0xd09de08a', // Call the increment function
  },
};

Next, you can write the script to get the encoded call data for the transaction. You'll take the following steps:

  1. Provide the input data for the call. This includes:
    • The Moonbase Alpha endpoint URL to create the provider
    • The value for the xcmTransaction parameter of the transact function
  2. Create the Polkadot.js API provider
  3. Craft the ethereumXcm.transact extrinsic with the xcmTransaction value
  4. Get the encoded call data for the extrinsic. You don't need to sign and send the transaction
import { ApiPromise, WsProvider } from '@polkadot/api'; // Version 9.13.6

// 1. Input data
const providerWsURL = 'wss://wss.api.moonbase.moonbeam.network';
const xcmTransaction = {
  V2: {
    gasLimit: 155000,
    action: { Call: '0xa72f549a1a12b9b49f30a7f3aeb1f4e96389c5d8' },
    value: 0,
    input: '0xd09de08a',
  },
};

const getEncodedCallData = async () => {
  // 2. Create Substrate API Provider
  const substrateProvider = new WsProvider(providerWsURL);
  const api = await ApiPromise.create({ provider: substrateProvider });

  // 3. Create the extrinsic
  const tx = api.tx.ethereumXcm.transact(xcmTransaction);

  // 4. Get the encoded call data
  const encodedCall = tx.method.toHex();
  console.log(`Encoded Calldata: ${encodedCall}`);

  api.disconnect();
};

getEncodedCallData();

Note

You can view an example of the output of the above script on Polkadot.js Apps using the following encoded call data: 0x260001785d02000000000000000000000000000000000000000000000000000000000000a72f549a1a12b9b49f30a7f3aeb1f4e96389c5d8000000000000000000000000000000000000000000000000000000000000000010d09de08a00.

You'll use the encoded call data in the Transact instruction in the following section.

Estimate Weight Required At Most

When using the Transact instruction, you'll need to define the requireWeightAtMost field, which is the required weight for the transaction. This field accepts two arguments: the refTime and proofSize. The refTime is the amount of computational time that can be used for execution, and the proofSize is the amount of storage in bytes that can be used.

To get an estimate for the refTime and proofSize, you can use the paymentInfo method of the Polkadot.js API. Since these weights are required for the Transact call data, you can extend the script from the previous section to add in the call to paymentInfo.

The paymentInfo method accepts the same parameters you would normally pass to the .signAndSend method, which is the sending account and, optionally, some additional values such as a nonce or signer.

To modify the encoded call data script, you'll need to add in logic to create a Keyring for the sender, which in this case is Alice. Then you'll simply need to take the tx and call the paymentInfo method and pass in Alice's Keyring.

Building the XCM for Remote XCM Execution

In this example, you'll build an XCM message to execute a remote EVM call in Moonbase Alpha from its relay chain through the Transact XCM instruction and the transact function of the Ethereum XCM pallet.

Now that you've generated the Ethereum XCM pallet encoded call data, you're going to use the XCM Pallet on the relay chain to perform a remote execution. To do so, you'll use the send function, which accepts two parameters: dest and message. You can start assembling these parameters by taking the following steps:

  1. Build the multilocation of the destination, which is Moonbase Alpha:

    const dest = { V3: { parents: 0, interior: { X1: { Parachain: 1000 } } } };

  2. Build the WithdrawAsset instruction, which will require you to define:

    • The multilocation of the DEV token on Moonbase Alpha
    • The amount of DEV tokens to withdraw
    const instr1 = {
  WithdrawAsset: [
    {
      id: { Concrete: { parents: 0, interior: { X1: { PalletInstance: 3 } } } },
      fun: { Fungible: 10000000000000000n }, // 0.01 DEV
    },
  ],
};

  3. Build the BuyExecution instruction, which will require you to define:

    • The multilocation of the DEV token on Moonbase Alpha
    • The amount of DEV tokens to buy for execution
    • The weight limit
    const instr2 = {
  BuyExecution: [
    {
      id: { Concrete: { parents: 0, interior: { X1: { PalletInstance: 3 } } } },
      fun: { Fungible: 10000000000000000n }, // 0.01 DEV
    },
    { Unlimited: null },
  ],
};

  4. Build the Transact instruction, which will require you to define:

    • The origin kind

    • The required weight for the transaction. You'll need to define a value for refTime, which is the amount of computational time that can be used for execution, and the proofSize, which is the amount of storage in bytes that can be used. Both figures can be calculated using the paymentInfo method of the Polkadot.js API. To calculate these values, you can modify the encoded call data script to call the paymentInfo method of the ethereumXcm.transact(xcmTransaction) transaction. To call the paymentInfo method, you'll need to pass in the senders account. You can pass in Alice's account on the relay chain: 5DV1dYwnQ27gKCKwhikaw1rz1bYdvZZUuFkuduB4hEK3FgDT:

      ...

