Developing Cross-Chain Applications

Summary

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

Each cross-chain application must implement its own messaging protocol. 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 Optics — 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
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 cross-chain application using a common language and set of rules. Applications of this kind may use Optics as the cross-chain courier for sending and receiving messages to each other.

Example Code

This repository has several examples one can use to build understanding around Cross-Chain Applications.

xApp Template

This is a template provided by the Optics team that shows the high-level components of an xApp, ready for one to fill in their own application logic and utilize an Optics channel for cross-chain communication.

To implement a xApp, define the actions you would like to execute across chains. For each type of action,

in the xApp Router
- implement a function like doTypeA to initiate the action from one domain to another (add your own parameters and logic)
- implement a corresponding _handle function to receive, parse, and execute this type of message on the remote domain
- add logic to the handle function to route incoming messages to the appropriate _handle function
in the Message library,
- implement functions to format the message to send to the other chain (encodes all necessary information for the action)
- implement functions to parse the message once it is received on the other chain (decode all necessary information for the action)

Ping Pong xApp

Important! The Ping Pong xApp is for reference only. Please do not deploy!

The PingPong xApp is capable of initiating PingPong "matches" between two chains. A match consists of "volleys" sent back-and-forth between the two chains via Optics.

The first volley in a match is always a Ping volley.

When a Router receives a Ping volley, it returns a Pong.
When a Router receives a Pong volley, it returns a Ping.

The Routers keep track of the number of volleys in a given match, and emit events for each Sent and Received volley so that spectators can watch.

Token Bridge xApp

See the full-length Token Bridge Documentation for in-depth details on Token Bridge operation and construction.

Link to Contracts

Cross-Chain Governance xApp

See the full-length Optics Governance Documentation for in-depth details on Governance xApp operation and construction.