Interop message passing tutorial

Follow the Installation Guide to install:

• Foundry for smart contract development (required in all cases)
• Supersim for local blockchain simulation (optional)

Verify your installation:

forge --version
./supersim --version

Setting up test networks

1. If you are using Supersim, go to the directory where Supersim is installed and start it with autorelay.

   ./supersim --interop.autorelay

   If you are using the devnets, just skip this step.

   SupersimDevnets

   Supersim creates three anvil blockchains:

   Role	ChainID	RPC URL
   L1	900	http://127.0.0.1:8545 (opens in a new tab)
   OPChainA	901	http://127.0.0.1:9545 (opens in a new tab)
   OPChainB	902	http://127.0.0.1:9546 (opens in a new tab)

2. In a separate shell, store the configuration in environment variables.

   SupersimDevnets

   Set these parameters for Supersim.

   PRIVATE_KEY=0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80
   USER_ADDRESS=0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266
   URL_CHAIN_A=http://127.0.0.1:9545
   URL_CHAIN_B=http://127.0.0.1:9546
   INTEROP_BRIDGE=0x4200000000000000000000000000000000000028

cast balance --ether $USER_ADDRESS --rpc-url $URL_CHAIN_A
cast balance --ether $USER_ADDRESS --rpc-url $URL_CHAIN_B

Create the contracts

1. Create a new Foundry project.

   mkdir onchain-code
   cd onchain-code
   forge init 

2. In src/Greeter.sol put this file. This is a variation on Hardhat's Greeter contract (opens in a new tab).

   //SPDX-License-Identifier: MIT
   pragma solidity ^0.8.0;
    
   contract Greeter {
       string greeting;
    
       event SetGreeting(
           address indexed sender,     // msg.sender
           string greeting
       ); 
    
       function greet() public view returns (string memory) {
           return greeting;
       }
    
       function setGreeting(string memory _greeting) public {
           greeting = _greeting;
           emit SetGreeting(msg.sender, _greeting);
       }
   }

3. Deploy the Greeter contract to Chain B and store the resulting contract address in the GREETER_B_ADDRESS environment variable.

   GREETER_B_ADDRESS=`forge create --rpc-url $URL_CHAIN_B --private-key $PRIVATE_KEY Greeter --broadcast | awk '/Deployed to:/ {print $3}'`
   Explanation

   The command that deploys the contract is:

   forge create --rpc-url $URL_CHAIN_B --private-key $PRIVATE_KEY Greeter --broadcast

   The command output gives us the deployer address, the address of the new contract, and the transaction hash:

   Deployer: 0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266
   Deployed to: 0x5FC8d32690cc91D4c39d9d3abcBD16989F875707
   Transaction hash: 0xf155d360ec70ee10fe0e02d99c16fa5d6dc2a0e79b005fec6cbf7925ff547dbf

   The awk (opens in a new tab) command looks for the line that has Deployed to: and writes the third word in that line, which is the address.

   awk '/Deployed to:/ {print $3}'

   Finally, in UNIX (including Linux and macOS) the when the command line includes backticks (```), the shell executes the code between the backticks and puts the output, in this case the contract address, in the command. So we get.

   GREETER_B_ADDRESS=<the address>

cast call --rpc-url $URL_CHAIN_B $GREETER_B_ADDRESS "greet()" | cast --to-ascii 
cast send --private-key $PRIVATE_KEY --rpc-url $URL_CHAIN_B $GREETER_B_ADDRESS "setGreeting(string)" Hello$$
cast call --rpc-url $URL_CHAIN_B $GREETER_B_ADDRESS "greet()" | cast --to-ascii

4. Install the Optimism Solidity libraries into the project.

   cd lib
   npm install @eth-optimism/contracts-bedrock
   cd ..
   echo @eth-optimism/=lib/node_modules/@eth-optimism/ >> remappings.txt

5. Create src/GreetingSender.sol.

   //SPDX-License-Identifier: MIT
   pragma solidity ^0.8.0;
    
   import { Predeploys } from "@eth-optimism/contracts-bedrock/src/libraries/Predeploys.sol";
   import { IL2ToL2CrossDomainMessenger } from "@eth-optimism/contracts-bedrock/src/L2/IL2ToL2CrossDomainMessenger.sol";
    
   import { Greeter } from "src/Greeter.sol";
    
   contract GreetingSender {
       IL2ToL2CrossDomainMessenger public immutable messenger =
           IL2ToL2CrossDomainMessenger(Predeploys.L2_TO_L2_CROSS_DOMAIN_MESSENGER);
    
       address immutable greeterAddress;
       uint256 immutable greeterChainId;
    
       constructor(address _greeterAddress, uint256 _greeterChainId) {
           greeterAddress = _greeterAddress;
           greeterChainId = _greeterChainId;
       }
    
       function setGreeting(string calldata greeting) public {
           bytes memory message = abi.encodeCall(
               Greeter.setGreeting,
               (greeting)
           );
           messenger.sendMessage(greeterChainId, greeterAddress, message);
       }
   }
   Explanation

       function setGreeting(string calldata greeting) public {
           bytes memory message = abi.encodeCall(
               Greeter.setGreeting,
               (greeting)
           );
           messenger.sendMessage(greeterChainId, greeterAddress, message);
       }

   This function encodes a call to setGreeting and sends it to a contract on another chain. abi.encodeCall(Greeter.setGreeting, (greeting)) constructs the calldata (opens in a new tab) by encoding the function selector and parameters. The encoded message is then passed to messenger.sendMessage, which forwards it to the destination contract (greeterAddress) on the specified chain (greeterChainId).

