pezkuwi-subxt/bridges/docs/high-level-overview.md

High-Level Bridge Documentation

This document gives a brief, abstract description of main components that may be found in this repository. If you want to see how we're using them to build Rococo <> Wococo (Kusama <> Polkadot) bridge, please refer to the Polkadot <> Kusama Bridge.

Purpose

This repo contains all components required to build a trustless connection between standalone Substrate chains, that are using GRANDPA finality, their parachains or any combination of those. On top of this connection, we offer a messaging pallet that provides means to organize messages exchange.

On top of that layered infrastructure, anyone may build their own bridge applications - e.g. XCM messaging, encoded calls messaging and so on.

Terminology

Even though we support (and require) two-way bridging, the documentation will generally talk about a one-sided interaction. That's to say, we will only talk about syncing finality proofs and messages from a source chain to a target chain. This is because the two-sided interaction is really just the one-sided interaction with the source and target chains switched.

The bridge has both on-chain (pallets) and offchain (relayers) components.

On-chain components

On-chain bridge components are pallets that are deployed at the chain runtime. Finality pallets require deployment at the target chain, while messages pallet needs to be deployed at both, source and target chains.

Bridge GRANDPA Finality Pallet

A GRANDPA light client of the source chain built into the target chain's runtime. It provides a "source of truth" about the source chain headers which have been finalized. This is useful for higher level applications.

The pallet tracks current GRANDPA authorities set and only accepts finality proofs (GRANDPA justifications), generated by the current authorities set. The GRANDPA protocol itself requires current authorities set to generate explicit justification for the header that enacts next authorities set. Such headers and their finality proofs are called mandatory in the pallet and relayer pays no fee for such headers submission.

The pallet does not require all headers to be imported or provided. The relayer itself chooses which headers he wants to submit (with the exception of mandatory headers).

More: pallet level documentation and code.

Bridge Parachains Finality Pallet

Parachains are not supposed to have their own finality, so we can't use bridge GRANDPA pallet to verify their finality proofs. Instead, they rely on their relay chain finality. The parachain header is considered final, when it is accepted by the paras pallet at its relay chain. Obviously, the relay chain block, where it is accepted, must also be finalized by the relay chain GRANDPA gadget.

That said, the bridge parachains pallet accepts storage proof of one or several parachain heads, inserted to the Heads map of the paras pallet. To verify this storage proof, the pallet uses relay chain header, imported earlier by the bridge GRANDPA pallet.

The pallet may track multiple parachains at once and those parachains may use different primitives. So the parachain header decoding never happens at the pallet level. For maintaining the headers order, the pallet uses relay chain header number.

More: pallet level documentation and code.

Bridge Messages Pallet

The pallet is responsible for queuing messages at the source chain and receiving the messages proofs at the target chain. The messages are sent to the particular lane, where they are guaranteed to be received in the same order they are sent. The pallet supports many lanes.

The lane has two ends. Outbound lane end is storing number of messages that have been sent and the number of messages that have been received. Inbound lane end stores the number of messages that have been received and also a map that maps messages to relayers that have delivered those messages to the target chain.

The pallet has three main entrypoints:

• the send_message may be used by the other runtime pallets to send the messages;
• the receive_messages_proof is responsible for parsing the messages proof and handing messages over to the dispatch code;
• the receive_messages_delivery_proof is responsible for parsing the messages delivery proof and rewarding relayers that have delivered the message.

Many things are abstracted by the pallet:

• the message itself may mean anything, the pallet doesn't care about its content;
• the message dispatch happens during delivery, but it is decoupled from the pallet code;
• the messages proof and messages delivery proof are verified outside of the pallet;
• the relayers incentivization scheme is defined outside of the pallet.

Outside of the messaging pallet, we have a set of adapters, where messages and delivery proofs are regular storage proofs. The proofs are generated at the bridged chain and require bridged chain finality. So messages pallet, in this case, depends on one of the finality pallets. The messages are XCM messages and we are using XCM executor to dispatch them on receival. You may find more info in Polkadot <> Kusama Bridge document.

More: pallet level documentation and code.

