[Preview] Integration guide for sub<>sub bridge developers

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Integration guide for sub<>sub bridge developers

Substrate-to-Substrate Cross-chain Message Protocol

Darwinia Substrate-to-Substrate Cross-chain Message Protocol is built on the design of Parity Bridges. It incorporates the mechanism of a Fee Market, which aims to financially incentivize users to participate in the stable running of bridges.

Darwinia has opened up message layer access to third-party blockchains developers, who can build applications on top of it. There have been several such use cases in place. Darwinia Wormhole has built a mapping token protocol for the cross-chain transfer of RING(native token for Darwinia network). The prospective cross-chain transfer of NFT will also adopt this design. Cross-chain exchange of tokens, staking, and message routing are among promising applications.

Since the message-layer protocol is only responsible for the delivery service of messages and guarantees the authenticity and verifiability of messages, the application layer needs to establish its protocol-layer security mechanisms. For instance, it needs to verify the identity information of the caller in the target chain.

The messages in this layer are transactional; each message has a unique ID and is delivered sequentially, returning a receipt(confirmation) of success or failure of execution to the target chain after a successful delivery. The message delivery and confirmation are similar to an RPC remote call, but there is no timeout, and the execution result is deterministic.

Connecting a Third-party Blockchain to Darwinia/Crab Network

Crab network has implemented Darwinia Virtual Machine(DVM) which is Ethereum-compatible; so will Darwinia network. Those blockchains connected to Darwinia/Crab networks can leverage DVM to pave the way to Ethereum and other public networks.

Steps of Integration:

Add following Pallets:

• pallet-bridge-grandpa

  This pallet synchronizes block headers between two blockchains at the frequency of once an era normally. However, it can synchronize the current block header when instant synchronization is necessary.

• pallet-bridge-messages

  This pallet is responsible for the sending, receiving and delivery confirmation of bridge messages.

  • Fetches messages from the application layer, assign IDs and cache to them, and ensures the messages are executed sequentially and message proofs are delivered to the target chain by a relayer;
  • Receives message proofs from the bridge-message pallet of the target chain, verifies and analyze messages, deliver them to the bridge-dispatch pallet;
  • Receives delivery proofs from the bridge-message pallet of the target chain and forwards them to the user.

Thanks to @xiaoch05 for drafting the preview.

More contents TBD.