pezkuwi-subxt/bridges/bin/runtime-common/README.md

Helpers for Message Lane Module Integration

The messages module of this crate contains a bunch of helpers for integrating message lane module into your runtime. Basic prerequisites of these helpers are:

• we're going to bridge Substrate-based chain with another Substrate-based chain;
• both chains have message lane module, Substrate bridge module and the call dispatch module;
• all message lanes are identical and may be used to transfer the same messages;
• the messages sent over the bridge are dispatched using call dispatch module;
• the messages are pallet_bridge_call_dispatch::MessagePayload structures, where call field is encoded Call of the target chain. This means that the Call is opaque to the message lane module instance at the source chain. It is pre-encoded by the message submitter;
• all proofs in the message lane module transactions are based on the storage proofs from the bridged chain: storage proof of the outbound message (value from the pallet_message_lane::Store::MessagePayload map), storage proof of the outbound lane state (value from the pallet_message_lane::Store::OutboundLanes map) and storage proof of the inbound lane state (value from the pallet_message_lane::Store::InboundLanes map);
• storage proofs are built at the finalized headers of the corresponding chain. So all message lane transactions with proofs are verifying storage proofs against finalized chain headers from Substrate bridge module.

IMPORTANT NOTE: after reading this document, you may refer to our test runtimes (rialto_messages.rs and/or millau_messages.rs) to see how to use these helpers.

Contents

• MessageBridge Trait
• ChainWithMessageLanes Trait
• Helpers for the Source Chain
• Helpers for the Target Chain

MessageBridge Trait

The essence of your integration will be a struct that implements a MessageBridge trait. Let's review every method and give some implementation hints here:

• MessageBridge::maximal_extrinsic_size_on_target_chain: you will need to return the maximal extrinsic size of the target chain from this function. This may be the constant that is updated when your runtime is upgraded, or you may use the message lane parameters functionality to allow the pallet owner to update this value more frequently (you may also want to use this functionality for all constants that are used in other methods described below).

• MessageBridge::weight_limits_of_message_on_bridged_chain: you'll need to return a range of dispatch weights that the outbound message may take at the target chain. Please keep in mind that our helpers assume that the message is an encoded call of the target chain. But we never decode this call at the source chain. So you can't simply get dispatch weight from pre-dispatch information. Instead there are two options to prepare this range: if you know which calls are to be sent over your bridge, then you may just return weight ranges for these particular calls. Otherwise, if you're going to accept all kinds of calls, you may just return range [0; maximal incoming message dispatch weight]. If you choose the latter, then you shall remember that the delivery transaction itself has some weight, so you can't accept messages with weight equal to maximal weight of extrinsic at the target chain. In our test chains, we reject all messages that have declared dispatch weight larger than 50% of the maximal bridged extrinsic weight.

• MessageBridge::weight_of_delivery_transaction: you will need to return the maximal weight of the delivery transaction that delivers a given message to the target chain. There are three main things to notice:

  1. weight, returned from this function is then used to compute the fee that the message sender needs to pay for the delivery transaction. So it shall not be a simple dispatch weight of delivery call - it should be the "weight" of the transaction itself, including per-byte "weight", "weight" of signed extras and etc.
  2. the delivery transaction brings storage proof of the message, not the message itself. So your transaction will include extra bytes. We suggest computing the size of single empty value storage proof at the source chain, increase this value a bit and hardcode it in the source chain runtime code. This size then must be added to the size of payload and included in the weight computation;
  3. before implementing this function, please take a look at the weight formula of delivery transaction. It adds some extra weight for every additional byte of the proof (everything above pallet_message_lane::EXPECTED_DEFAULT_MESSAGE_LENGTH), so it's not trivial. Even better, please refer to our implementation for test chains for details.

• MessageBridge::weight_of_delivery_confirmation_transaction_on_this_chain: you'll need to return the maximal weight of a single message delivery confirmation transaction on this chain. All points from the previous paragraph are also relevant here.

• MessageBridge::this_weight_to_this_balance: this function needs to convert weight units into fee units on this chain. Most probably this can be done by calling pallet_transaction_payment::Config::WeightToFee::calc() for passed weight.

• MessageBridge::bridged_weight_to_bridged_balance: this function needs to convert weight units into fee units on the target chain. The best case is when you have the same conversion formula on both chains - then you may just call the same formula from the previous paragraph. Otherwise, you'll need to hardcode this formula into your runtime.

