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.


Input: CollationGenerationMessage

fn main() {
enum CollationGenerationMessage {

No more than one initialization message should ever be sent to the collation generation subsystem.

Output: CollationDistributionMessage


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:

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(
    ) -> 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: 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.

On ActiveLeavesUpdate:

  • If there is no collation generation config, ignore.
  • Otherwise, for each activated head in the update:
    • Determine if the para is scheduled on any core by fetching the availability_cores Runtime API.
    • Determine an occupied core assumption to make about the para. Scheduled cores can make OccupiedCoreAssumption::Free.
    • Use the Runtime API subsystem to fetch the full validation data.
    • Invoke the collator, and use its outputs to produce a CandidateReceipt, signed with the configuration's key.
    • Dispatch a CollatorProtocolMessage::DistributeCollation(receipt, pov).