pezkuwi-subxt/polkadot/modules/messages

Messages Module

The messages module is used to deliver messages from source chain to target chain. Message is (almost) opaque to the module and the final goal is to hand message to the message dispatch mechanism.

Contents

• Overview
• Message Workflow
• Integrating Message Lane Module into Runtime
• Non-Essential Functionality
• Weights of Module Extrinsics

Overview

Message lane is an unidirectional channel, where messages are sent from source chain to the target chain. At the same time, a single instance of messages module supports both outbound lanes and inbound lanes. So the chain where the module is deployed (this chain), may act as a source chain for outbound messages (heading to a bridged chain) and as a target chain for inbound messages (coming from a bridged chain).

Messages module supports multiple message lanes. Every message lane is identified with a 4-byte identifier. Messages sent through the lane are assigned unique (for this lane) increasing integer value that is known as nonce ("number that can only be used once"). Messages that are sent over the same lane are guaranteed to be delivered to the target chain in the same order they're sent from the source chain. In other words, message with nonce N will be delivered right before delivering a message with nonce N+1.

Single message lane may be seen as a transport channel for single application (onchain, offchain or mixed). At the same time the module itself never dictates any lane or message rules. In the end, it is the runtime developer who defines what message lane and message mean for this runtime.

Message Workflow

The message "appears" when its submitter calls the send_message() function of the module. The submitter specifies the lane that he's willing to use, the message itself and the fee that he's willing to pay for the message delivery and dispatch. If a message passes all checks, the nonce is assigned and the message is stored in the module storage. The message is in an "undelivered" state now.

We assume that there are external, offchain actors, called relayers, that are submitting module related transactions to both target and source chains. The pallet itself has no assumptions about relayers incentivization scheme, but it has some callbacks for paying rewards. See Integrating Messages Module into runtime for details.

Eventually, some relayer would notice this message in the "undelivered" state and it would decide to deliver this message. Relayer then crafts receive_messages_proof() transaction (aka delivery transaction) for the messages module instance, deployed at the target chain. Relayer provides his account id at the source chain, the proof of message (or several messages), the number of messages in the transaction and their cumulative dispatch weight. Once a transaction is mined, the message is considered "delivered".

Once a message is delivered, the relayer may want to confirm delivery back to the source chain. There are two reasons why he would want to do that. The first is that we intentionally limit number of "delivered", but not yet "confirmed" messages at inbound lanes (see What about other Constants in the Messages Module Configuration Trait for explanation). So at some point, the target chain may stop accepting new messages until relayers confirm some of these. The second is that if the relayer wants to be rewarded for delivery, he must prove the fact that he has actually delivered the message. And this proof may only be generated after the delivery transaction is mined. So relayer crafts the receive_messages_delivery_proof() transaction (aka confirmation transaction) for the messages module instance, deployed at the source chain. Once this transaction is mined, the message is considered "confirmed".

The "confirmed" state is the final state of the message. But there's one last thing related to the message - the fact that it is now "confirmed" and reward has been paid to the relayer (or at least callback for this has been called), must be confirmed to the target chain. Otherwise, we may reach the limit of "unconfirmed" messages at the target chain and it will stop accepting new messages. So relayer sometimes includes a nonce of the latest "confirmed" message in the next receive_messages_proof() transaction, proving that some messages have been confirmed.

Integrating Messages Module into Runtime

As it has been said above, the messages module supports both outbound and inbound message lanes. So if we will integrate a module in some runtime, it may act as the source chain runtime for outbound messages and as the target chain runtime for inbound messages. In this section, we'll sometimes refer to the chain we're currently integrating with, as this chain and the other chain as bridged chain.

Messages module doesn't simply accept transactions that are claiming that the bridged chain has some updated data for us. Instead of this, the module assumes that the bridged chain is able to prove that updated data in some way. The proof is abstracted from the module and may be of any kind. In our Substrate-to-Substrate bridge we're using runtime storage proofs. Other bridges may use transaction proofs, Substrate header digests or anything else that may be proved.

IMPORTANT NOTE: everything below in this chapter describes details of the messages module configuration. But if you interested in well-probed and relatively easy integration of two Substrate-based chains, you may want to look at the bridge-runtime-common crate. This crate is providing a lot of helpers for integration, which may be directly used from within your runtime. Then if you'll decide to change something in this scheme, get back here for detailed information.