const tx = api.tx.ethereumXcm.transact(xcmTransaction);
const alice = '5DV1dYwnQ27gKCKwhikaw1rz1bYdvZZUuFkuduB4hEK3FgDT';
const info = await tx.paymentInfo(alice);
console.log(`Required Weight: ${info.weight.toString()}`);
      Complete modified script 
      import { ApiPromise, WsProvider } from '@polkadot/api'; // Version 9.13.6

// 1. Input data
const providerWsURL = 'wss://wss.api.moonbase.moonbeam.network';
const xcmTransaction = {
  V2: {
    gasLimit: 155000,
    action: { Call: '0xa72f549a1a12b9b49f30a7f3aeb1f4e96389c5d8' },
    value: 0,
    input: '0xd09de08a',
  },
};

const getEncodedCallData = async () => {
  // 2. Create Substrate API Provider
  const substrateProvider = new WsProvider(providerWsURL);
  const api = await ApiPromise.create({ provider: substrateProvider });

  // 3. Create the extrinsic
  const tx = api.tx.ethereumXcm.transact(xcmTransaction);

  // 4. Get the encoded call data
  const encodedCall = tx.method.toHex();
  console.log(`Encoded Calldata: ${encodedCall}`);

  // 5. Estimate the required weight
  const alice = '5DV1dYwnQ27gKCKwhikaw1rz1bYdvZZUuFkuduB4hEK3FgDT';
  const info = await tx.paymentInfo(alice);
  console.log(`Required Weight: ${info}`);

  api.disconnect();
};

getEncodedCallData();

      The script, at the time of writing, returns an estimate of 3900000000 for refTime and 38750 for proofSize.

    • The encoded call data, which you generated in the Ethereum XCM Transact Call Data section

    const instr3 = {
  Transact: {
    originKind: 'SovereignAccount',
    requireWeightAtMost: { refTime: 3900000000n, proofSize: 38750n },
    call: {
      encoded:
        '0x260001785d02000000000000000000000000000000000000000000000000000000000000a72f549a1a12b9b49f30a7f3aeb1f4e96389c5d8000000000000000000000000000000000000000000000000000000000000000010d09de08a00',
    },
  },
};

  5. Combine the XCM instructions into a versioned XCM message:

    const message = { V3: [instr1, instr2, instr3] };

Now that you have the values for each of the parameters, you can write the script for the execution. You'll take the following steps:

  1. Provide the input data for the call. This includes:
    • The relay chain endpoint URL to create the provider
    • The values for each of the parameters of the send function
  2. Create a Keyring instance that will be used to send the transaction
  3. Create the Polkadot.js API provider
  4. Craft the xcmPallet.send extrinsic with the dest and message values
  5. Send the transaction using the signAndSend extrinsic and the Keyring instance you created in the second step

Remember

This is for demo purposes only. Never store your private key in a JavaScript file.

import { ApiPromise, WsProvider, Keyring } from '@polkadot/api'; // Version 9.13.6
import { cryptoWaitReady } from '@polkadot/util-crypto';

// 1. Input data
const providerWsURL =
  'wss://frag-moonbase-relay-rpc-ws.g.moonbase.moonbeam.network';
const privateKey = 'INSERT_PRIVATE_KEY';
const dest = { V3: { parents: 0, interior: { X1: { Parachain: 1000 } } } };
const instr1 = {
  WithdrawAsset: [
    {
      id: { Concrete: { parents: 0, interior: { X1: { PalletInstance: 3 } } } },
      fun: { Fungible: 10000000000000000n }, // 0.01 DEV
    },
  ],
};
const instr2 = {
  BuyExecution: [
    {
      id: { Concrete: { parents: 0, interior: { X1: { PalletInstance: 3 } } } },
      fun: { Fungible: 10000000000000000n }, // 0.01 DEV
    },
    { Unlimited: null },
  ],
};
const instr3 = {
  Transact: {
    originKind: 'SovereignAccount',
    requireWeightAtMost: { refTime: 3900000000n, proofSize: 38750n },
    call: {
      encoded:
        '0x260001785d02000000000000000000000000000000000000000000000000000000000000a72f549a1a12b9b49f30a7f3aeb1f4e96389c5d8000000000000000000000000000000000000000000000000000000000000000010d09de08a00',
    },
  },
};
const message = { V3: [instr1, instr2, instr3] };

// 2. Create Keyring instance
await cryptoWaitReady();
const keyring = new Keyring({ type: 'sr25519' });
const alice = keyring.addFromUri(privateKey);

const sendXcmMessage = async () => {
  // 3. Create Substrate API Provider
  const substrateProvider = new WsProvider(providerWsURL);
  const api = await ApiPromise.create({ provider: substrateProvider });

  // 4. Create the extrinsic
  const tx = api.tx.xcmPallet.send(dest, message);

  // 5. Send the transaction
  const txHash = await tx.signAndSend(alice);
  console.log(`Submitted with hash ${txHash}`);

  api.disconnect();
};

sendXcmMessage();

Note

You can view an example of the output of the above script on Polkadot.js Apps using the following encoded call data: 0x630003000100a10f030c00040000010403000f0000c16ff28623130000010403000f0000c16ff2862300060103004775e87a5d02007901260001785d02000000000000000000000000000000000000000000000000000000000000a72f549a1a12b9b49f30a7f3aeb1f4e96389c5d8000000000000000000000000000000000000000000000000000000000000000010d09de08a00.

