cosmos-sdk/docs/sdk/core/messages.md

Messages

Users can create messages containing arbitrary information by implementing the Msg interface:

type Msg interface {

	// Return the message type.
	// Must be alphanumeric or empty.
	Type() string

	// Get the canonical byte representation of the Msg.
	GetSignBytes() []byte

	// ValidateBasic does a simple validation check that
	// doesn't require access to any other information.
	ValidateBasic() error

	// Signers returns the addrs of signers that must sign.
	// CONTRACT: All signatures must be present to be valid.
	// CONTRACT: Returns addrs in some deterministic order.
	GetSigners() []Address
}

Messages must specify their type via the Type() method. The type should correspond to the messages handler, so there can be many messages with the same type.

Messages must also specify how they are to be authenticated. The GetSigners() method return a list of addresses that must sign the message, while the GetSignBytes() method returns the bytes that must be signed for a signature to be valid.

Addresses in the SDK are arbitrary byte arrays that are hex-encoded when displayed as a string or rendered in JSON.

Messages can specify basic self-consistency checks using the ValidateBasic() method to enforce that message contents are well formed before any actual logic begins.

For instance, the Basecoin message types are defined in x/bank/tx.go:

type MsgSend struct {
	Inputs  []Input  `json:"inputs"`
	Outputs []Output `json:"outputs"`
}

type MsgIssue struct {
	Banker  sdk.Address `json:"banker"`
	Outputs []Output       `json:"outputs"`
}

Each specifies the addresses that must sign the message:

func (msg MsgSend) GetSigners() []sdk.Address {
	addrs := make([]sdk.Address, len(msg.Inputs))
	for i, in := range msg.Inputs {
		addrs[i] = in.Address
	}
	return addrs
}

func (msg MsgIssue) GetSigners() []sdk.Address {
	return []sdk.Address{msg.Banker}
}