General Information

The messages module supports instances. Every module instance is supposed to bridge this chain and some bridged chain. To bridge with another chain, using another instance is suggested (this isn't forced anywhere in the code, though).

Message submitters may track message progress by inspecting module events. When Message is accepted, the MessageAccepted event is emitted in the send_message() transaction. The event contains both message lane identifier and nonce that has been assigned to the message. When a message is delivered to the target chain, the MessagesDelivered event is emitted from the receive_messages_delivery_proof() transaction. The MessagesDelivered contains the message lane identifier, inclusive range of delivered message nonces and their single-bit dispatch results.

Please note that the meaning of the 'dispatch result' is determined by the message dispatcher at the target chain. For example, in case of immediate call dispatcher it will be the true if call has been successfully dispatched and false if it has only been delivered. This simple mechanism built into the messages module allows building basic bridge applications, which only care whether their messages have been successfully dispatched or not. More sophisticated applications may use their own dispatch result delivery mechanism to deliver something larger than single bit.

How to plug-in Messages Module to Send Messages to the Bridged Chain?

The pallet_bridge_messages::Config trait has 3 main associated types that are used to work with outbound messages. The pallet_bridge_messages::Config::TargetHeaderChain defines how we see the bridged chain as the target for our outbound messages. It must be able to check that the bridged chain may accept our message - like that the message has size below maximal possible transaction size of the chain and so on. And when the relayer sends us a confirmation transaction, this implementation must be able to parse and verify the proof of messages delivery. Normally, you would reuse the same (configurable) type on all chains that are sending messages to the same bridged chain.

The pallet_bridge_messages::Config::LaneMessageVerifier defines a single callback to verify outbound messages. The simplest callback may just accept all messages. But in this case you'll need to answer many questions first. Who will pay for the delivery and confirmation transaction? Are we sure that someone will ever deliver this message to the bridged chain? Are we sure that we don't bloat our runtime storage by accepting this message? What if the message is improperly encoded or has some fields set to invalid values? Answering all those (and similar) questions would lead to correct implementation.

There's another thing to consider when implementing type for use in pallet_bridge_messages::Config::LaneMessageVerifier. It is whether we treat all message lanes identically, or they'll have different sets of verification rules? For example, you may reserve lane#1 for messages coming from some 'wrapped-token' pallet - then you may verify in your implementation that the origin is associated with this pallet. Lane#2 may be reserved for 'system' messages and you may charge zero fee for such messages. You may have some rate limiting for messages sent over the lane#3. Or you may just verify the same rules set for all outbound messages - it is all up to the pallet_bridge_messages::Config::LaneMessageVerifier implementation.

The last type is the pallet_bridge_messages::Config::MessageDeliveryAndDispatchPayment. When all checks are made and we have decided to accept the message, we're calling the pay_delivery_and_dispatch_fee() callback, passing the corresponding argument of the send_message function. Later, when message delivery is confirmed, we're calling pay_relayers_rewards() callback, passing accounts of relayers and messages that they have delivered. The simplest implementation of this trait is in the instant_payments.rs module and simply calls Currency::transfer() when those callbacks are called. So Currency units are transferred between submitter, 'relayers fund' and relayers accounts. Other implementations may use more or less sophisticated techniques - the whole relayers incentivization scheme is not a part of the messages module.

I have a Messages Module in my Runtime, but I Want to Reject all Outbound Messages. What shall I do?

You should be looking at the bp_messages::source_chain::ForbidOutboundMessages structure bp_messages::source_chain. It implements all required traits and will simply reject all transactions, related to outbound messages.

How to plug-in Messages Module to Receive Messages from the Bridged Chain?

The pallet_bridge_messages::Config trait has 2 main associated types that are used to work with inbound messages. The pallet_bridge_messages::Config::SourceHeaderChain defines how we see the bridged chain as the source or our inbound messages. When relayer sends us a delivery transaction, this implementation must be able to parse and verify the proof of messages wrapped in this transaction. Normally, you would reuse the same (configurable) type on all chains that are sending messages to the same bridged chain.

The pallet_bridge_messages::Config::MessageDispatch defines a way on how to dispatch delivered messages. Apart from actually dispatching the message, the implementation must return the correct dispatch weight of the message before dispatch is called.

I have a Messages Module in my Runtime, but I Want to Reject all Inbound Messages. What

shall I do?

You should be looking at the bp_messages::target_chain::ForbidInboundMessages structure from the bp_messages::target_chain module. It implements all required traits and will simply reject all transactions, related to inbound messages.