   This ensures that setGreeting is executed remotely with the provided greeting value (as long as there is an executing message to relay it).

6. Deploy GreetingSender to chain A.

   CHAIN_ID_B=`cast chain-id --rpc-url $URL_CHAIN_B`
   GREETER_A_ADDRESS=`forge create --rpc-url $URL_CHAIN_A --private-key $PRIVATE_KEY --broadcast GreetingSender --constructor-args $GREETER_B_ADDRESS $CHAIN_ID_B | awk '/Deployed to:/ {print $3}'`

Send a message

Send a greeting from chain A to chain B.

cast call --rpc-url $URL_CHAIN_B $GREETER_B_ADDRESS "greet()" | cast --to-ascii 
cast send --private-key $PRIVATE_KEY --rpc-url $URL_CHAIN_A $GREETER_A_ADDRESS "setGreeting(string)" "Hello from chain A"
sleep 4
cast call --rpc-url $URL_CHAIN_B $GREETER_B_ADDRESS "greet()" | cast --to-ascii

The sleep call is because it can take up to two seconds until the transaction is included in chain A, and then up to two seconds until the relay transaction is included in chain B.

Modify the Greeter contract

1. Modify src/Greeter.sol to this code.

   //SPDX-License-Identifier: MIT
   pragma solidity ^0.8.0;
    
   import { Predeploys } from "@eth-optimism/contracts-bedrock/src/libraries/Predeploys.sol";
    
   interface IL2ToL2CrossDomainMessenger {
       function crossDomainMessageContext() external view returns (address sender_, uint256 source_);        
   }
    
   contract Greeter {
    
       IL2ToL2CrossDomainMessenger public immutable messenger =
           IL2ToL2CrossDomainMessenger(Predeploys.L2_TO_L2_CROSS_DOMAIN_MESSENGER);
    
       string greeting;
    
       event SetGreeting(
           address indexed sender,     // msg.sender
           string greeting
       ); 
    
       event CrossDomainSetGreeting(
           address indexed sender,   // Sender on the other side
           uint256 indexed chainId,  // ChainID of the other side
           string greeting
       );
    
       function greet() public view returns (string memory) {
           return greeting;
       }
    
       function setGreeting(string memory _greeting) public {
           greeting = _greeting;
           emit SetGreeting(msg.sender, _greeting);
    
           if (msg.sender == Predeploys.L2_TO_L2_CROSS_DOMAIN_MESSENGER) {
               (address sender, uint256 chainId) =
                   messenger.crossDomainMessageContext();              
               emit CrossDomainSetGreeting(sender, chainId, _greeting);
           }
       }
   }
   Explanation

   interface IL2ToL2CrossDomainMessenger {
     function crossDomainMessageContext() external view returns (address sender_, uint256 source_);        
   }          

   This definition isn't part of the npmjs package (opens in a new tab) at writing, so we just add it here.

           if (msg.sender == Predeploys.L2_TO_L2_CROSS_DOMAIN_MESSENGER) {
               (address sender, uint256 chainId) =
                   messenger.crossDomainMessageContext();              
               emit CrossDomainSetGreeting(sender, chainId, _greeting);
           }

   If we see that we got a message from L2ToL2CrossDomainMessenger, we call L2ToL2CrossDomainMessenger.crossDomainMessageContext (opens in a new tab).

2. Redeploy the contracts. Because the address of Greeter is immutable in GreetingSender, we need to redeploy both contracts.

   GREETER_B_ADDRESS=`forge create --rpc-url $URL_CHAIN_B --private-key $PRIVATE_KEY Greeter --broadcast | awk '/Deployed to:/ {print $3}'`
   GREETER_A_ADDRESS=`forge create --rpc-url $URL_CHAIN_A --private-key $PRIVATE_KEY --broadcast GreetingSender --constructor-args $GREETER_B_ADDRESS $CHAIN_ID_B | awk '/Deployed to:/ {print $3}'`

Verify you can see cross chain sender information

1. Set the greeting through GreetingSender.

   cast call --rpc-url $URL_CHAIN_B $GREETER_B_ADDRESS "greet()" | cast --to-ascii 
   cast send --private-key $PRIVATE_KEY --rpc-url $URL_CHAIN_A $GREETER_A_ADDRESS "setGreeting(string)" "Hello from chain A, with a CrossDomainSetGreeting event"
   sleep 4
   cast call --rpc-url $URL_CHAIN_B $GREETER_B_ADDRESS "greet()" | cast --to-ascii

2. Read the log entries.

   cast logs --rpc-url $URL_CHAIN_B 'CrossDomainSetGreeting(address,uint256,string)'
   echo $GREETER_A_ADDRESS
   echo 0x385=`echo 0x385 | cast --to-dec`
   echo 0x190a85c0=`echo 0x190a85c0 | cast --to-dec`

   See that the second topic (the first indexed log parameter) is the same as $GREETER_A_ADDRESS. The third topic can be either 0x385=901, which is the chain ID for supersim chain A, or 0x190a85c0=420120000, which is the chain ID for devnet alpha 0.