Building xApps

A xApp (pronounced "zap") is a cross-chain application built on top of the Nomad Protocol.

Nomad sends messages from one chain to another in the form of raw bytes. A xApp that wishes to use Nomad will need to define the rules for sending and receiving messages for its use case.

Each cross-chain application must implement its own messaging interface. By convention, we call the contracts that implement this protocol the application's Router contracts. These Router contracts must:

• Maintain a permissioned set of the contract(s) on remote chains from which it will accept messages via Nomad — this could be a single owner of the application on one chain; it could be a registry of other applications implementing the same rules on various chains

• Maintain a permissioned registry of connections via the XappConnectionManager contract (see Connection Management).

• Encode messages in a standardized format, so they can be decoded by the Router contract on the destination chain

• Handle messages from remote Router contracts

• Dispatch messages to remote Router contracts

By implementing these pieces of functionality within a Router contract and deploying it across multiple chains, we create a working xApp using a common language and set of rules. Applications of this kind may use Nomad message passing channels as the cross-chain courier for sending and receiving messages between chains.

Connection Management

The router implements the XappConnectionClient abstract contract. This contract provides convenience functions for working with a XAppConnectionManager.

The XCM is the primary permissioning point for channels. It provides functions by which

• xApp administrators can enroll or unenroll Replica contracts for inbound messages

• xApp administrators can enroll or unenroll a Home contract for outbound messages

• xApp administrators can permission or de-permission watchers

• watchers can unenroll Replica contracts

When deploying a xApp Router, the xApp administrators must select an existing XappConnectionManager, or deploy their own. The address of the XappConnectionManager must be passed to the router's initialization method.