What about other Constants in the Messages Module Configuration Trait?

Message is being stored in the source chain storage until its delivery will be confirmed. After that, we may safely remove the message from the storage. Lane messages are removed (pruned) when someone sends a new message using the same lane. So the message submitter pays for that pruning. To avoid pruning too many messages in a single transaction, there's pallet_bridge_messages::Config::MaxMessagesToPruneAtOnce configuration parameter. We will never prune more than this number of messages in the single transaction. That said, the value should not be too big to avoid waste of resources when there are no messages to prune.

To be able to reward the relayer for delivering messages, we store a map of message nonces range => identifier of the relayer that has delivered this range at the target chain runtime storage. If a relayer delivers multiple consequent ranges, they're merged into single entry. So there may be more than one entry for the same relayer. Eventually, this whole map must be delivered back to the source chain to confirm delivery and pay rewards. So to make sure we are able to craft this confirmation transaction, we need to: (1) keep the size of this map below a certain limit and (2) make sure that the weight of processing this map is below a certain limit. Both size and processing weight mostly depend on the number of entries. The number of entries is limited with the pallet_bridge_messages::ConfigMaxUnrewardedRelayerEntriesAtInboundLane parameter. Processing weight also depends on the total number of messages that are being confirmed, because every confirmed message needs to be read. So there's another pallet_bridge_messages::Config::MaxUnconfirmedMessagesAtInboundLane parameter for that.

When choosing values for these parameters, you must also keep in mind that if proof in your scheme is based on finality of headers (and it is the most obvious option for Substrate-based chains with finality notion), then choosing too small values for these parameters may cause significant delays in message delivery. That's because there are too many actors involved in this scheme: 1) authorities that are finalizing headers of the target chain need to finalize header with non-empty map; 2) the headers relayer then needs to submit this header and its finality proof to the source chain; 3) the messages relayer must then send confirmation transaction (storage proof of this map) to the source chain; 4) when the confirmation transaction will be mined at some header, source chain authorities must finalize this header; 5) the headers relay then needs to submit this header and its finality proof to the target chain; 6) only now the messages relayer may submit new messages from the source to target chain and prune the entry from the map.

Delivery transaction requires the relayer to provide both number of entries and total number of messages in the map. This means that the module never charges an extra cost for delivering a map - the relayer would need to pay exactly for the number of entries+messages it has delivered. So the best guess for values of these parameters would be the pair that would occupy N percent of the maximal transaction size and weight of the source chain. The N should be large enough to process large maps, at the same time keeping reserve for future source chain upgrades.

Non-Essential Functionality

Apart from the message related calls, the module exposes a set of auxiliary calls. They fall in two groups, described in the next two paragraphs.

There may be a special account in every runtime where the messages module is deployed. This account, named 'module owner', is like a module-level sudo account - he's able to halt all and result all module operations without requiring runtime upgrade. The module may have no message owner, but we suggest to use it at least for initial deployment. To calls that are related to this account are:

• fn set_owner(): current module owner may call it to transfer "ownership" to another account;
• fn halt_operations(): the module owner (or sudo account) may call this function to stop all module operations. After this call, all message-related transactions will be rejected until further resume_operations call'. This call may be used when something extraordinary happens with the bridge;
• fn resume_operations(): module owner may call this function to resume bridge operations. The module will resume its regular operations after this call.

Apart from halting and resuming the bridge, the module owner may also tune module configuration parameters without runtime upgrades. The set of parameters needs to be designed in advance, though. The module configuration trait has associated Parameter type, which may be e.g. enum and represent a set of parameters that may be updated by the module owner. For example, if your bridge needs to convert sums between different tokens, you may define a 'conversion rate' parameter and let the module owner update this parameter when there are significant changes in the rate. The corresponding module call is fn update_pallet_parameter().

Weights of Module Extrinsics

The main assumptions behind weight formulas is:

• all possible costs are paid in advance by the message submitter;
• whenever possible, relayer tries to minimize cost of its transactions. So e.g. even though sender always pays for delivering outbound lane state proof, relayer may not include it in the delivery transaction (unless messages module on target chain requires that);
• weight formula should incentivize relayer to not to submit any redundant data in the extrinsics arguments;
• the extrinsic shall never be executing slower (i.e. has larger actual weight) than defined by the formula.