Once the transaction is processed, you can check the relevant extrinsics and events in the relay chain and Moonbase Alpha.

In the relay chain, the extrinsic is xcmPallet.send, and the associated event is xcmPallet.Sent (among others related to the fee). In Moonbase Alpha, the XCM execution happens within the parachainSystem.setValidationData extrinsic, and there are multiple associated events that can be highlighted:

  • parachainSystem.DownwardMessagesReceived — event that signals that a message from the relay chain was received. With the current XCM implementation, messages from other parachains will show the same event
  • balances.Withdraw — event related to the withdrawing of tokens to pay for the execution of the call. Note that the who address is the multilocation-derivative account calculated before
  • ethereum.Executed — event associated with the execution of the remote EVM call. It provides the from, to, transactionHash (calculated with the non-standard signature and global pallet nonce), and the exitReason. Currently, some common EVM errors, like out of gas, will show Reverted in the exit reason
  • polkadotXcm.AssetsTrapped — event that is emitted when part of the tokens withdrawn from the account (for fees) are not used. Generally, when there are leftover tokens in the registry that are not allocated to an account. These tokens are temporarily burned and can be retrieved through a democracy proposal. A combination of both RefundSurplus and DepositAsset XCM instructions can prevent assets from getting trapped

To verify that the remote EVM call through XCM was successful, you can head to the contract's page in Moonscan and verify the new value for the number and its timestamp.

Remote EVM Call Transaction by Hash

As mentioned before, there are some differences between regular and remote XCM EVM calls. Some main differences can be seen when retrieving the transaction by its hash using the Ethereum JSON-RPC.

To do so, you first need to retrieve the transaction hash you want to query. For this example, you can use the transaction hash from the previous section, which is 0x753588d6e59030eeffd31aabccdd0fb7c92db836fcaa8ad71512cf3a7d0cb97f. Open the terminal, and execute the following command:

curl --location --request POST 'https://rpc.api.moonbase.moonbeam.network' \
--header 'Content-Type: application/json' \
--data-raw '{
    "jsonrpc":"2.0",
    "id":1,
    "method":"eth_getTransactionByHash",
    "params": ["0x753588d6e59030eeffd31aabccdd0fb7c92db836fcaa8ad71512cf3a7d0cb97f"]
  }
'

If the JSON-RPC request is sent correctly, the response should look like this:

{
    "jsonrpc": "2.0",
    "result": {
        "hash": "0x753588d6e59030eeffd31aabccdd0fb7c92db836fcaa8ad71512cf3a7d0cb97f",
        "nonce": "0x129",
        "blockHash": "0xeb8222567e434215f472f0c53f68a606c77ea8f475e5fbc3a5b715db6cce8887",
        "blockNumber": "0x46c268",
        "transactionIndex": "0x0",
        "from": "0xdd2399f3b5ca0fc584c4637283cda4d73f6f87c0",
        "to": "0xa72f549a1a12b9b49f30a7f3aeb1f4e96389c5d8",
        "value": "0x0",
        "gasPrice": "0x0",
        "maxFeePerGas": "0x0",
        "maxPriorityFeePerGas": "0x0",
        "gas": "0x25d78",
        "input": "0xd09de08a",
        "creates": null,
        "raw": "0x02eb820507820129808083025d7894a72f549a1a12b9b49f30a7f3aeb1f4e96389c5d88084d09de08ac0010101",
        "publicKey": "0x14745b9075ac0f0426c61c9a2895f130ea6f3b964e8f49cefdb4e2d248306f19396361d877f8b9ad60a94a5ec28325a1b9baa2ae59e7a9f6fe1731caec130ab4",
        "chainId": "0x507",
        "standardV": "0x1",
        "v": "0x1",
        "r": "0x1",
        "s": "0x1",
        "accessList": [],
        "type": "0x2"
    },
    "id": 1
}

Note that the v-r-s values are set to 0x1, and the gas price-related fields are set to 0x0. In addition, the nonce field corresponds to a global nonce of the Ethereum XCM pallet, and not the transaction count of the dispatcher account.

Note

You might be able to find some transaction hash collisions in the Moonbase Alpha TestNet, as early versions of remote EVM calls through XCM did not use a global nonce of the Ethereum XCM pallet.

Last update: September 28, 2023
| Created: May 25, 2022