Collation Generation
The collation generation subsystem is executed on collator nodes and produces candidates to be distributed to validators. If configured to produce collations for a para, it produces collations and then feeds them to the Collator Protocol subsystem, which handles the networking.
Protocol
Collation generation for Parachains currently works in the following way:
- A new relay chain block is imported.
- The collation generation subsystem checks if the core associated to the parachain is free and if yes, continues.
- Collation generation calls our collator callback, if present, to generate a PoV. If none exists, do nothing.
- Authoring logic determines if the current node should build a PoV.
- Build new PoV and give it back to collation generation.
Messages
Incoming
ActiveLeaves
- Notification of a change in the set of active leaves.
- Triggers collation generation procedure outlined in "Protocol" section.
CollationGenerationMessage::Initialize
- Initializes the subsystem. Carries a config.
- No more than one initialization message should ever be sent to the collation generation subsystem.
- Sent by a collator to initialize this subsystem.
CollationGenerationMessage::SubmitCollation
- If the subsystem isn't initialized or the relay-parent is too old to be relevant, ignore the message.
- Otherwise, use the provided parameters to generate a [
CommittedCandidateReceipt
] - Submit the collation to the collator-protocol with
CollatorProtocolMessage::DistributeCollation
.
Outgoing
CollatorProtocolMessage::DistributeCollation
- Provides a generated collation to distribute to validators.
Functionality
The process of generating a collation for a parachain is very parachain-specific. As such, the details of how to do so are left beyond the scope of this description. The subsystem should be implemented as an abstract wrapper, which is aware of this configuration:
#![allow(unused)] fn main() { /// The output of a collator. /// /// This differs from `CandidateCommitments` in two ways: /// /// - does not contain the erasure root; that's computed at the Polkadot level, not at Cumulus /// - contains a proof of validity. pub struct Collation { /// Messages destined to be interpreted by the Relay chain itself. pub upward_messages: Vec<UpwardMessage>, /// The horizontal messages sent by the parachain. pub horizontal_messages: Vec<OutboundHrmpMessage<ParaId>>, /// New validation code. pub new_validation_code: Option<ValidationCode>, /// The head-data produced as a result of execution. pub head_data: HeadData, /// Proof to verify the state transition of the parachain. pub proof_of_validity: PoV, /// The number of messages processed from the DMQ. pub processed_downward_messages: u32, /// The mark which specifies the block number up to which all inbound HRMP messages are processed. pub hrmp_watermark: BlockNumber, } /// Result of the [`CollatorFn`] invocation. pub struct CollationResult { /// The collation that was build. pub collation: Collation, /// An optional result sender that should be informed about a successfully seconded collation. /// /// There is no guarantee that this sender is informed ever about any result, it is completely okay to just drop it. /// However, if it is called, it should be called with the signed statement of a parachain validator seconding the /// collation. pub result_sender: Option<oneshot::Sender<CollationSecondedSignal>>, } /// Signal that is being returned when a collation was seconded by a validator. pub struct CollationSecondedSignal { /// The hash of the relay chain block that was used as context to sign [`Self::statement`]. pub relay_parent: Hash, /// The statement about seconding the collation. /// /// Anything else than `Statement::Seconded` is forbidden here. pub statement: SignedFullStatement, } /// Collation function. /// /// Will be called with the hash of the relay chain block the parachain block should be build on and the /// [`ValidationData`] that provides information about the state of the parachain on the relay chain. /// /// Returns an optional [`CollationResult`]. pub type CollatorFn = Box< dyn Fn( Hash, &PersistedValidationData, ) -> Pin<Box<dyn Future<Output = Option<CollationResult>> + Send>> + Send + Sync, >; /// Configuration for the collation generator pub struct CollationGenerationConfig { /// Collator's authentication key, so it can sign things. pub key: CollatorPair, /// Collation function. See [`CollatorFn`] for more details. pub collator: Option<CollatorFn>, /// The parachain that this collator collates for pub para_id: ParaId, } }
The configuration should be optional, to allow for the case where the node is not run with the capability to collate.
Summary in plain English
-
Collation (output of a collator)
- Contains the PoV (proof to verify the state transition of the parachain) and other data.
-
Collation result
- Contains the collation, and an optional result sender for a collation-seconded signal.
-
Collation seconded signal
- The signal that is returned when a collation was seconded by a validator.
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Collation function
- Called with the relay chain block the parablock will be built on top of.
- Called with the validation data.
- Provides information about the state of the parachain on the relay chain.
-
Collation generation config
- Contains collator's authentication key, optional collator function, and parachain ID.