How to build xApps - League of Bridges, Cross Chain Salon Workshop brought to you by Celo


In this video, we will learn how to build cross-chain applications called zaps. Zaps are applications that utilize cross-domain messaging channels to perform actions across different chains. We will explore the basics of zaps, the components involved, and how to design and build your own zap. This tutorial aims to provide you with the intuition and knowledge to start building your own cross-chain applications.

Understanding Zaps and Cross-Chain Messaging

What is a Zap?

  • A zap is a cross-chain application that utilizes cross-domain messaging channels.
  • Cross-chain messaging channels allow communication between different domains or environments, including various blockchains, layer ones, side chains, and layer twos.

Cross-Domain Messaging Channels and Terminology

  • Cross-domain messaging channels refer to the channels that connect a local domain to a remote domain.
  • Messages sent through these channels encode information used to execute actions on-chain.
  • Local and remote domains refer to the current and different domains in the context of a specific chain.

Introduction to Optics

  • Optics is a standard for building cross-chain messaging channels.
  • It provides a modular and reusable design, making it easier to deploy on new chains.
  • Optics solves the problems faced by developers when building cross-chain applications.

Existing Designs for Cross-Chain Messaging Channels

  • Light clients and custodial channels are two existing designs.
  • Light clients are complex and challenging to develop for new chains.
  • Custodial channels are run by trusted admins and lack the same level of security as light clients.
  • Optics provides a more efficient and reusable design for cross-chain messaging channels.

Building Your Own Zap

Understanding Cross-Chain Applications

  • Cross-chain applications define a format for encoding useful information on a local chain and sending it to a remote chain.
  • Zaps are built on top of cross-chain messaging channels to facilitate communication between domains.

Intuition for Cross-Chain Applications

  • Review existing code examples and documentation to develop intuition for the design space of cross-chain applications.
  • Explore code examples provided by the tutorial presenter to gain a deeper understanding of application-specific logic.

Governance Router

  • The governance router is a more complex level of zaps that builds upon the original template.
  • It enables more specific and customized functionality within the cross-chain application.

Implementation Steps for Your Own Zap

  1. Build intuition about the design space for cross-chain applications.
  2. Review existing code examples and documentation.
  3. Study the code examples provided by the presenter to gain a deeper understanding.
  4. Explore the governance router for more advanced functionality.
  5. Follow the provided links to access the code and resources for building your own zap.

Examples of Zaps

Token Bridge

  • A token bridge is a zap that allows users to transfer tokens across different domains.
  • With a token bridge, you can trade cello tokens on Ethereum mainnet without relying on centralized intermediaries.
  • Token bridges are currently in high demand as a type of zap.

Optics Contracts

  • Optics contracts are governed by a cross-chain application that controls upgradeability and other permissioned roles.
  • Optics contracts offer various possibilities for cross-chain applications.

Limitations and Considerations

Asynchronicity and Standardized Formats

  • Cross-chain messages introduce asynchronicity to on-chain interactions.
  • Zaps are limited to problems that can be solved asynchronously.
  • Messages must encode meaningful information that can be interpreted on the receiving side.

Outgoing Messages and Incoming Message Authentication

  • Zaps need to address outgoing messages and authenticate incoming messages.
  • Outgoing messages must be constructed correctly to ensure secure and accurate communication.

Off-Chain Components and Interoperability

  • Cross-chain applications often involve off-chain components that facilitate communication and provide additional functionality.
  • Off-chain components can include relayers, oracles, and other services that help bridge the gap between different chains.
  • Interoperability between different chains is a crucial consideration when building cross-chain applications.
  • Different chains may have different smart contract languages, virtual machine architectures, or consensus mechanisms, requiring careful design and compatibility considerations.

Security Considerations

  • Building secure cross-chain applications is essential to protect user funds and ensure the integrity of the system.
  • When designing and implementing zaps, consider security best practices and conduct thorough code reviews.
  • Take into account potential attack vectors, such as message manipulation, replay attacks, and vulnerabilities in smart contracts.

Testing and Auditing

  • Thorough testing and auditing are critical to ensure the reliability and security of cross-chain applications.
  • Write comprehensive test cases and perform both unit tests and integration tests to verify the functionality of your zap.
  • Engage external security auditors to conduct a thorough audit of your codebase and identify any potential vulnerabilities.

Community and Collaboration

  • Joining communities and engaging with other developers in the cross-chain space can provide valuable insights, guidance, and support.
  • Collaboration and knowledge sharing can help improve the design and implementation of your zap.
  • Participate in forums, developer communities, and events to connect with like-minded individuals and learn from their experiences.

Resources and Documentation

  • Optics provides extensive documentation, code examples, and tutorials to help you understand and build cross-chain applications.
  • Consult the official Optics documentation and GitHub repository for detailed information and resources.
  • Stay updated with the latest developments and releases in the cross-chain ecosystem to leverage new features and improvements.


In this workshop, we covered the basics of building cross-chain applications called zaps. We explored the concept of cross-chain messaging channels, the Optics standard, and the components involved in building zaps. We discussed the steps to build your own zap, examples of existing zaps like token bridges and optics contracts, and important considerations such as security, testing, and collaboration. By following the provided resources and engaging with the cross-chain community, you can start building powerful and interoperable applications across different domains. Happy coding!