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Reply

The reply entrypoint is another special one.

It relates to CosmWasm's actor model which dictates how you call other contracts and interact with them.

tip

If you are not familiar with the actor model, you can find out more about it on its own page.

This endpoint gets invoked when you receive a response to a message you sent out over the chain. The reply parameter then allows you to inspect and work with the response that was produced.

Request a reply​

To request a reply, you need to use the reply_on field in the message you send and set it to one of the following values:

  • ReplyOn::Always
  • ReplyOn::Error
  • ReplyOn::Success
tip

The reply_on value has an impact on when a transaction is cancelled. For more info, check the page about transactions.

contract.rs
const CONTRACT_ADDR: &str = "other_contract";
const SUBMSG_ID: u64 = 1; // This is a unique identifier so we can associate a reply with a specific submessage. It can be any numeric value.

#[cfg_attr(not(feature = "library"), entry_point)]
pub fn reply(deps: DepsMut, env: Env, msg: cosmwasm_std::Reply) -> StdResult<Response> {
    if msg.id != SUBMSG_ID {
        return Err(StdError::generic_err("Invalid submsg id"));
    }

    // We received a message! From the contract we invoked earlier.

    Ok(Response::default())
}

#[cfg_attr(not(feature = "library"), entry_point)]
pub fn execute(deps: DepsMut, env: Env, msg: ExecuteMsg) -> StdResult<Response> {
    let msg = WasmMsg::Execute {
        contract_addr: CONTRACT_ADDR.into(),
        msg: to_json_binary(&OtherContractMsg::DoSomething {}).unwrap(),
        funds: vec![],
    };

    let submsg = SubMsg::reply_always(msg, SUBMSG_ID);
    Ok(Response::new().add_submessage(submsg))
}

Definition​

contract.rs
#[cfg_attr(not(feature = "library"), entry_point)]
pub fn reply(
    deps: DepsMut, 
    env: Env, 
    msg: cosmwasm_std::Reply) -> StdResult<Response> {
    // TODO: Put you reply logic here.
    Ok(Response::default())
}

Handling the reply​

Once the submessage handling is finished, the caller will get a chance to handle the result. It will get the original id of the subcall and the Result of the execution, both success and error. Note that it includes all events returned by the submessage, which applies to native SDK modules (like bank) as well as the contracts. If you need more state, you must save it in the local store during the execute entrypoint call, and load it in the reply entrypoint.

The reply call may return Err itself, in which case it is treated like the caller errored, and aborting the sub-transaction. However, on successful processing, reply may return a normal Response, which will be processed as normal - events added to the EventManager, and all messages dispatched as described above. When Err is returned by a message handler, all changes made by the handler up to the reply entrypoint that returns the Ok response are reverted. More information can be found in the following section.

The responses emitted by the submessage are gathered in the msg_responses field of the SubMsgResponse structure. wasmd allows chains to translate a single contract message into multiple SDK messages. In that case all the message responses from each are concatenated into this flattened Vec.

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