「Understanding Darwinia Bridge 1–4」The New Message Protocol for the Ethereum-Darwinia Bridge

This post was initially printed on Darwinia

Within the layered construction of a cross-chain bridge, a great message protocol must be as consensus-agnostic and application-agnostic as attainable. On this article, we’ll introduce our new model of the design of Ethereum-Darwinia bridge on this route.

Layers of the Ethereum-Darwinia Bridge


Table of Contents

Supply chain and goal chain

The supply chain is the community from which messages are generated, whereas the goal chain refers back to the community the place messages are delivered.


An software within the supply chain makes use of a message to name a operate with parameters within the goal chain. Like a distant process name (RPC), a message comprises information that identifies the supply software, the goal software, and a sound encoded that the goal software expects to grasp.


A channel is answerable for the unidirectional supply of messages from the supply chain to the goal chain, comprising an outbound lane within the supply chain and an inbound lane within the goal chain. Customers or functions eager to switch messages to a different blockchain should submit the messages to a selected channel. The message layer consists of a number of channels and gives quite a lot of protections, together with replay safety and ordered or unordered supply of messages.


A relayer is an off-chain actor who connects to each the supply chain and the goal chain and faithfully transfers the information between the supply chain and the goal chain.

Message Workflow

The message protocol will be applied with sensible contracts or a module within the runtime. It’s on the discretion of builders. Since our DVM(Darwinia Digital Machine) is Ethereum suitable, we’ve got applied the protocol in sensible contracts on either side of the bridge. We are going to clarify the whole lifecycle of a cross-chain message along with a time sequence diagram and the information construction of message.

The info construction for message is as follows:

It comprises two structs in it, key(MessageKey) and information(MessageData). From its members, we are able to see that the struct MessageKey uniquely identifies a message in storage. The struct MessageData represents an asynchronous name {that a} submitter(a consumer or an software) desires to make on a sensible contract in one other blockchain.

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A message is generated by a sensible contract(within the supply chain) after which delivered to a sensible contract (within the goal chain) and ultimately processed by this distant sensible contract. Theoretically, encoded within the struct MessageData will be something as a result of the message protocol doesn’t apply any syntactic or format restriction on it. In our design, it encapsulates the information comprehensible to the sensible contract on the receiving finish. The receiving sensible contract will decode it into the information kind it wants. We apply ABI encoding for encoded as a result of it’s handy for EVM to generate and ship and the fuel payment is low.

Following is the sequence diagram of a whole means of a message supply from Ethereum to Darwinia.

Within the first section, the message is delivered from Ethereum(the supply chain) to Darwinia(the goal chain).

  • First, a consumer or an software in Ethereum(supply chain) calls send_message() within the outbound lane to provoke a message to name a Darwinia operate. The submitter specifies the popular channel, the message itself and the payment that they’re keen to pay for the message supply and dispatch;
  • The outbound lane in Ethereum checks and saves the message and emits an occasion of MessgeAccepted. If the message passes all checks, the nonce is assigned and the message is saved within the contrac storage. The message is now within the accepteded state.
  • As soon as the relayer notices an accepteded message, it requests Ethereum for the receipt_proofor storage_proof;
  • Upon receiving the message proof, the relayer calls receive_messages_proof() within the inbound lane in Darwinia to append the message. There’s a verification of the message proof verify_message_proof() concerned right here;
  • After the Ethereum mild shopper verifies the message, the inbound lane in Darwinia dispatches it and emits an occasion of MessageDispatched, and therefore the state of the message turns into delivered .
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Up until now, the message has been transferred and dispatched efficiently. Nevertheless, the supply chain doesn’t find out about this. Due to this fact within the second section (affirmation), the relayer must notify the outbound lane on the Ethereum facet.

The affirmation section is critical for 2 causes:

  1. With it, the variety of messages delivered but not confirmed can be utilized for circulation management;
  2. If there may be an incentivization scheme for the relayer’s work, the relayer must show they’ve truly delivered the message and the proof must be accepted by the supply chain.

The steps are much like these within the first section. The Darwinia mild shopper in Ethereum verifies the proof of message supply submitted by the relayer, after which the inbound lane emits an occasion of MessageDelivered. The state of the message is now confirmed.

If the relayer will get any reward for his or her work, there might be an additional step, cost of the relayer payment. For the reason that message doesn’t exist any extra, rewarded just isn’t an actual state of the message. It’s only for the comfort of clarification.

However the story doesn’t cease right here. The relayer ought to notify the goal chain(Darwinia) that the message supply is confirmed within the supply chain(Ethereum) for the supply iteration to proceed. In any other case, messages will pile up within the inbound lane, and it’ll cease accepting new messages. The relayer features a nonce of the most recent confirmed message in subsequent receive_messages_proof() name to realize this objective.

Above is a message supply from Ethereum to Darwinia. For a message supply in the wrong way (Darwinia to Ethereum), the method is symmetric.

The steps of relay header within the diagram is represented with dashed line as a result of this isn’t a part of the message protocol in a strict sense. Nevertheless, it’s indispensable for the verification of messages.


The protocol has no assumptions about relayers incentivization scheme. We’re working the relayer nodes on our personal at current. Nevertheless, we’ve got devised a scheme of Relayer Charge Market, which was launched in another article, to draw extra builders to take part within the ecosystem.

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Though we take the Ethereum-Darwinia bridge for instance to clarify the brand new protocol, builders can apply this design to implement different cross-chain bridges. As a matter of reality, we use the identical scheme in our Substrate-to-Substrate bridges.

The inbound lane can filter obtained messages with a Cross-Chain Filter interface applied within the sensible contracts on the applying degree within the goal chain. Thus the sensible contracts can determine on receiving messages from which chains or which functions.

Associated repos

Collection Articles

「Understanding Darwinia Bridge 1–1」Darwinia Relayer Incentive Scheme — Fee Market

「Understanding Darwinia Bridge 1–2」Mapping Token Factory

「Understanding Darwinia Bridge 1–3」The Token Bridge Solution

About Darwinia Community

Darwinia Community is a decentralized cross-chain bridge community constructing on Substrate, which is the “cross-chain bridge hub” of the Internet 3.0 Metaverse. It gives a protected and basic bridging resolution, connects to Polkadot, Ethereum, TRON, and different heterogeneous chains by cross-chain switch of property and distant chain calls.

Darwinia Community has gained excessive repute and recognition alongside the way in which to construct the decentralized cross-chain bridge protocol. In 2020, Darwinia was written in Polkadot light-paper as one of many associates of Polkadot and Substrate. And Darwinia was chosen to hitch Substrate Builder Program and Web3.0 Bootcamp, and for the excellent work in Substrate Builder Program, Darwinia Community was formally awarded the Stage 2 badge by Parity. The merchandise and instruments developed by Darwinia have been rewarded three W3F Grants.

Darwinia has been contributing to the compatibility and interoperability of the Metaverse.

The applying areas of Darwinia Community embody DeFi, cross-chain NFT buying and selling, video games, and many others. Darwinia additionally develops the Metaverse sport Evolution Land.

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