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How to send messages

The Substrate-to-Substrate relay comes with a command line interface (CLI) which is implemented by the substrate-relay binary.

Substrate-to-Substrate relay

USAGE:
    substrate-relay <SUBCOMMAND>

FLAGS:
    -h, --help
            Prints help information

    -V, --version
            Prints version information


SUBCOMMANDS:
    help              Prints this message or the help of the given subcommand(s)
    init-bridge       Initialize on-chain bridge pallet with current header data
    relay-headers     Start headers relay between two chains
    relay-messages    Start messages relay between two chains
    send-message      Send custom message over the bridge

The relay related commands relay-headers and relay-messages are basically continously running a sync loop between the Millau and Rialto chains. The init-bridge command submitts initialization transactions. An initialization transaction brings an initial header and authorities set from a source chain to a target chain. The header synchronization then starts from that header.

For sending custom messages over an avialable bridge, the send-message command is used.

Send custom message over the bridge.

Allows interacting with the bridge by sending messages over `Messages` component. The message is being sent to the
source chain, delivered to the target chain and dispatched there.

USAGE:
    substrate-relay send-message <SUBCOMMAND>

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

SUBCOMMANDS:
    help                Prints this message or the help of the given subcommand(s)
    MillauToRialto      Submit message to given Millau -> Rialto lane
    RialtoToMillau      Submit message to given Rialto -> Millau lane

Messages are send from a source chain to a target chain using a so called message lane. Message lanes handle both, message transport and message dispatch. There is one command for submitting a message to each of the two available bridges, namely MillauToRialto and RialtoToMillau.

Submitting a message requires a number of arguments to be provided. Those arguments are essentially the same for both submit message commands, hence only the output for MillauToRialto is shown below.

Submit message to given Millau -> Rialto lane

USAGE:
    substrate-relay send-message MillauToRialto [OPTIONS] --lane <lane> --source-host <source-host> --source-port <source-port> --source-signer <source-signer> --origin <origin> --target-signer <target-signer> <SUBCOMMAND>

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

OPTIONS:
        --fee <fee>
            Delivery and dispatch fee. If not passed, determined automatically

        --lane <lane>                                        Hex-encoded lane id
        --source-host <source-host>                          Connect to Source node at given host
        --source-port <source-port>                          Connect to Source node websocket server at given port
        --source-signer <source-signer>
            The SURI of secret key to use when transactions are submitted to the Source node

        --source-signer-password <source-signer-password>
            The password for the SURI of secret key to use when transactions are submitted to the Source node

        --origin <origin>
            The origin to use when dispatching the message on the target chain [possible values: Target, Source]

        --target-signer <target-signer>
            The SURI of secret key to use when transactions are submitted to the Target node

        --target-signer-password <target-signer-password>
            The password for the SURI of secret key to use when transactions are submitted to the Target node


SUBCOMMANDS:
    help        Prints this message or the help of the given subcommand(s)
    remark      Make an on-chain remark (comment)
    transfer    Transfer the specified `amount` of native tokens to a particular `recipient`

As can be seen from the output, there are two types of messages available: remark and transfer. A remark is some opaque message which will be placed on-chain. For basic testing, a remark is the easiest to go with.

Usage of the arguments is best explained with an example. Below you can see, how a remark would look like:

substrate-relay send-message MillauToRialto \
		--source-host=127.0.0.1 \
		--source-port=10946 \
		--source-signer=//Dave \
		--target-signer=//Dave \
		--lane=00000000 \
		--origin Target \
		remark

Messages are basically regular transactions. That means, they have to be signed. In order to send a message, you have to control an account private key on both, the source and the target chain. Those accounts are specified using the --source-signer and --target-signer arguments in the example above.

Message delivery and dispatch requires a fee to be paid. In the example above, we have not specified the --fee argument. Hence, the fee will be estimated automatically. Note that in order to pay the fee, the message sender account has to have sufficient funds available.

The --origin argument allows to denote under which authority the message will be dispatched on the target chain. Accepted values are Target and Source.

Although not strictly necessary, it is recommended, to use one of the well-known development accounts (Alice, Bob, Charlie, Dave, Eve) for message sending. Those accounts are endowed with funds for fee payment. In addtion, the development Seed URI syntax (like //Dave) for the signer can be used, which will remove the need for a password.