# GRANDPA Finality Relay

Source Chain is running GRANDPA Finality Gadget. Bridge GRANDPA finality pallet is deployed at Target Chain runtime. Relayer is configured to relay Source Chain finality to Target Chain.

sequenceDiagram participant Source Chain participant Relayer participant Target Chain Note left of Source Chain: Best: 500, Finalized: 480, Authorities Set Index: 42 Note right of Target Chain: Uninitialized Source Chain ->> Relayer: Read Initialization Data Relayer ->> Target Chain: Initialize Bridge GRANDPA Finality Pallet Note right of Target Chain: Finalized: 480, Authorities Set Index: 42 Source Chain ->> Source Chain: Import Block 501 Source Chain ->> Source Chain: Import Block 502 Source Chain ->> Source Chain: Finalize Block 495 Source Chain ->> Relayer: Read Finality Proof of Block 495 Relayer ->> Target Chain: Finality Proof of Block 495 Note right of Target Chain: Finalized: 495, Authorities Set Index: 42 Source Chain ->> Source Chain: Import Block 503 that changes Authorities Set to 43 Source Chain ->> Source Chain: Finalize Block 500 Note left of Relayer: Relayer Misses Finality Notification for Block 500 Source Chain ->> Source Chain: Import Block 504 Source Chain ->> Source Chain: Finalize Mandatory Block 503 Source Chain ->> Source Chain: Finalize Block 504 Source Chain ->> Relayer: Read Finality Proof of Mandatory Block 503 Relayer ->> Target Chain: Finality Proof of Block 503 Note right of Target Chain: Finalized: 503, Authorities Set Index: 43