Weight of send_message call

Related benchmarks

Benchmark	Description
send_minimal_message_worst_case	Sends 0-size message with worst possible conditions
send_1_kb_message_worst_case	Sends 1KB-size message with worst possible conditions
send_16_kb_message_worst_case	Sends 16KB-size message with worst possible conditions

Weight formula

The weight formula is:

Weight = BaseWeight + MessageSizeInKilobytes * MessageKiloByteSendWeight

Where:

Component	How it is computed?	Description
SendMessageOverhead	send_minimal_message_worst_case	Weight of sending minimal (0 bytes) message
MessageKiloByteSendWeight	(send_16_kb_message_worst_case - send_1_kb_message_worst_case)/15	Weight of sending every additional kilobyte of the message

Weight of receive_messages_proof call

Related benchmarks

Benchmark	Description*
receive_single_message_proof	Receives proof of single EXPECTED_DEFAULT_MESSAGE_LENGTH message
receive_two_messages_proof	Receives proof of two identical EXPECTED_DEFAULT_MESSAGE_LENGTH messages
receive_single_message_proof_with_outbound_lane_state	Receives proof of single EXPECTED_DEFAULT_MESSAGE_LENGTH message and proof of outbound lane state at the source chain
receive_single_message_proof_1_kb	Receives proof of single message. The proof has size of approximately 1KB**
receive_single_message_proof_16_kb	Receives proof of single message. The proof has size of approximately 16KB**

* - In all benchmarks all received messages are dispatched and their dispatch cost is near to zero

** - Trie leafs are assumed to have minimal values. The proof is derived from the minimal proof by including more trie nodes. That's because according to receive_message_proofs_with_large_leaf and receive_message_proofs_with_extra_nodes benchmarks, increasing proof by including more nodes has slightly larger impact on performance than increasing values stored in leafs.

Weight formula

The weight formula is:

Weight = BaseWeight + OutboundStateDeliveryWeight
       + MessagesCount * MessageDeliveryWeight
       + MessagesDispatchWeight
       + Max(0, ActualProofSize - ExpectedProofSize) * ProofByteDeliveryWeight

Where:

Component	How it is computed?	Description
BaseWeight	2*receive_single_message_proof - receive_two_messages_proof	Weight of receiving and parsing minimal proof
OutboundStateDeliveryWeight	receive_single_message_proof_with_outbound_lane_state - receive_single_message_proof	Additional weight when proof includes outbound lane state
MessageDeliveryWeight	receive_two_messages_proof - receive_single_message_proof	Weight of of parsing and dispatching (without actual dispatch cost) of every message
MessagesCount		Provided by relayer
MessagesDispatchWeight		Provided by relayer
ActualProofSize		Provided by relayer
ExpectedProofSize	EXPECTED_DEFAULT_MESSAGE_LENGTH * MessagesCount + EXTRA_STORAGE_PROOF_SIZE	Size of proof that we are expecting. This only includes EXTRA_STORAGE_PROOF_SIZE once, because we assume that intermediate nodes likely to be included in the proof only once. This may be wrong, but since weight of processing proof with many nodes is almost equal to processing proof with large leafs, additional cost will be covered because we're charging for extra proof bytes anyway
ProofByteDeliveryWeight	(receive_single_message_proof_16_kb - receive_single_message_proof_1_kb) / (15 * 1024)	Weight of processing every additional proof byte over ExpectedProofSize limit

Why for every message sent using send_message we will be able to craft receive_messages_proof transaction?

We have following checks in send_message transaction on the source chain:

• message size should be less than or equal to 2/3 of maximal extrinsic size on the target chain;
• message dispatch weight should be less than or equal to the 1/2 of maximal extrinsic dispatch weight on the target chain.

Delivery transaction is an encoded delivery call and signed extensions. So we have 1/3 of maximal extrinsic size reserved for:

• storage proof, excluding the message itself. Currently, on our test chains, the overhead is always within EXTRA_STORAGE_PROOF_SIZE limits (1024 bytes);
• signed extras and other call arguments (relayer_id: SourceChain::AccountId, messages_count: u32, dispatch_weight: u64).

On Millau chain, maximal extrinsic size is 0.75 * 2MB, so 1/3 is 512KB (524_288 bytes). This should be enough to cover these extra arguments and signed extensions.

