parachains

Parachains Finality Relay

The parachains finality relay works with two chains - source relay chain and target chain (which may be standalone chain, relay chain or a parachain). The source chain must have the paras pallet deployed at its runtime. The target chain must have the bridge parachains pallet deployed at its runtime.

The relay is configured to submit heads of one or several parachains. It pokes source chain periodically and compares parachain heads that are known to the source relay chain to heads at the target chain. If there are new heads, the relay submits them to the target chain.

More: Parachains Finality Relay Sequence Diagram.

How to Use the Parachains Finality Relay

There are only two traits that need to be implemented. The SourceChain implementation is supposed to connect to the source chain node. It must be able to read parachain heads from the Heads map of the paras pallet. It also must create storage proofs of Heads map entries, when required.

The TargetChain implementation connects to the target chain node. It must be able to return the best known head of given parachain. When required, it must be able to craft and submit parachains finality delivery transaction to the target node.

The main entrypoint for the crate is the run function, which takes source and target clients and ParachainSyncParams parameters. The most important parameter is the parachains - it is the set of parachains, which relay tracks and updates. The other important parameter that may affect the relay operational costs is the strategy. If it is set to Any, then the finality delivery transaction is submitted if at least one of tracked parachain heads is updated. The other option is All. Then the relay waits until all tracked parachain heads are updated and submits them all in a single finality delivery transaction.

Parachain Finality Relay Metrics

Every parachain in Polkadot is identified by the 32-bit number. All metrics, exposed by the parachains finality relay have the parachain label, which is set to the parachain id. And the metrics are prefixed with the prefix, that depends on the name of the source relay and target chains. The list below shows metrics names for Rococo (source relay chain) to BridgeHubWestend (target chain) parachains finality relay. For other chains, simply change chain names. So the metrics are:

• Rococo_to_BridgeHubWestend_Parachains_best_parachain_block_number_at_source - returns best known parachain block number, registered in the paras pallet at the source relay chain (Rococo in our example);

• Rococo_to_BridgeHubWestend_Parachains_best_parachain_block_number_at_target - returns best known parachain block number, registered in the bridge parachains pallet at the target chain (BridgeHubWestend in our example).

If relay operates properly, you should see that the Rococo_to_BridgeHubWestend_Parachains_best_parachain_block_number_at_target tries to reach the Rococo_to_BridgeHubWestend_Parachains_best_parachain_block_number_at_source. And the latter one always increases.