Header Sync
Header pallet on the target chain, keeps track of the forks, but requires finality for blocks that perform authority set changes. That means, it won't sync a fork with authority set change unless that change finalized.
sequenceDiagram
participant Source Chain
participant Relayer
participant Target Chain
Note right of Target Chain: Best: 0, Finalized: 0
Source Chain ->> Source Chain: Import Block 1
Source Chain ->> Source Chain: Import Block 2
Relayer ->> Target Chain: Submit Block 1
Note right of Target Chain: Best: 1, Finalized: 0
Relayer ->> Target Chain: Submit Block 2
Note right of Target Chain: Best: 2, Finalized: 0
Source Chain ->> Source Chain: Import Block 2'
Relayer ->> Target Chain: Submit Block 2'
Note right of Target Chain: Best: 2 or 2', Finalized: 0
Source Chain ->> Source Chain: Finalize Block 2'
Relayer ->> Target Chain: Submit Finality of Block 2'
Note right of Target Chain: Best: 2', Finalized: 2'
Message Delivery (single lane)
Pending messages are stored on-chain (source) so the relayer code is completely stateless - it can read all the details from the chain.
Delivering pending messages requires finality first.
sequenceDiagram
participant Source Chain
participant Relayer
participant Target Chain
Source Chain ->> Source Chain: Queue Message 1
Source Chain ->> Source Chain: Queue Message 2
Source Chain ->> Source Chain: Queue Message 3
Note left of Source Chain: Queued Messages: [1, 2, 3, ]
Note left of Source Chain: Reward for [1, 2, 3, ] reserved
Relayer ->> Target Chain: Deliver Messages 1..2
Note right of Target Chain: Target chain dispatches the messages.
To Confirm: {1..2 => relayer_1}
Relayer ->> Source Chain: Delivery Confirmation of 1..2
Note left of Source Chain: Queued Messages: [3, ]
Note left of Source Chain: Reward payout for [1, 2, ]
Relayer -->> Target Chain: Confirmed Messages 1..2
Note right of Target Chain: To Confirm: {}
Note over Relayer, Target Chain: (this is not a separate transaction,
it's bundled with the "Deliver Messages" proof)