Information requests to the Foreign side

Recent versions (6.0.0+) of the AMB contracts allow any contract deployed on the Home chain to request and receive information from the Foreign chain asynchronously. For example, a contract on the Sokol chain can request data from the Kovan chain through the Kovan-Sokol AMB bridge, or a contract from the xDai chain can request information from Rinkeby or BSC.

Information Types

Everything that can be returned by the following JSON RPC API calls can be requested from the Foreign chain:

• eth_call

• eth_getStorageAt

• eth_getBalance

• eth_getBlockByNumber

• eth_getBlockByHash

• eth_getTransactionReceipt

• eth_getTransactionByHash

Process

1. T contract that receives the information should invoke the following method of the AMB contract:

    function requireToGetInformation(bytes32 _requestSelector, 
                                     bytes calldata _data) external returns (bytes32);

   where _requestSelector is one of the following identifiers:

   • 0x88b6c755140efe88bff94bfafa4a7fdffe226d27d92bd45385bb0cfa90986650 for eth_call

   • 0xed99ae50430a72b5fd8de83256893be815884aa6e59d3e25e754c5269620e964 for eth_getBalance

   • 0x771ca5f7250ac7deadb5ecd3a00e0d901edb92aff583f55c8db58ced9a140926 for eth_getStorageAt

   • 0x21bed3a725245055f5aa932a62078981ec55f2464167839ea001f45c839c8808 for eth_getBlockByNumber

   • 0x31deda753ce7d0fdf0af4d4d03a69913616c4b1377be882d407fbed7cd5a7a98 for eth_getBlockByHash

   • 0x0f47e1863d0e05e42b4a2aeee1322f349562a50b6166c14622210ade49a0b274 for eth_getTransactionReceipt

   • 0x05c2b13240a3dff2b14abb3b793a0a5763a11c190af35bd2b5279c5bac8f2a94 for eth_getTransactionByHash

   and _data is an ABI encoded set of arguments for the corresponding call:

   • eth_call: (address _to, bytes _calldata)

   • eth_getBalance: (address _account)

   • eth_getStorageAt: (address _contract, bytes32 _slot)

   • eth_getBlockByNumber: (uint256 _number)

   • eth_getBlockByHash: (bytes32 _hash)

   • eth_getTransactionReceipt: (bytes32 _txhash)

   • eth_getTransactionByHash: (bytes32 _txhash)

   The method requireToGetInformation returns a message id assigned by the AMB contract for this information request.

2. The AMB contract generates a message for the bridge oracles.

3. Based on the timestamp of the Home chain's block with the transaction for the information request, the oracles calculates a block number (for eth_call, eth_getBalance and eth_getStorageAt) to access the blockchain state.

4. The oracles call the target RPC API and receive the result of the API method execution.

5. The oracles sends a transaction to the AMB contract with the confirmInformation invocation.

    function confirmInformation(bytes32 _messageId, 
                                bool _status, 
                                bytes _result) external;

6. The AMB contract invokes onInformationReceived of the contract which originates the information request from step 1.

    function onInformationReceived(bytes32 messageId, 
                                   bool status, 
                                   bytes calldata result) external;

   where

   • messageId is the message id associated with the request which the information is received for

   • status is the status of the request execution

   • result is ABI-encoded response of the JSON RPC node on the information request.

Available Bridges

Currently the cross-chain information requests are available on the following bridges:

• xDai-ETH, requests from xDai to the Ethereum Mainnet (will be available after the current security audit)

• ​Kovan-Sokol AMB, requests from Sokol to Kovan

• ​Rinkeby-xDai AMB, requests from xDai to Rinkeby

• ​BSC-xDai AMB, requests from xDai to Binance Smart Chain

• ​ETH-BSC AMB, requests from Binance Smart Chain to the Ethereum Mainnet

Examples

The following examples use contracts deployed on the Sokol chain. The Kovan-Sokol AMB is used to demonstrate how information can be received from Kovan with transactions initiated on Sokol.

struct Block {
    uint256 blockNumber;
    bytes32 blockHash;
    address miner;
    uint256 gasUsed;
    uint256 gasLimit;
    bytes32 parentHash;
    bytes32 receiptRoot;
    bytes32 stateRoot;
    bytes32 transactionRoot;
    uint256 timestamp;
    uint256 difficulty;
    uint256 totalDifficulty;
}

struct Log {
    address from;
    bytes32[] topics;
    bytes data;
}
    
struct TransactionReceipt {
    bytes32 txHash;
    uint256 blockNumber;
    bytes32 blockHash;
    uint256 transactionIndex;
    address from;
    address to;
    uint256 gasUsed;
    bool status;
    Log[] logs;
}

struct Transaction {
    bytes32 txHash;
    uint256 blockNumber;
    bytes32 blockHash;
    uint256 transactionIndex;
    address from;
    address to;
    uint256 value;
    uint256 nonce;
    uint256 gas;
    uint256 gasPrice;
    bytes input;
}