Bridge Relayers Pallet

The pallet is quite simple. It just registers relayer rewards and has an entrypoint to collect them. When the rewards are registered and the reward amount is configured outside of the pallet.

More: pallet level documentation and code.

Offchain Components

Offchain bridge components are separate processes, called relayers. Relayers are connected both to the source chain and target chain nodes. Relayers are reading state of the source chain, compare it to the state of the target chain and, if state at target chain needs to be updated, submits target chain transaction.

GRANDPA Finality Relay

The task of relay is to submit source chain GRANDPA justifications and their corresponding headers to the Bridge GRANDPA Finality Pallet, deployed at the target chain. For that, the relay subscribes to the source chain GRANDPA justifications stream and submits every new justification it sees to the target chain GRANDPA light client. In addition, relay is searching for mandatory headers and submits their justifications - without that the pallet will be unable to move forward.

More: GRANDPA Finality Relay Sequence Diagram, pallet level documentation and code.

Parachains Finality Relay

The relay connects to the source relay chain and the target chain nodes. It doesn't need to connect to the tracked parachain nodes. The relay looks at the Heads map of the paras pallet in source chain, and compares the value with the best parachain head, stored in the bridge parachains pallet at the target chain. If new parachain head appears at the relay chain block B, the relay process waits until header B or one of its ancestors appears at the target chain. Once it is available, the storage proof of the map entry is generated and is submitted to the target chain.

As its on-chain component (which requires bridge GRANDPA pallet to be deployed nearby), the parachains finality relay requires GRANDPA finality relay to be running in parallel. Without it, the header B or any of its children's finality at source won't be relayed at target, and target chain won't be able to verify generated storage proof.

More: Parachains Finality Relay Sequence Diagram, code.

Messages Relay

Messages relay is actually two relays that are running in a single process: messages delivery relay and delivery confirmation relay. Even though they are more complex and have many caveats, the overall algorithm is the same as in other relays.

Message delivery relay connects to the source chain and looks at the outbound lane end, waiting until new messages are queued there. Once they appear at the source block B, the relay start waiting for the block B or its descendant appear at the target chain. Then the messages storage proof is generated and submitted to the bridge messages pallet at the target chain. In addition, the transaction may include the storage proof of the outbound lane state - that proves that relayer rewards have been paid and this data (map of relay accounts to the delivered messages) may be pruned from the inbound lane state at the target chain.

Delivery confirmation relay connects to the target chain and starts watching the inbound lane end. When new messages are delivered to the target chain, the corresponding source chain account is inserted to the map in the inbound lane data. Relay detects that, say, at the target chain block B and waits until that block or its descendant appears at the source chain. Once that happens, the relay crafts a storage proof of that data and sends it to the messages pallet, deployed at the source chain.

As you can see, the messages relay also requires finality relay to be operating in parallel. Since messages relay submits transactions to both source and target chains, it requires both source-to-target and target-to-source finality relays. They can be GRANDPA finality relays or GRANDPA+parachains finality relays, depending on the type of connected chain.

More: Messages Relay Sequence Diagram, pallet level documentation and code.

Complex Relay

Every relay transaction has its cost. The only transaction, that is "free" to relayer is when the mandatory GRANDPA header is submitted. The relay that feeds the bridge with every relay chain and/or parachain head it sees, will have to pay a (quite large) cost. And if no messages are sent through the bridge, that is just waste of money.

We have a special relay mode, called complex relay, where relay mostly sleeps and only submits transactions that are required for the messages/confirmations delivery. This mode starts two message relays (in both directions). All required finality relays are also started in a special on-demand mode. In this mode they do not submit any headers without special request. As always, the only exception is when GRANDPA finality relay sees the mandatory header - it is submitted without such request.

The message relays are watching their lanes and when, at some block B, they see new messages/confirmations to be delivered, they are asking on-demand relays to relay this block B. On-demand relays does that and then message relay may perform its job. If on-demand relay is a parachain finality relay, it also runs its own on-demand GRANDPA relay, which is used to relay required relay chain headers.

More: Complex Relay Sequence Diagram, code.