Let's exclude message dispatch cost from single message delivery transaction weight formula:

Weight = BaseWeight + OutboundStateDeliveryWeight + MessageDeliveryWeight
       + Max(0, ActualProofSize - ExpectedProofSize) * ProofByteDeliveryWeight

So we have 1/2 of maximal extrinsic weight to cover these components. BaseWeight, OutboundStateDeliveryWeight and MessageDeliveryWeight are determined using benchmarks and are hardcoded into runtime. Adequate relayer would only include required trie nodes into the proof. So if message size would be maximal (2/3 of MaximalExtrinsicSize), then the extra proof size would be MaximalExtrinsicSize / 3 * 2 - EXPECTED_DEFAULT_MESSAGE_LENGTH.

Both conditions are verified by pallet_bridge_messages::ensure_weights_are_correct and pallet_bridge_messages::ensure_able_to_receive_messages functions, which must be called from every runtime's tests.

Post-dispatch weight refunds of the receive_messages_proof call

Weight formula of the receive_messages_proof call assumes that the dispatch fee of every message is paid at the target chain (where call is executed), that every message will be dispatched and that dispatch weight of the message will be exactly the weight that is returned from the MessageDispatch::dispatch_weight method call. This isn't true for all messages, so the call returns actual weight used to dispatch messages.

This actual weight is the weight, returned by the weight formula, minus:

• the weight of undispatched messages, if we have failed to dispatch because of different issues;
• the unspent dispatch weight if the declared weight of some messages is less than their actual post-dispatch weight;
• the pay-dispatch-fee weight for every message that had dispatch fee paid at the source chain.

The last component is computed as a difference between two benchmarks results - the receive_single_message_proof benchmark (that assumes that the fee is paid during dispatch) and the receive_single_prepaid_message_proof (that assumes that the dispatch fee is already paid).

Weight of receive_messages_delivery_proof call

Related benchmarks

Benchmark	Description
receive_delivery_proof_for_single_message	Receives proof of single message delivery
receive_delivery_proof_for_two_messages_by_single_relayer	Receives proof of two messages delivery. Both messages are delivered by the same relayer
receive_delivery_proof_for_two_messages_by_two_relayers	Receives proof of two messages delivery. Messages are delivered by different relayers

Weight formula

The weight formula is:

Weight = BaseWeight + MessagesCount * MessageConfirmationWeight
       + RelayersCount * RelayerRewardWeight
       + Max(0, ActualProofSize - ExpectedProofSize) * ProofByteDeliveryWeight
       + MessagesCount * (DbReadWeight + DbWriteWeight)

Where:

Component	How it is computed?	Description
BaseWeight	2*receive_delivery_proof_for_single_message - receive_delivery_proof_for_two_messages_by_single_relayer	Weight of receiving and parsing minimal delivery proof
MessageDeliveryWeight	receive_delivery_proof_for_two_messages_by_single_relayer - receive_delivery_proof_for_single_message	Weight of confirming every additional message
MessagesCount		Provided by relayer
RelayerRewardWeight	receive_delivery_proof_for_two_messages_by_two_relayers - receive_delivery_proof_for_two_messages_by_single_relayer	Weight of rewarding every additional relayer
RelayersCount		Provided by relayer
ActualProofSize		Provided by relayer
ExpectedProofSize	EXTRA_STORAGE_PROOF_SIZE	Size of proof that we are expecting
ProofByteDeliveryWeight	(receive_single_message_proof_16_kb - receive_single_message_proof_1_kb) / (15 * 1024)	Weight of processing every additional proof byte over ExpectedProofSize limit. We're using the same formula, as for message delivery, because proof mechanism is assumed to be the same in both cases

Post-dispatch weight refunds of the receive_messages_delivery_proof call

Weight formula of the receive_messages_delivery_proof call assumes that all messages in the proof are actually delivered (so there are no already confirmed messages) and every messages is processed by the OnDeliveryConfirmed callback. This means that for every message, we're adding single db read weight and single db write weight. If, by some reason, messages are not processed by the OnDeliveryConfirmed callback, or their processing is faster than that additional weight, the difference is refunded to the submitter.

Why we're always able to craft receive_messages_delivery_proof transaction?

There can be at most <PeerRuntime as pallet_bridge_messages::Config>::MaxUnconfirmedMessagesAtInboundLane messages and at most <PeerRuntime as pallet_bridge_messages::Config>::MaxUnrewardedRelayerEntriesAtInboundLane unrewarded relayers in the single delivery confirmation transaction.

We're checking that this transaction may be crafted in the pallet_bridge_messages::ensure_able_to_receive_confirmation function, which must be called from every runtime' tests.