• MessageBridge::bridged_balance_to_this_balance: this may be the easiest method to implement and the hardest to maintain at the same time. If you don't have any automatic methods to determine conversion rate, then you'll probably need to maintain it by yourself (by updating conversion rate, stored in runtime storage). This means that if you're too late with an update, then you risk to accept messages with lower-than-expected fee. So it may be wise to have some reserve in this conversion rate, even if that means larger delivery and dispatch fees.

ChainWithMessageLanes Trait

Apart from its methods, MessageBridge also has two associated types that are implementing the ChainWithMessageLanes trait. One is for this chain and the other is for the bridged chain. The trait is quite simple and can easily be implemented - you just need to specify types used at the corresponding chain. There are two exceptions, though. Both may be changed in the future. Here they are:

• ChainWithMessageLanes::Call: it isn't a good idea to reference bridged chain runtime from your runtime (cyclic references + maintaining on upgrades). So you can't know the type of bridged chain call in your runtime. This type isn't actually used at this chain, so you may use () instead.

• ChainWithMessageLanes::MessageLaneInstance: this is used to compute runtime storage keys. There may be several instances of message lane pallet, included in the Runtime. Every instance stores messages and these messages stored under different keys. When we are verifying storage proofs from the bridged chain, we should know which instance we're talking to. This is fine, but there's significant inconvenience with that - this chain runtime must have the same message lane pallet instance. This does not necessarily mean that we should use the same instance on both chains - this instance may be used to bridge with another chain/instance, or may not be used at all.

Helpers for the Source Chain

The helpers for the Source Chain reside in the source submodule of the messages module. The structs are: FromThisChainMessagePayload, FromBridgedChainMessagesDeliveryProof, FromThisChainMessageVerifier. And the helper functions are: maximal_message_size, verify_chain_message, verify_messages_delivery_proof and estimate_message_dispatch_and_delivery_fee.

FromThisChainMessagePayload is a message that the sender sends through our bridge. It is the pallet_bridge_call_dispatch::MessagePayload, where call field is encoded target chain call. So at this chain we don't see internals of this call - we just know its size.

FromThisChainMessageVerifier is an implementation of bp_message_lane::LaneMessageVerifier. It has following checks in its verify_message method:

1. it'll verify that the used outbound lane is enabled in our runtime;

2. it'll reject messages if there are too many undelivered outbound messages at this lane. The sender need to wait while relayers will do their work before sending the message again;

3. it'll reject a message if it has the wrong dispatch origin declared. Like if the submitter is not the root of this chain, but it tries to dispatch the message at the target chain using pallet_bridge_call_dispatch::CallOrigin::SourceRoot origin. Or he has provided wrong signature in the pallet_bridge_call_dispatch::CallOrigin::TargetAccount origin;

4. it'll reject a message if the delivery and dispatch fee that the submitter wants to pay is lesser than the fee that is computed using the estimate_message_dispatch_and_delivery_fee function.

estimate_message_dispatch_and_delivery_fee returns a minimal fee that the submitter needs to pay for sending a given message. The fee includes: payment for the delivery transaction at the target chain, payment for delivery confirmation transaction on this chain, payment for Call dispatch at the target chain and relayer interest.

FromBridgedChainMessagesDeliveryProof holds the lane identifier and the storage proof of this inbound lane state at the bridged chain. This also holds the hash of the target chain header, that was used to generate this storage proof. The proof is verified by the verify_messages_delivery_proof, which simply checks that the target chain header is finalized (using Substrate bridge module) and then reads the inbound lane state from the proof.

verify_chain_message function checks that the message may be delivered to the bridged chain. There are two main checks:

1. that the message size is less than or equal to the 2/3 of maximal extrinsic size at the target chain. We leave 1/3 for signed extras and for the storage proof overhead;

2. that the message dispatch weight is less than or equal to the 1/2 of maximal normal extrinsic weight at the target chain. We leave 1/2 for the delivery transaction overhead.

Helpers for the Target Chain

The helpers for the target chain reside in the target submodule of the messages module. The structs are: FromBridgedChainMessagePayload, FromBridgedChainMessagesProof, FromBridgedChainMessagesProof. And the helper functions are: maximal_incoming_message_dispatch_weight, maximal_incoming_message_size and verify_messages_proof.

FromBridgedChainMessagePayload corresponds to the FromThisChainMessagePayload at the bridged chain. We expect that messages with this payload are stored in the OutboundMessages storage map of the message lane module. This map is used to build FromBridgedChainMessagesProof. The proof holds the lane id, range of message nonces included in the proof, storage proof of OutboundMessages entries and the hash of bridged chain header that has been used to build the proof. Additionally, there's storage proof may contain the proof of outbound lane state. It may be required to prune relayers entries at this chain (see message lane module documentation for details). This proof is verified by the verify_messages_proof function.