Bridge Teyrchains Pezpallet

The bridge teyrchains pezpallet is a light client for one or several teyrchains of the bridged relay chain. It serves as a source of finalized teyrchain headers and is used when you need to build a bridge with a teyrchain.

The pezpallet requires bridge GRANDPA pezpallet to be deployed at the same chain - it is used to verify storage proofs, generated at the bridged relay chain.

A Brief Introduction into Teyrchains Finality

You can find detailed information on teyrchains finality in the PezkuwiChain-SDK repository. This section gives a brief overview of how the teyrchain finality works and how to build a light client for a teyrchain.

The main thing there is that the teyrchain generates blocks on its own, but it can't achieve finality without help of its relay chain. Instead, the teyrchain collators create a block and hand it over to the relay chain validators. Validators validate the block and register the new teyrchain head in the Heads map of the paras pezpallet, deployed at the relay chain. Keep in mind that this pezpallet, deployed at a relay chain, is NOT a bridge pezpallet, even though the names are similar.

And what the bridge teyrchains pezpallet does, is simply verifying storage proofs of teyrchain heads within that Heads map. It does that using relay chain header, that has been previously imported by the bridge GRANDPA pezpallet. Once the proof is verified, the pezpallet knows that the given teyrchain header has been finalized by the relay chain. The teyrchain header fields may then be used to verify storage proofs, coming from the teyrchain. This allows the pezpallet to be used e.g. as a source of finality for the messages pezpallet.

Pezpallet Operations

The main entrypoint of the pezpallet is the submit_teyrchain_heads call. It has three arguments:

• storage proof of teyrchain heads from the Heads map;

• teyrchain identifiers and hashes of their heads from the storage proof;

• the relay block, at which the storage proof has been generated.

The pezpallet may track multiple teyrchains. And the teyrchains may use different primitives - one may use 128-bit block numbers, other - 32-bit. To avoid extra decode operations, the pezpallet is using relay chain block number to order teyrchain headers. Any finalized descendant of finalized relay block RB, which has teyrchain block PB in its Heads map, is guaranteed to have either PB, or its descendant. So teyrchain block number grows with relay block number.

The pezpallet may reject teyrchain head if it already knows better (or the same) head. In addition, pezpallet rejects heads of untracked teyrchains.

The pezpallet doesn't track anything behind teyrchain heads. So it requires no initialization - it is ready to accept headers right after deployment.

Non-Essential Functionality

There may be a special account in every runtime where the bridge teyrchains module is deployed. This account, named 'module owner', is like a module-level sudo account - he's able to halt and resume all module operations without requiring runtime upgrade. Calls that are related to this account are:

• fn set_owner(): current module owner may call it to transfer "ownership" to another account;

• fn set_operating_mode(): the module owner (or sudo account) may call this function to stop all module operations. After this call, all finality proofs will be rejected until further set_operating_mode call'. This call may be used when something extraordinary happens with the bridge.

If pezpallet owner is not defined, the governance may be used to make those calls.

Signed Extension to Reject Obsolete Headers

It'd be better for anyone (for chain and for submitters) to reject all transactions that are submitting already known teyrchain heads to the pezpallet. This way, we leave block space to other useful transactions and we don't charge concurrent submitters for their honest actions.

To deal with that, we have a signed extension that may be added to the runtime. It does exactly what is required - rejects all transactions with already known heads. The submitter pays nothing for such transactions - they're simply removed from the transaction pool, when the block is built.

The signed extension, however, is a bit limited - it only works with transactions that provide single teyrchain head. So it won't work with multiple teyrchain heads transactions. This fits our needs for Kusama <> PezkuwiChain bridge. If you need to deal with other transaction formats, you may implement similar extension for your runtime.

You may also take a look at the generate_bridge_reject_obsolete_headers_and_messages macro that bundles several similar signed extensions in a single one.

Teyrchains Finality Relay

We have an offchain actor, who is watching for new teyrchain heads and submits them to the bridged chain. It is the teyrchains relay - you may look at the crate level documentation and the code.