# Complex Relay

Both Source Chain and Target Chains have Bridge Messages pallets deployed. They also have required finality pallets deployed - we don't care about finality type here - they can be either Bridge GRANDPA, or Bridge Parachains finality pallets, or any combination of those.

There are 4-6 relayer subprocesses inside the Complex Relayer. They include two message relayers, serving the lane in both directions and 2-4 Complex Relayers (depending on the finality type of Source and Target Chains).

The following diagram shows the way the complex relayer serves the lane in single direction. Everything below may be applied to the opposite direction if you'll swap the Source and Target Chains.

sequenceDiagram participant Source Chain participant Complex Relayer participant Target Chain Note right of Source Chain: Finalized: 480, Target Finalized: 50, Sent Messages: 42, Confirmed Messages: 42 Note left of Target Chain: Finalized: 60, Source Finalized: 420, Received Messages: 42 Source Chain ->> Source Chain: someone Sends Message 43 Source Chain ->> Source Chain: Import and Finalize Block 481 Source Chain ->> Complex Relayer: notes new outbound message 43 at Source Chain Block 481 Note right of Complex Relayer: can't deliver message 43, Source Chain Block 481 is not relayed Complex Relayer ->> Complex Relayer: asks on-demand Finality Relayer to relay Source Chain Block 481 Source Chain ->> Complex Relayer: Read Finality Proof of Block 481 Complex Relayer ->> Target Chain: Submit Finality Proof of Block 481 Target Chain ->> Target Chain: Import and Finalize Block 61 Note left of Target Chain: Finalized: 61, Source Finalized: 481, Received Messages: 42 Source Chain ->> Complex Relayer: Read Proof of Message 43 at Block 481 Complex Relayer ->> Target Chain: Submit Proof of Message 43 at Block 481 Target Chain ->> Target Chain: Import and Finalize Block 62 Note left of Target Chain: Finalized: 62, Source Finalized: 481, Received Messages: { rewarded: 42, messages-relayer-account: [43] } Target Chain ->> Complex Relayer: notes new unrewarded relayer at Target Chain Block 62 Note right of Complex Relayer: can't relay delivery confirmations because Target Chain Block 62 is not relayed Complex Relayer ->> Complex Relayer: asks on-demand Finality Relayer to relay Target Chain Block 62 Target Chain ->> Complex Relayer: Read Finality Proof of Block 62 Complex Relayer ->> Source Chain: Submit Finality Proof of Block 62 Source Chain ->> Source Chain: Import and Finalize Block 482 Note right of Source Chain: Finalized: 482, Target Finalized: 62, Confirmed Messages: 42 Target Chain ->> Complex Relayer: Read Proof of Message 43 Delivery at Block 62 Complex Relayer ->> Source Chain: Submit Proof of Message 43 Delivery at Block 612 Source Chain ->> Source Chain: rewards messages-relayer-account for delivering message [43] Source Chain ->> Source Chain: prune delivered message 43 from runtime storage Note right of Source Chain: Finalized: 482, Target Finalized: 61, Confirmed Messages: 43 Source Chain ->> Source Chain: someone Sends Message 44 Source Chain ->> Source Chain: Import and Finalize Block 483 Source Chain ->> Complex Relayer: notes new outbound message 44 at Source Chain Block 483 and new confirmed message 43 Note right of Complex Relayer: can't deliver message 44, Source Chain Block 483 is not relayed Complex Relayer ->> Complex Relayer: asks on-demand Finality Relayer to relay Source Chain Block 483 Source Chain ->> Complex Relayer: Read Finality Proof of Block 483 Complex Relayer ->> Target Chain: Submit Finality Proof of Block 483 Target Chain ->> Target Chain: Import and Finalize Block 63 Note left of Target Chain: Finalized: 63, Source Finalized: 483, Received Messages: { rewarded: 42, messages-relayer-account: [43] } Source Chain ->> Complex Relayer: Read Proof of Message 44 and Proof of Message 43 reward at Block 483 Complex Relayer ->> Target Chain: Submit Proof of Message 44 and Proof of Message 43 reward at Block 483 Target Chain ->> Target Chain: Import and Finalize Block 64 Note left of Target Chain: Finalized: 64, Source Finalized: 483, Received Messages: { rewarded: 43, messages-relayer-account: [44] }-->