Asynchronous interaction between Vara programs is similar to the usual asynchronous request in using await and implemented by sending a message.

Program entry points

If a program's logic implies asynchronous messaging, its main executable functions must differ from those used in synchronous communications.

async init()

In case of an asynchronous call in the program initialization, the async init() must be used instead of init(). Also, it should be preceded by the gstd::async_init macro:

#[gstd::async_init]
async fn init() {
    gstd::debug!("Hello world!");
}

async main()

The same for asynchronous messages, the async main() must be used instead of handle() and handle_reply(). Also, it should be preceded by the gstd::async_main macro:

#[gstd::async_main]
async fn main() {
    gstd::debug!("Hello world!");
}

Cross-program message

To send a message to a Vara program, use the msg::send_for_reply() function. In this function:

• program - the address of the program to send the message for;
• payload - the message to the program;
• value - the funds attached to the message (zero if no value is attached);
• reply_deposit - used to provide gas for future reply handling (skipped if zero).

To get an encoded reply from another actor use the msg::send_for_reply_as() function.

pub fn send_for_reply_as<E: Encode, D: Decode>(
    program: ActorId,
    payload: E,
    value: u128,
    reply_deposit: u64
) -> Result<CodecMessageFuture<D>>

Usage example:

let reply: SomeEvent = msg::send_for_reply_as(
    receiver_id,
    SomeAction {
        command: 42,
    },
    0,
    0,
)
.expect("Unable to send message")
.await
.expect("Error in receiving reply");